Appendix A
Guidance for Aerial Mapping and Surveying  
A.1	Introduction
This Appendix presents Federal Emergency Management Agency (FEMA) aerial mapping and 
surveying guidelines and specifications that have been established to specify the quality of the 
spatial data products to be produced, including work maps and base maps used in the production of 
Flood Insurance Rate Maps (FIRMs), Digital Flood Insurance Rate Maps (DFIRMs), and DFIRM-
Digital Line Graphs (DFIRM-DLGs).  The term "FIRM" is used generically hereinafter to specify 
this "family" of FEMA spatial products. 
This Appendix also includes guidance for ground surveys of control points, cross sections, and 
hydraulic structures; topographic mapping using photogrammetry, LIght Detection and Ranging 
(LIDAR) or other airborne remote-sensing technologies, consistent with FEMA and industry 
standards.  
[February 2002]
A.1.1	Base Maps
A FIRM base map is a planimetric map, in digital or hardcopy format, showing the georeferenced 
horizontal location of mapped features, without depiction of elevation data such as contour lines. 
Georeferencing means that the map depicts the spherical earth projected as a plane map, normally 
with Universal Transverse Mercator (UTM) or State Plane coordinates, with or without tick marks 
or lines that depict parallels (lines of equal latitude) or meridians (lines of equal longitude). Base 
maps may be categorized as either vector maps or raster image maps, depending on how they are 
produced. 
Vector maps result from linear features (e.g., roads, railroads, streams) digitized as single-line 
centerlines or, alternatively, as dual-lines showing the outer extremities of linear features (e.g., left 
and right banks of streams, curb lines on both sides of streets).  Vector maps are digitized in a way 
so that a Geographic Information System (GIS) can automatically derive intelligence from the 
vector features and perform analyses (e.g., adjacency analyses, proximity analyses, and connectivity 
analyses).  
Raster image maps result from digital scanning of paper maps, map negatives, aerial photographs, 
and orthorectification of those images so that they are accurately georeferenced with distortions 
removed.  Raster image maps rely on human interpretation of scanned or imaged features to derive 
intelligence therefrom.  The most common form of raster image map is the digital orthophoto, 
especially the standard digital orthophoto quarter-quad (DOQ) produced by the U.S. Geological 
Survey (USGS). 
For a more detailed discussion of FIRM base maps, see Appendix K of these Guidelines.  
[February 2002]
A.1.2	Work Maps
FIRM work maps are discussed in Volume I, Subsection 1.4.2.3.4, as part of the discussion of 
requirements for flood hazard analyses performed as part of a Flood Map Project.  
[February 2002]
A.1.3	Manual and Automated Analysis Procedures
A distinction is made in this Appendix between manual and automated hydrologic and hydraulic 
analysis procedures.  Manual procedures rely on maps with contours for human interpretation and a 
relatively few number of cross sections selected to be representative of average conditions in 
reaches that are as long as possible without permitting excessive conveyance change between cross 
sections.  Automated procedures normally use high-density elevation points (in lieu of contours) 
and computer generation of a potentially larger number of cross sections representative of shorter 
reaches.
[February 2002]

A.2	Industry Geospatial Standards
In 1998, the Federal Geographic Data Committee (FGDC) published Geospatial Positioning 
Accuracy Standards, that replaced both the United States National Map Accuracy Standards 
(NMAS) published by the Office of Management and Budget in 1947 (Bureau of the Budget, 1947) 
and the ASPRS Accuracy Standards for Large-Scale Maps (ASPRS, 1990) published in 1990 by the 
American Society for Photogrammetry and Remote Sensing (ASPRS).  Designed specifically for 
digital spatial data products, this new FGDC standard has three parts:
*	Part 1, Reporting Methodology (FGDC-STD-007.1-1998)
*	Part 2, Standards for Geodetic Networks (FGDC-STD-007.2-1998); and
*	Part 3, National Standard for Spatial Data Accuracy (FGDC-STD-007.3-1998)
FGDC-STD-007.1-1998-STD-007.1- provides a common methodology for reporting the accuracy 
of horizontal and vertical coordinate values of digital geospatial products. Specifically, the 
reporting standard in the horizontal component (Accuracyr) is the radius of a circle of uncertainty, 
such that the true or theoretical location of the point falls within that circle 95-percent of the time. 
The reporting standard in the vertical component (Accuracyz) is a linear uncertainty value, such that 
the true or theoretical location of the point falls within ? of that linear uncertainty value 95-percent 
of the time. It also defines the meanings of "local accuracy" and "network accuracy" and other terms 
used in the FGDC standard. Part 1 of the Geospatial Positioning Accuracy Standards is available 
on-line at: www.fgdc.gov/standards/documents/standards/chapter1.pdf.  
FGDC-STD-007.2-1998 provides a common methodology for determining and reporting the 
accuracy of horizontal and vertical coordinate values for geodetic control points represented by 
survey monuments, such as brass disks and rod marks. It provides a means to directly compare the 
accuracy of coordinate values obtained by one method, e.g., a classical line-of-sight traverse, with 
the accuracy of coordinate values obtained by another method, e.g., a Global Positioning System 
(GPS) geodetic network survey, for the same point. It explains how "network accuracy" is achieved 
by properly connecting survey and mapping data to control points in the National Spatial Reference 
System (NSRS).  Part 2 of the Geospatial Positioning Accuracy Standards is available on the 
FGDC website at www.fgdc.gov/standards/documents/standards/chapter2.pdf.

FGDC-STD-007.3-1998 implements a statistical and testing methodology for estimating the 
positional accuracy of points on maps and in digital geospatial data, with respect to georeferenced 
ground positions of higher accuracy.  If errors have a normal distribution and if systematic errors 
have been eliminated as best as possible, the National Standard for Spatial Data Accuracy 
(NSSDA) uses root-mean-square error (RMSE) to estimate positional accuracy of x, y and z 
coordinates (RMSEx, RMSEy and RMSEz respectively).  FGDC-STD-007.3-1998 defines RMSE as 
the square root of the average of the set of squared differences between dataset coordinate values 
and coordinate values from an independent source of higher accuracy for identical points and it 
defines (horizontal) radial accuracy in terms of RMSEr computed as a function of RMSEx and 
RMSEy.  FGDC-STD-007.3-1998 provides NSSDA testing guidelines, it relates Accuracyr and 
Accuracyz (horizontal and vertical accuracies at the 95-percent confidence level) to RMSEr and 
RMSEz, and it documents the statistical relationship between the NSSDA and the prior National 
Map Accuracy Standard (NMAS) and ASPRS 1990 standards.  Part 3 is available on-line at 
www.fgdc.gov/standards/documents/standards/chapter3.pdf.
Other industry standards relevant to this Appendix include:
*	National Oceanic and Atmospheric Administration Technical Memorandum NOS NGS-58, 
Guidelines for Establishing GPS-Derived Ellipsoid Heights (Standards: 2 cm and 5 cm), 
version 4.3, (NOAA, 1997) which is available on-line at www.ngs.noaa.gov/PUBS_LIB/NGS-
58.html  and is referred to hereinafter as "NGS-58;" 
*	Digital Elevation Model Technologies and Applications: The DEM Users Manual (ASPRS, 
2001);
*	ASPRS Draft Aerial Photography Standards, "Photogrammetric Engineering & Remote 
Sensing"  (ASPRS, 1995);
*	U.S. Army Corps of Engineers, Engineer Manual 1110-1-1000, Photogrammetric Mapping 
(USACE, 1993); and
*	U.S. Army Corps of Engineers, Engineer Manual 1110-2-1003, Hydrographic Surveying 
(USACE, 1994)
[February 2002]

A.3	Accuracy Guidelines
For decades, when topographic maps were produced photogrammetrically and printed in hardcopy 
form, users became accustomed to specifying map requirements in terms of the published map 
scale and contour interval.  Users generally understood the meaning of those terms, and 
photogrammetrists had well-established procedures for determining the flying height, forward 
overlap and sidelap, aerial triangulation, and map compilation procedures necessary to consistently 
produce a map to the required map scale and contour interval, in conformance with the National 
Map Accuracy Standard (NMAS).  
With the new NSSDA designed for digital geospatial data, the defining parameters are much more 
complex. What once had been a simple standard printed on one page (i.e., the NMAS) has become 
a complex standard (i.e., the NSSDA) that is nearly one inch thick.  The more complex standard 
was necessary to accommodate the variables and options available with digital geospatial data and 
the increased complexity of digital data that are subject to use (and misuse) in many different ways.
FEMA has chosen to define its digital topographic data accuracy requirements in terms of map 
scale and contour interval equivalent to NMAS terminology, but also cross-referenced to radial 
root-mean-square-error (RMSEr) and/or Accuracyr (to define horizontal accuracy at the 95-percent 
confidence level), and RMSEz and/or Accuracyz (to define vertical accuracy at the 95-percent 
confidence level) as defined by the NSSDA, assuming the errors have a normal distribution. The 
NSSDA does not say what to do when errors do not follow a normal distribution, as is often the 
case with LIDAR datasets. ASPRS, 2001, Chapter 12 (DEM Quality Assessment) proposes a 5-step 
approach that includes the use of the 95th percentile error as the next best method for estimating 
Accuracyz when it is determined (through statistical "outliers," discussed in Section A.8.3.1.3) that 
checkpoint errors do not follow a normal distribution.  
As shown in Tables A-1 and A-2 below, these NMAS and NSSDA terms are also cross-referenced 
to ASPRS, 1990 Class 1, Class 2, and Class 3 standards -- terms still used by the industry.  To 
simplify the terminology, FEMA has reduced the complex requirements to two standard choices for 
digital elevation data, expressed as equivalent contour intervals:
*	Two-foot equivalent contour interval for flat terrain (Accuracyz = 1.2 foot at 95% confidence 
level). This means that 95% of the elevations in the dataset will have an error with respect to 
true ground elevation that is equal to or smaller than 1.2 ft.
*	Four-foot equivalent contour interval for rolling to hilly terrain (Accuracyz = 2.4 ft at 95% 
confidence level.) This means that 95% of the elevations in the dataset will have an error with 
respect to true ground elevation that is equal to or smaller than 2.4 ft.
The FEMA Lead for a Flood Map Project -- usually, the Regional Project Officer (RPO) or the 
Project Officer (PO) at FEMA Headquarters -- may select non-standard alternatives when valid and 
compelling reasons for specifying other accuracy standards exist.
[February 2002]
A.3.1	Horizontal Accuracy Criteria 
Several accuracy standards are still in use today for the mapping industry. The following is a 
description and comparison of the different standards used and which apply. 
The NMAS (Bureau of the Budget, 1947) states: "Horizontal accuracy: For maps on publication 
scales larger than 1:20,000, not more than 10% of the points tested shall be in error by more than 
1/30th of an inch, measured on the publication scale."  This 1/30 inch standard for large-scale maps 
is called the Circular Map Accuracy Standard (CMAS).  The NMAS became obsolete for digital 
mapping products because computers can easily change the scale of a map, and maps do not 
become more accurate just because the computer enables users to "zoom in" on the map to display 
it at a larger scale.    
To prevent abuse of digital mapping data, the mapping industry operated during much of the 1990's 
under ASPRS, 1990. ASPRS 1990 standards established limiting RMSEs for three classes of maps 
(Class 1, Class 2, Class 3), along with typical map scales associated with the limiting errors. Three 
times the "limiting RMSE" was essentially a 100-percent confidence level standard.
In 1998, the FGDC published NSSDA which superseded both the NMAS and ASPRS 1990 
standards for digital mapping products.  NSSDA implemented a statistical and testing methodology 
for estimating the positional accuracy of points on maps and in digital geospatial data, with respect 
to georeferenced ground positions of higher accuracy.  Radial RMSE (RMSEr) calculations were 
established, and radial accuracy (Accuracyr) at the 95-percent confidence level was established as 
1.7308 x RMSEr.  Accuracyr is defined as "the radius of a circle of uncertainty, such that the true or 
theoretical location of the point falls within that circle 95-percent of the time."  NSSDA specifies 
horizontal errors at the 95-percent confidence level, whereas the NMAS specified horizontal errors 
at the 90-percent confidence level, and ASPRS 1990 specified horizontal errors at nearly the 100-
percent confidence level.  When assuming all horizontal errors have a normal distribution, the 
NSSDA/NMAS conversion factor is as follows:  
Accuracyr = CMAS x 1.1406.
With NSSDA, RMSEr is defined in terms of feet or meters at ground scale rather than in inches or 
millimeters at the target map scale.  The RMSEr of a DFIRM panel is the cumulative result of all 
errors including those introduced by Mapping Partners in performing ground surveys, aerial 
triangulation, map compilation, and digitization activities.  The RMSEr and Accuracyr values 
shown in Table A-1 are the maximum permissible values established by NSSDA for base maps 
compiled at 1"=500' and 1"=1000' under NMAS.  Table A-1 serves as a "crosswalk" between the 
NMAS, NSSDA, and ASPRS 1990 horizontal accuracy standards.  Note that RMSEr = square root 
of (RMSEx2 + RMSEy2).

Table A-1 Comparison of Horizontal Accuracy Standards
NMAS
Map Scale
NMAS
CMAS
90% confidence level
NSSDA
Accuracyr
95% confidence level
NSSDA
RMSEr
ASPRS  1990
Class 1/2/3
Limiting RMSEr
1" = 500'
16.7 feet
19.0 feet
11.0 feet
7.1 feet (Class 1)
14.1 feet (Class 2)
21.2 feet (Class 3)
1" = 1,000'
33.3 feet
38.0 feet
22.0 feet
14.1 feet (Class 1)
28.3 feet (Class 2)
42.4 feet (Class 3)
1" = 2,000'
40.0 feet
45.6 feet
26.3 feet
28.3 feet (Class 1)
56.5 feet (Class 2)
84.9 feet (Class 3)

Thus, when FEMA specifies a base map at 1" = 500', for example, this is the same as FEMA 
specifying that a digital base map should have a horizontal (radial) RMSEr of 11 feet or Accuracyr 
of 19 feet at the 95-percent confidence level, for consistency with the new NSSDA.
When a base map is compiled at 1"=1,000' and is published at a hardcopy map scale of 1"=500', 
the horizontal accuracy remains that of the 1"=1,000' map scale. Therefore, such a 1"=500' map 
would be compiled to meet 38-foot horizontal accuracy at 95-percent confidence level, rather than 
19-foot horizontal accuracy at 95-percent confidence level as is normally expected of maps 
published at a scale of 1"=500'. This is an example where "zooming in" on a map image does not 
make the map any more accurate.
[February 2002]
A.3.2	Vertical Accuracy Criteria  
Several standards for computing vertical accuracy also are in use by the mapping industry. These 
are discussed and compared below.  
The NMAS (Bureau of the Budget, 1947) states: 
Vertical accuracy, as applied to contour maps on all publication scales, shall be such that not more 
than 10 percent of the elevations tested shall be in error more than one-half the contour interval.  In 
checking elevations taken from the map, the apparent vertical error may be decreased by assuming a 
horizontal displacement within the permissible horizontal error for a map of that scale. 
This one-half contour interval under NMAS is called the Vertical Map Accuracy Standard 
(VMAS).  NMAS became obsolete for digital mapping products because computers can easily 
change the scale and contour interval of a map, and maps don't get more accurate just because the 
computer can take 10-foot contours, for example, and regenerate 5-, 2- or 1-foot contours for higher 
resolution display purposes.    
Under ASPRS 1990 standards, which were used during much of the 1990s, the mapping industry 
operated with vertical RMSEs equal to one-third of the contour interval for ASPRS Class 1 maps, 
two-thirds of the contour interval for ASPRS Class 2 maps, and full contour interval for ASPRS 
Class 3 maps.  Again, three times the limiting RMSEz was a standard at almost the 100-percent 
confidence level (99.75-percent).
In 1998, NSSDA also superseded both the NMAS and ASPRS 90 vertical accuracy standards for 
digital mapping products.  Vertical RMSE (RMSEz) calculations were established, and vertical 
accuracy (Accuracyz) at the 95-percent confidence level was established as 1.9600 x RMSEz when 
errors follow a normal distribution.  Accuracyz is defined as "the linear uncertainty value, such that 
the true or theoretical location of the point falls within ? of that linear uncertainty value 95-percent 
of the time." Again, note that NSSDA specifies vertical errors at the 95-percent confidence level 
whereas NMAS specified vertical errors at the 90-percent confidence level, and the ASPRS 1990 
specified vertical errors at nearly the 100-percent confidence level. When assuming all vertical 
errors have a normal distribution, the NSSDA/NMAS conversion factor is as follows:  
Accuracyz = VMAS x 1.1916
Contrary to NMAS, NSSDA does not specifically state that the apparent vertical error may be 
decreased by assuming a horizontal displacement within the permissible horizontal error.
With NSSDA, RMSEz is defined in terms of feet or meters at ground scale rather than in terms of 
the contour interval of the published map.  The RMSEz and Accuracyz values shown in Table A-2 
are the maximum permissible values established by the NSSDA for digital elevation data 
equivalent to maps compiled with 2-foot and 4-foot contours compiled under NMAS.  Table A-2 
serves as a "crosswalk" between the NMAS, NSSDA and ASPRS 1990 vertical accuracy standards. 
This comparison is only statistically valid, however, when vertical errors have a normal 
distribution. 
Table A-2 Comparison of Vertical Accuracy Standards
NMAS
Contour 
Interval
NMAS
VMAS
90% confidence level
NSSDA
Accuracyz
95% confidence level
NSSDA
RMSEz
ASPRS  1990
Class 1/2/3
Limiting RMSEz
2 feet
1 foot
1.2 feet
0.6 foot
18.5 cm
0.7 foot (Class 1)
1.3 feet (Class 2)
2.0 feet (Class 3)
4 feet
2 feet
2.4 feet
1.2 feet
37.0 cm
1.3 feet (Class 1)
2.7 feet (Class 2)
4.0 feet (Class 3)

Thus, when FEMA specifies 2-foot contour interval mapping, for example, this is the same as 
specifying that digital topographic data, regardless of whether the data are digital contours, mass 
points and breaklines, Triangulated Irregular Networks (TINs), or Digital Elevation Models 
(DEMs), should have vertical RMSEz of 0.6 foot (18.5 cm) or Accuracyz of 1.2 feet (37 cm) at the 
95-percent confidence level, for consistency with the NSSDA. 
[February 2002]
A.3.3	Accuracy Labeling  
Since contours or DEMs are not printed on DFIRMs, their vertical accuracy does not need to be 
labeled on DFIRMs, but the vertical accuracy is still recorded in the metadata of elevation datasets 
used for hydrologic and hydraulic modeling.  If tested to a vertical RMSE of 0.6 foot (18.5 
centimeters), for example, and recognizing that Accuracyz = 1.9600 x RMSEz when errors have a 
normal distribution, the metadata would read as follows: 
Tested 1.2-foot vertical accuracy at 95-percent confidence level
DFIRMs published at 1"=500' scale with community-provided base maps or digital orthophotos 
compiled at 1"=500' or smaller scales (e.g., 1"=400', 1"=200', 1"=100') would be labeled as 
follows: 
Compiled to meet 19-foot horizontal accuracy at 95-percent confidence level
DFIRMs published at either 1"=500' or 1"=1,000' with default USGS DOQs, compiled at 
1"=1000', would be labeled as follows:
Compiled to meet 38-foot horizontal accuracy at 95-percent confidence level
Similarly, DFIRMs published at 1"=2,000' would be labeled as follows:
Compiled to meet 45.6-feet horizontal accuracy at 95-percent confidence level
[February 2002]

A.4	Data Requirements
Requirements for topographic mapping, cross-sections and hydraulic structure surveys will vary 
from Flood Map Project to Flood Map Project, depending on the flood hazard being addressed 
(e.g., riverine, coastal, alluvial fan) and the option that is selected for generating flood hazard data. 
Detailed information on the options for generating flood hazard data is presented in Volume 1, 
Section 1.4 and in Appendix C of these Guidelines. 
New flood hazard data may be generated through detailed or approximate analyses. Updated flood 
hazard data may be developed through detailed or approximate analyses or redelineation of 
floodplain boundaries from the effective FIRM using more up-to-date and/or accurate topographic 
data than were used to prepare the effective FIRM. The topographic mapping, cross section, and 
hydraulic survey requirements for each analysis option are summarized below.
*	For a detailed flood hazard analysis, which will generally include a detailed hydraulic analysis, 
digital topographic data, cross sections (to include underwater elevations), and surveys of 
hydraulic structures are required. Ground surveys and either photogrammetric mapping or 
LIDAR-generated mapping are normally required unless suitable topographic information is 
already available from other sources.
*	For an approximate flood hazard analysis, cross sections may be interpolated from contours on 
topographic maps, and underwater elevations may be interpolated from upstream/downstream 
data, assuming the channel bottom information has not changed significantly.  Hydraulic 
structure surveys are not required.  
*	For a redelineation of floodplain boundaries using more up-to-date or more accurate 
topographic data, the topographic data needed to update the floodplain boundaries are required, 
but no new cross sections or hydraulic structure surveys are required. 
The requirements summarized in this Appendix are based on the assumption that suitable data do 
not exist from alternative sources and that new ground/aerial surveys will be required.
[February 2002]
A.4.1	Map Scale of DFIRMs and Base Maps
Detailed information on how DFIRM map scales are determined is presented in Appendix K of 
these Guidelines.
[February 2002]
A.4.2	Data Models and Surfaces
Mapping Partners may produce digital elevation datasets in many forms.  For purposes of the 
National Flood Insurance Program (NFIP), the default surface is the bare-earth terrain, devoid of 
vegetation and manmade structures.  The FEMA Lead for the Flood Map Project (usually, the 
Regional Project Officer (RPO) or Project officer at FEMA Headquarters) may specify the use of 
one or more of the following data models:
*	Digital contours;
*	Mass points;
*	Breaklines;
*	TINs; and
*	DEMs
These models are discussed in Subsections A.4.2.1 through A.4.2.5.
[February 2002]
A.4.2.1	Digital Contours
Digital contours, or hardcopy plots of such contours, are normally used in manual hydrologic and 
hydraulic analyses.  Contours normally depict breakline features of interest to hydraulic engineers.
[February 2002]
A.4.2.2	Mass Points
LIDAR produces irregularly spaced mass points. "First-return" LIDAR data provide elevation mass 
points on reflective surfaces (e.g., treetops, rooftops, towers). "Last-return" LIDAR data provide 
elevation mass points of the bare-earth terrain, but only after successful completion of automated 
and manual post processing for vegetation removal and cleaning (removal) of manmade features 
and artifacts.  Mass points can also be produced when using photogrammetric methods.
[February 2002]
A.4.2.3	Breaklines
Breaklines are linear features that describe a change in the slope, smoothness, or continuity of a 
surface.  "Soft breaklines" ensure that known z-values along a linear feature are maintained, and 
they ensure that linear features and polygon edges are maintained in a Triangulated Irregular 
Network (TIN) surface model, by enforcing the breakline as TIN edges; but they do not define 
interruptions in surface smoothness.  "Hard breaklines," which define interruptions in surface 
smoothness, are used to define streams, shorelines, dams, ridges, building footprints, sea walls, and 
other locations with abrupt surface changes. Automated techniques for generating digital elevation 
data, from LIDAR data for example, normally do not do a good job of generating breaklines. 

Breaklines are best produced by either stereo photogrammetric procedures where three-dimensional 
breaklines are produced (coordinate line strings having x/y/z coordinates) or from digital 
orthophotography where shorelines, stream centerlines and other breakline features are digitized as 
two-dimensional breaklines (coordinate line strings having x/y coordinates but no z-values).  
Regardless of the technology used, the Mapping Partner shall normally produce breaklines for 
stream centerlines, drainage ditches, tops and bottoms of streambanks, ridge lines, road crowns, 
levees, bulkheads, seawalls, road/highway embankments, and selected manmade features that 
constrict or control the flow of water (e.g., curb lines).  The Mapping Partner also shall specify the 
sources and accuracy of breakline data.

[February 2002]

A.4.2.4	Triangulated Irregular Networks (TINs)
A TIN is a set of adjacent, non-overlapping triangles, computed from irregularly spaced points with 
x/y coordinates and z-values.  The TIN data structure is based on irregularly-spaced point, line, and 
polygon data interpreted as mass points and breaklines.  The TIN model stores the topological 
relationship between triangles and their adjacent neighbors (i.e., which points define each triangle 
and which triangles are adjacent to each other). This data structure allows for the efficient 
generation of surface models for the analysis and display of terrain and other types of surfaces while 
preserving the continuous structure of features such as levees and stream banks that are critical in 
hydrologic and hydraulic analyses.  
Because breakline information is important for hydraulic analyses, TINs are normally required for 
floodplains and surrounding buffer zones where automated hydraulic analyses and modeling are 
performed.  TINs are also ideal for performing independent Quality Assurance/Quality Control 
(QA/QC) reviews, because TIN surfaces are easily interpolated to determine elevation z-values at 
checkpoint x/y coordinates that fall somewhere between the TIN triangle corners.
[February 2002]
A.4.2.5	Digital Elevation Models (DEMs)
DEMs model the elevation of the land (z-values) at regularly spaced intervals in x and y directions 
(eastings and northings).  DEMs are usually displayed as uniformly-spaced grids. Because of the 
uniform point spacing, DEMs can "jump over" breaklines without identifying ditches, stream 
centerlines, steep banks, and other similar features.  However, DEMs are simple data models, easy 
to store, and suitable for automated hydrologic analyses and modeling where breakline information 
is unimportant. 
DEMs are normally produced by interpolation from surrounding mass points or TIN corners. DEMs 
are normally required for entire watersheds, for which automated hydrologic analyses and modeling 
will be performed. Because they are interpolated, DEMs are slightly less accurate than the TINs, 
mass points, or breaklines from which they are derived.
[February 2002]
A.4.3	Vertical Accuracy 
As explained in Section A.3, one of the fundamental requirements that the FEMA Lead must 
establish is to which of the following standards the digital topographic data will be compiled:
*	Standard 2-foot equivalent contour interval accuracy (Accuracyz = 1.2 foot) appropriate for flat 
terrain, 
*	Standard 4-foot equivalent contour interval accuracy (Accuracyz = 2.4 foot) appropriate for 
rolling to hilly terrain, or 
*	Some alternative standard. 
The specified accuracy of FIRM work maps produced by Mapping Partners must be sufficient to 
ensure that the final FIRMs produced by FEMA can be reliably used for the purpose intended. 
However, the accuracy and resolution requirements of a mapping product must not surpass that 
required for its intended functional use.  Specifying map accuracies in excess of those required 
results in increased costs, delays in project completion, and reduction in the total numbers of new or 
revised products that the Mapping Partner may generate. Mapping accuracy requirements must 
originate from functional and realistic accuracy requirements.
[February 2002]
A.4.3.1	Vertical Accuracy as a Function of Horizontal Accuracy
The vertical accuracy of mass points, TINs or DEMs also is a function of horizontal accuracy 
because NMAS traditionally has allowed apparent vertical errors to be decreased by assuming a 
horizontal displacement within the permissible horizontal error for a map of that scale.  As 
indicated in Table A-1, according to NMAS, 90 percent of horizontal test points are to be accurate 
within 16.7 feet for maps published at a scale of 1" = 500', the most common scale for DFIRM 
panels. This equates to a horizontal (radial) RMSE (RMSEr) of 11.0 feet.  Nevertheless, LIDAR 
vendors normally advertise that their systems deliver data with a RMSEr of 1-meter or less. This 
horizontal accuracy is difficult to verify, except for calibration test ranges where coordinates of the 
four corners of rooftops of several buildings are accurately surveyed (in addition to ground control 
points) and compared with LIDAR calibration flights flown over the calibration area from four 
cardinal directions (north, south, east, west). The horizontal accuracy with which these building 
breaklines can be determined provides a good estimate of the overall horizontal accuracy of LIDAR 
datasets obtained under similar conditions.
[February 2002]
A.4.3.2	Vertical Accuracy as a Function of Horizontal Resolution
The vertical accuracy of mass points, TINs or DEMs also is a function of the horizontal resolution 
of the digital topographic data. Horizontal resolution refers to the average point spacing of 
irregularly spaced mass points (from LIDAR for example) or uniformly spaced DEM points, and is 
more commonly known as the DEM "post spacing."  To provide multiple surrounding points for 
interpolation of DEM elevation posts, the horizontal resolution of mass points normally is narrower 
than the DEM resolution.  For example, to derive a DEM with uniform post spacing of 5 meters, it 
is common for LIDAR dataset mass points to have average post spacings of 3 to 4 meters, a denser 
dataset from which some points will be eliminated as a result of post-processing to remove points 
that impinged on manmade structures or dense vegetation.
No established rules directly correlate the horizontal resolution of digital elevation data with 
vertical accuracy, even though there is general agreement that TINs or DEMs equivalent to 2-foot 
contours ought to have narrower post spacing than TINs or DEMs equivalent to 4-foot contours. 
Cartographers typically associate map scale with contour intervals as follows:
*	Maps compiled at 1"=100' with 1-foot  contours need DEM post spacing of 1 meter 
*	Maps compiled at 1"=200' with 2-foot contours need DEM post spacing of 2 meters
*	Maps compiled at 1"=500' with 5-foot contours need DEM post spacing of 5 meters
*	Maps compiled at 1"=1000' with 10-foot contours need DEM post spacing of 10 meters
*	Maps compiled at 1"=2000' with 20-foot contours need DEM post spacing of 20 meters. 
USGS 7.5-minute-series topographic quadrangle maps are generally prepared at a scale of 
1"=2,000', and DEMs have traditionally been produced by USGS with post spacing of 30 meters. 
However, new standard USGS DEMs, produced from the contour lines on quadrangle maps, are 
being produced with 10-meter post spacing.  Depending on the ruggedness of the terrain, the USGS 
may have compiled these quadrangle maps with contour intervals of 5, 10, 20, or 40 feet -- all 
published at a scale of 1"=2,000'.  Thus, no rigid rules that link DEM resolution to map scale or 
contour interval exist.
However, mass points and breaklines, or TINs derived therefrom, are typically used for hydraulic 
modeling.  When breakline data are available (e.g., at the tops and bottoms of stream banks), the 
average mass point or DEM post spacing can be wider than indicated above.  For example, if 
Mapping Partners are producing DFIRM workmaps in flat terrain with digital topographic data 
equivalent to 2-foot contours, 2-meter post spacing may be appropriate if no supplemental 
breaklines are provided, but 5-meter post spacing is appropriate if supplemental breaklines are 
provided.
[February 2002]
A.4.4	File Size, Tile Size and Buffers
The size of the data file will have an effect on how the Mapping Partner will be able to manage and 
manipulate the DEM or TIN. File size could have limiting factors such as data storage, transfer 
media size, and software limitations.  A DEM file with 2-meter post spacing will be 6.25 times 
larger than a DEM file with 5-meter post spacing; the same ratio applies to the average point 
spacing of mass points and TINs.  File sizes shall not exceed 1 gigabyte unless otherwise specified 
by the FEMA Lead.
In some cases, Mapping Partners may choose to limit files by "tile" size (e.g., 10,000-meter x 
10,000-meter tiles, 5,000-meter x 5,000-meter tiles) by DFIRM panel size, by USGS DOQ size, or 
by the size of community-provided base maps.  Depending on the horizontal resolution of data 
within these "tiles," the file sizes will vary.  
Supplemental to the exact tile size, a buffer surrounding the tile may be required in the event that 
individual tiles are to be mosaicked together.  Due to the interpolation and extrapolation at tile 
edges, single tiles may not "fit" adjoining tiles without overlapping buffers.  For these reasons, a 
buffer zone equal to five percent of the tile size must be incorporated around adjoining tiles so that 
a seamless interpolation will occur between them. No new data are required as a result of buffering, 
but file sizes for individual tiles with such buffers will be 5 percent larger than without such buffers.
[February 2002]
A.4.5	Mapping Area
High-accuracy digital elevation data are not required for hydrologic analyses.  If no better form of 
digital elevation data is available, Mapping Partners may use USGS "off-the-shelf" DEMs for 
hydrologic analyses and modeling of entire watersheds. A Mapping Partner can purchase an "off-
the-shelf" DEM from USGS at a nominal cost.   
Hydraulic analyses require high-accuracy contours, mass points and breaklines, TINs or DEMs, but 
of floodplain areas only, and not of entire watersheds.  The area to be mapped normally consists of 
the projected 0.2-percent-annual-chance (500-year) floodplain, plus a specified buffer of 1000 feet 
beyond the 0.2-percent-annual-chance floodplain, as shown in Figure A-1.  Although data for the 
entire study area normally are acquired by an aerial survey contractor using photogrammetric 
methods or LIDAR, only the polygon areas within the projected 0.2-percent-annual-chance 
floodplain and buffer zones are to be processed to produce the high-accuracy elevation data 
required for hydraulic analyses. 

Figure A-1  Sample Location Map
[February 2002]

A.4.6	Cross Sections
For detailed flood hazard analyses, the assigned Mapping Partner shall survey cross sections 
immediately upstream and downstream of bridges and culverts, using field survey methods, to 
include survey of channel invert elevations (the elevation at the deepest part of a channel cross 
section).  The assigned Mapping Partner shall survey Intermediate cross sections when bridges or 
culverts are more than 1,000 feet apart, especially where a significant change in conveyance occurs 
between cross sections.  The assigned Mapping Partner may "cut" Intermediate cross sections from 
stereophotogrammetric models or from LIDAR datasets so long as no significant change in the 
stream channel geometry below the water level occurs. When flooding sources have little change in 
conveyance, the assigned Mapping Partner may determine whether fewer cross sections are needed. 
The assigned Mapping Partner shall determine cross section elevations and stations at those points 
that represent significant breaks in ground slope and at changes in the hydraulic characteristics of 
the floodplain.  The assigned Mapping Partner shall ensure that each cross section crosses the entire 
0.2-percent-annual-chance floodplain.
For hydrologic and hydraulic analyses performed using manual techniques, the assigned Mapping 
Partner shall select ground surveyed or photogrammetric cross section locations carefully to ensure 
they are representative of reaches that are as long as possible, without permitting excessive 
conveyance change between cross sections. When the assigned Mapping Partner compiles elevation 
data photogrammetrically, the elevations shall be taken to the nearest 0.5 foot at the three most 
significant gradient breaks on each bank and at enough intermediate points to satisfy the following 
three criteria:
1.	No adjacent points separated vertically by more than 20 percent of range;
2.	No adjacent points separated horizontally by more than 5 percent of the complete channel cross 
section width; and 
3.	No adjacent points in the main channel separated by more than 10 percent of main channel 
width or 2 feet, whichever is greater. 
Specified spacing is illustrated in Figure A-2.  The assigned Mapping Partner shall read elevations 
and stations at each edge of water.  The Zero Station (initial point) for each cross section must be 
the finally adopted terminus on the left bank (looking downstream).  Stations must be the distance 
to the nearest foot measured along the straight, curved, or zigzag alignment of the cross section.  
With automatic hydrologic and hydraulic analyses and LIDAR datasets, the cross sections 
established by the assigned Mapping Partner can be more numerous and represent shorter reaches. 
Because they can be easily "cut" from high-density datasets, multiple LIDAR cross sections enable 
the Mapping Partner to determine cross sections that are more truly representative of reaches.    
For surveying cross sections beneath the water level, the method to be used depends upon the depth 
and velocity of the stream. When water is shallow and can be waded, traditional differential 
leveling is used with conventional elevation rod (including rod extensions), using an attached 
bottom plate as necessary to prevent the rod from penetrating silt and mud. When water is deeper or 
too swift for wading, various hydrographic survey methods described in Chapter 8 of U.S. Army 
Corps of Engineers EM 1110-2-1003 (USACE, 1994) may be used. These may include small work 
boats with sounding poles or lead lines, or specialized hydrographic survey vessels with echo 
sounders, either single (vertical-beam) or multi-beam transducers.
Figure A-2  Examples of Cross Section Ground Point Spacing

[February 2002]
A.4.7	Hydraulic Structures
For detailed flood hazard analyses, the assigned Mapping Partner shall obtain the required 
dimensions and elevations of all hydraulic structures and underwater sections adjacent to the 
structures from available sources or by field survey where necessary.  The assigned Mapping 
Partner shall not establish dimensions and elevations of hydraulic structures by aerial survey 
methods (i.e., photogrammetry, LIDAR).
For bridges, sufficient data for input to modeling software usually includes the following: 
*	Size and shape of the opening; 
*	Upstream and downstream channel invert elevations; 
*	Entrance conditions (e.g., wingwalls, vertical abutments)
*	Bridge deck thickness, low-steel elevation, and bridge parapet type (i.e., solid railing, open 
railing); 
*	Roadway embankment side-slope rate; 
*	Type and width of roadway pavement; and 
*	Top-of-road section for sufficient length for weir-flow calculations.  
For culverts, sufficient data for input to modeling software or Federal Highway Administration 
culvert nomographs usually includes the following: 
*	Size and shape; 
*	Upstream and downstream channel invert elevations; 
*	Entrance conditions (i.e., headwall, wingwalls, mitered to slope, projecting); 
*	Roadway embankment side-slope rate; 
*	Type and width of roadway pavement; and 
*	Top-of-road section for sufficient length for weir-flow calculations.
For dams and weirs, sufficient data for input to modeling software usually includes the following:
*	Top of dam elevation; 
*	Normal pool elevation; 
*	Principal spillway type, inlet and outlet elevations, and dimensions; and 
*	Emergency spillway (if applicable) type, elevation, and dimensions.
For approximate flood hazard analyses, the assigned Mapping Partner may estimate the bridge, 
culvert, dam, and weir data from photographs, orthophotos, or existing topographic mapping 
without performing field surveys.
[February 2002]
A.4.8	Datums, Projections, Coordinate Systems, Coordinates
The horizontal and vertical datums, projection and coordinate system for a Flood Map Project must 
be determined during the Project Scoping phase.  Details are discussed in Appendices B and K of 
these Guidelines.
In addition to determining whether coordinates are to be in feet or meters, the assigned Mapping 
Partner must determine the number of decimal places to be used for x/y/z coordinates.  Normally, 
feet are specified to two decimal places, and meters are specified to either two or three decimal 
places.
When dealing with decimal degrees, one arc degree equals approximately 367,000 feet. Therefore, 
decimal degrees to six decimal places would record latitude and longitude to the nearest 0.367 foot.
When dealing with degrees/minutes/seconds, one arc second equals approximately 102 feet.  
Therefore, seconds to two decimal places would record latitude and longitude to the nearest 1 foot.
[February 2002]
A.4.9	Data Formats
The following digital file formats are acceptable with contours, breaklines, and other vector data, 
but additional software-specific formats may also be acceptable if preferred by Mapping Partners:
*	DGN: MicroStation design file, internal proprietary drawing data base format;
*	DO: Digital Line Graph (DLG) Optional file format for vector data (USGS);
*	DWG: AutoCAD drawing file, internal proprietary drawing data base format;
*	DXF: Drawing Exchange (Interchange) file, American Standard Code for Information 
Interchange (ASCII) or binary file format used to transfer data between CAD and GIS; 
*	E00: ARC/INFO Export (Interchange) file of either binary coverages or grids;
*	MIF/MID: MapInfo data set, which contains vector drawings and tables (databases);
*	SHP: ArcView Shape file, collection of files used in ArcView for vector drawings and database 
storage;
*	SDTS: Spatial Data Transfer Standard, ASCII or binary file format designed to handle earth-
referenced spatial data between dissimilar computer systems; and
*	VPF: Vector Product Format, A published vector format used by NIMA.
The following digital file formats are acceptable with mass points and TINs, but other software-
specific formats may also be acceptable if preferred by Mapping Partners:
*	ASCII x, y, z: Predominant character set encoding of computers;
*	ASCII x, y, z with Attributes: Additional attributes such as acquisition dates, sensor make and 
model (when different sensors are used on a project), or LIDAR intensity values;
*	BIN: Binary encoding of ASCII data; and
*	TIN Arc/Info Export file format (other GIS software programs have similar formats).
The following digital file formats are acceptable with uniformly-spaced DEMs, but other software-
specific formats may also be acceptable if preferred by Mapping Partners:
*	BIL: Band Interleaved by Line format, treats each line of pixels as separate units and then stores 
by lines;
*	BIP: Band Interleaved by Pixel format, treats pixels as separate storage units
*	BSQ: Band Sequential format, all data for a single band (in this case DEM) is written to one 
file;
*	DEM: is a raster format used by USGS to record elevation data. Cell values reflect elevation 
data and not pixel brightness;
*	ESRI Float Grid: 32 bit floating point raster grids for ESRI products;
*	ESRI Integer Grid: 16 bit integer format;
*	GeoTiff: Georeferenced Tagged Image File Format, one of the most widely supported file 
formats for storing bit-mapped images. Can be used with TWF (Tagged World File) for 
georeferencing; and
*	RLE: Run Length Encoding, a band sequential format that stores the cell value and the number 
of times it occurs along a given raster line.
[February 2002]
A.4.10	Hydrologically Enforced Elevation Data
It is normal for uniformly spaced DEM points to "jump" across a flooding source and fail to show 
that a flooding source passes through.  Similarly, it is possible for irregularly spaced mass points 
(from LIDAR for example) to fail to capture the lowest elevations of ditches and shorelines. 
Artificial "puddles" may then be created with no apparent outlet drain. When this occurs, breaklines 
are necessary to depict the true three-dimensional shape of the terrain, especially drainage features 
used for hydrologic and hydraulic modeling. 
A hydro-enforced TIN uses drainage system breaklines to form triangle edges in the TIN data 
structure. A hydro-enforced DEM uses breaklines to lower selected DEM cells to enforce natural 
drainage in the Digital Terrain Model (DTM). The horizontal locations of these breaklines are 
determined from hydrographic vector data or from imagery.  The z-values assigned to these 
breaklines start with the lowest elevation upstream from a "puddle" and slope downstream to an 
elevation where the natural drainage system resumes, as depicted by the TIN or DEM. 
A similar situation occurs with bridges and culverts. Elevations on the tops of bridges and culverts 
will appear identical to a dam, so a hydraulic model would erroneously indicate that water can't pass 
through these dams. Hydro-enforced TINs, DEMs or contours ensure that top surfaces of bridges 
and culverts are cut by stream breaklines so that computer models will accurately represent drainage 
flow. Hydro-enforcement normally requires human intervention to "cut" breaklines so that 
TIN/DEM elevations and contours will correctly follow drainage patterns, rather than erroneously 
model the terrain to depict dams or puddles that don't actually exist. 
[February 2002]
A.4.11	Other Digital Topographic Data Requirements
Other requirements in the Digital Topographic Data Requirements Checklist (Table A-3), unique to 
LIDAR data, will be addressed in Section A.8.

Table A-3  Digital Topographic Data Requirements Checklist
Surface Description (choose one)                        	Reflective surface (if using LIDAR)
0	Bare-earth surface (FEMA default) 				0	First          0 Last (FEMA default)        0	All
0	Top surface  (e.g., treetops/rooftops)			0  LIDAR intensity returns
0	Bathymetric surface							0  Other simultaneous imagery					
Vertical Accuracy (choose one)
0	1' contour equiv. (Accuracyz = 0.6 ft.)			0	5' contour equiv. (Accuracyz = 3.0 ft.)
0	2' contour equiv. (Accuracyz = 1.2 ft.)			0	Other: Accuracyz = ___ ft. 
0	4' contour equiv. (Accuracyz = 2.4 ft.
Vertical accuracy at the 95% confidence level (Accuracyz) = RMSEz x 1.9600 with normal distribution
Horizontal Accuracy (choose one)
0  1" = 500' equiv. (Accuracyr = 11' or 3.35 m)		0  RMSEr = 1 m
0  1" = 1000' equiv. (Accuracyr = 22' or 6.7 m)		0  RMSEr = _______
Horizontal accuracy at the 95% confidence level (Accuracyr) = RMSEr x 1.7308
Data Model (choose one or more)
0	Contours			0	Mass points			0	TIN (average point spacing = ___meters) *
0	Cross section	s		0	Breaklines			0	DEM (post spacing = ___meters)
* FEMA's standard DEM post spacing is 5-meters when mass points are supplemented with breaklines for 
hydraulic modeling.  The TIN point spacing is typically smaller than the DEM post spacing to allow a denser 
network of irregularly-spaced points for interpolation of the uniformly-spaced DEM.
Horizontal Datum (choose one)                                   Vertical Datum (choose one)
0	NAD 27		0	 NAD 83 (default) 			0	NGVD 29			0	NAVD 88 (default)
Coordinate System (choose one)
0	UTM				0	State Plane			0	Geographic
Units Note: For feet and meters, vertical (V) units may differ from horizontal (H) units 
0	Feet to ___  decimal places	0 V	0 H	0	Decimal degrees to ___ decimal places	
0	Meters to ___ decimal places 	0 V	0 H	0  DDDMMSS to ___ decimal places
Feet are assumed to be U.S. Survey Feet unless specified to the contrary
Data Format (choose one or more)
Digital contour lines and breaklines         Mass points and TINs                    DEMs
0	.DGN							0	ASCII x/y/z					0	ASCII x/y/z
0	.DO (DLG Optional)				0	ASCII with attribute data		0	.BIL 
0	.DWG							0	BIN							0	.BIP 
0	.DXF							0	TIN Arc/Info Export File		0	.BSQ  
0	.E00							0  Other ____________			0	.DEM (USGS standard) 
0	.MIF/.MID														0	ESRI Float Grid 
0	.SHP															0	ESRI Integer Grid 
0	SDTS															0	GeoTiff 
0	VPF 															0	.RLE 
0  Other ___________												0	Other ____________		
		 							File size or Tile size (choose one)
									0  File size ____ MB or 1 GB (max)
									0  Tile size _______ x ______ (specify feet or meters)
									0  Other tile size: _____________________________
									0  Buffer size: ________________________________
Other Quality Factors (optional, explain on separate page)
0	Cleanness from artifacts
0	Limits on size/location of void areas where there are no elevation data shown
0  How elevations are to be shown for void areas
0	Hydro-enforcement					Bridges/culverts removed? 	0Yes	0 No
0	Other requirements

[February 2002]

A.5	Ground Control
Ground control is used to control the network accuracy of derived ground and aerial surveys so that 
data are correctly registered to the horizontal and vertical datums.  When properly controlled, 
DFIRM work maps (including topographic data, cross sections, and hydraulic structure surveys) 
will accurately register to DOQs or other DFIRM base maps that are similarly registered to our 
National horizontal and vertical datums in compliance with applicable FGDC standards.
Elevation Reference Marks (ERMs), used by FEMA in the past, are not suitable for NFIP ground 
control as defined herein because ERMs lack both the minimum accuracy and stability needed for 
three-dimensional ground control points.  ERMs are suitable only for Temporary Bench Marks 
(TBMs) established during the performance of a Flood Map Project, and the assigned Mapping 
Partner shall provide documentation for the TBMs in the Technical Support Data Notebook.  ERMs 
must not be shown on DFIRMs.  The only ground control points printed on DFIRMs are those that 
are included in the National Spatial Reference System (NSRS).
[February 2002]
A.5.1	National Spatial Reference System 
Aerial mapping and surveying projects, and products derived therefrom, must be connected to 
NSRS monuments.  Mapping Partners can search the NSRS database by accessing the National 
Geodetic Survey (NGS) home page at www.ngs.noaa.gov, selecting the "Data Sheets" option, and 
retrieving NGS data sheets by using either the "Rectangular Search" or "Radial Search" option. 
With the "Rectangular Search" option, the user must define the minimum and maximum values for 
the latitudes and longitudes that bound the area of interest.  For the "Radial Search" option, the user 
must specify the search radius and latitude/longitude for a point near the center of the area of 
interest, or at selected points elsewhere. From all available NGS Data Sheets, users can select data 
sheets from the following options:
*	Any Horz. And/or Vert. Control;
*	GPS Sites Only;
*	Any Horizontal Control;
*	Horizontal Order-A;
*	Horizontal Order-B or Better (HARN);
*	Horizontal Order-1 or Better;
*	Horizontal Order-2 or Better;
*	Any Vertical Control;
*	Vertical Order-1;
*	Vertical Order-2 or Better;
*	FBN and/or CBN; or
*	PAC and/or SAC.
"HARN" stands for High Accuracy Reference Network, some of which have high horizontal 
accuracy only.  "FBN" stands for the Federal Base Network of control monuments.  "CBN" stands 
for the Cooperative Base Network, established by states, counties and/or municipalities.  "PAC" 
stands for Primary Airport Control stations. "SAC" stands for Secondary Airport Control stations.
The "GPS Sites Only" option provides three-dimensional control points that have been surveyed 
using GPS and have "network accuracy" relative to the national geodetic datum.  These control 
points have "Ellipsoid Order" specified on the NGS Data Sheets.   The "Any Horz. And/or Vert. 
Control" option provides conventional horizontal control points and vertical bench marks, as well 
as GPS control points.  If these control points and bench marks do not specify "Ellipsoid Order," 
they have "local accuracy" relative to the coordinates of other directly connected, adjacent control 
points, but have not yet been  surveyed using GPS relative to the national geodetic datum.  Such 
local control points can be connected to the NSRS, to determine their "network accuracy," by 2 to 3 
days of GPS surveys (in accordance with NGS-58 procedures for Secondary or Primary Base 
surveys) relative to NSRS monument(s) for which "Ellipsoid Order" has been specified.
If the rectangular or radial search area is so large that more than 100 NSRS monuments are 
identified within the defined area, an error message will specify that the rectangular area must be 
subdivided, or the radius of the circular search area must be reduced, so that there will be fewer 
than 100 NSRS monuments within the specified area.  
GPS is the preferred method for extending any survey control from NSRS monuments to other 
control points and bench marks so that all control used for a Flood Map Project will have "network 
accuracy" relative to the NSRS and the horizontal and vertical datums used.
[February 2002]
A.5.2	Horizontal Control  
At a minimum, all horizontal control must be to an accuracy level of NGS Second Order or better. 
This horizontal accuracy is easily achievable when NGS-58 5-centimeter local network accuracy 
procedures are used and when NSRS monuments are used for GPS base stations.
[February 2002]
A.5.3	Vertical Control
The assigned Mapping Partner must perform necessary field surveys to maintain vertical control, 
with all elevations referred to the National Geodetic Vertical Datum of 1929 (NGVD29) or the 
successor North American Vertical Datum of 1988 (NAVD88).  The vertical datum may be either 
NGVD29 or NAVD88, but not mixed within a single Flood Map Project.  It is recommended that 
all new Flood Map Projects be referenced to NAVD88. See Appendix B of these Guidelines for 
additional information on the use of NGVD29 and NAVD88. During the Project Scoping phase of 
a Flood Map Project, the assigned Mapping Partner must clarify whether to use NGVD29 or 
NAVD88 before beginning any survey work.     
For vertical control surveys, the assigned Mapping Partner must use 5-centimeter or better GPS 
procedures (as specified in NGS-58 for local network accuracy) or Third-Order (or better) 
differential leveling, or trigonometric leveling for short distances. The assigned Mapping Partner 
shall establish vertical control points, for leveling of photogrammetric stereo models, with 
elevations accurate to within  5 centimeters, relative to the NSRS bench marks of Second-Order or 
higher accuracy used for the survey.  For county-size and smaller areas, GPS is preferred by FEMA 
for establishing vertical control points if the assigned Mapping Partner uses precise differential GPS 
(DGPS) techniques , with the base station receiver simultaneously measuring the elevation of a 
local NSRS bench mark of Second-Order or higher accuracy.  
NGS has published a national geoid model and PC-based computer software, called GEOID99, 
which converts from ellipsoid heights (derived from GPS) to orthometric heights (derived from 
precise leveling) nationwide with a standard deviation of 1 centimeter for points spaced 10 
kilometers apart. When the assigned Mapping Partner uses DGPS techniques within a single 
county, this correction for variable height of the geoid above the ellipsoid in non-mountainous areas 
is insignificant.  GEOID99 was designed to be used with elevations referenced to NAVD88, not 
NGVD29.  NGS has also published another PC-based software program, called VERTCON, which 
converts elevations referenced to NGVD29  to NAVD 88 for any horizontal coordinate pair entered 
(longitude/latitude or easting/northing).  Both GEOID99 and VERTCON programs may be 
downloaded from the NGS portion of the NOAA website at www.ngs.noaa.gov.   
[February 2002]

A.6	Ground Surveys
Monuments selected for differential GPS base stations for all surveys listed in this section will have 
the best available Horizontal Order, Vertical Order and/or Ellipsoid Order, and Stability C or better. 
Where possible, these monuments must be the same NSRS monuments used for airborne GPS 
control of mapping aircraft for photogrammetric or LIDAR surveys. This eliminates the possibility 
that discrepancies may be caused by inconsistencies between different control points. 
[February 2002]
A.6.1	Photogrammetric Control Surveys
Primary photogrammetric control will consist of a network of control points adequate to produce 
maps with a 4-foot (rolling/hilly terrain) or 2-foot (flat terrain) contour interval.  The assigned 
Mapping Partner shall include enough points in the primary network so that no stereomodel 
ground-surveyed control point (picture point) is farther than 15,000 feet from the nearest primary 
control point in that network. The assigned Mapping Partner shall use sufficient points to produce a 
stable aerotriangulation solution.  
Points must be located in areas where they can be read from as many stereomodels as possible, 
except in cases where the point lies within 0.33 inch of the edge of the stereomodel.  The assigned 
Mapping Partner shall ensure that no more than two stereomodels are without vertical control 
points unless direct georeferencing is used (See Subsection A.7.3).  The assigned Mapping Partner 
shall locate and number the points on the image side of the contact print and shall number and 
describe the points on the reverse side. To ensure sufficient and accurate selection, points must be 
selected under the supervision of an ASPRS certified photogrammetrist.
[February 2002]
A.6.2	Cross Section Surveys
The assigned Mapping Partner must "anchor" terminal ends of cross sections by GPS surveys 
relative to NSRS monuments having both "Ellipsoid Order" and "Vertical Order" specified on the 
NGS data sheets. The assigned Mapping Partner may use traditional traverse/trigonometric leveling 
procedures between the terminal points to determine x/y/z coordinates of points that represent 
significant breaks in ground slope and at changes in the hydraulic characteristics of the floodplain. 
Each cross section must cross the entire 0.2-percent-annual-chance floodplain. Elevations must be 
determined at each edge of water, and invert elevations must be determined at the deepest part of 
the stream.  Underwater surveys should be performed per instructions in section A.4.6.
[February 2002]
A.6.3	Hydraulic Structure Surveys

For bridges and culverts, survey information to support hydraulic modeling includes the following:
*	Size and shape of bridge/culvert openings;
*	Upstream and downstream channel invert elevations;
*	Entrance conditions (vertical abutments, headwalls, wingwalls, mitered to slope, projecting);
*	Bridge deck thickness, low steel elevation, and bridge parapet type (i.e., solid, open railing);
*	Roadway embankment side-slope rate;
*	Type and width of roadway pavement; and
*	Top-of-road section for sufficient length for weir-flow calculations
For dams and weirs, survey information to support hydraulic modeling includes the following:
*	Top of dam elevation;
*	Normal pool elevation;
*	Principal spillway type;
*	Inlet and outlet elevations and dimensions; and
*	Emergency spillway type, elevation and dimensions.
[February 2002]
A.6.4	Checkpoint Surveys
Checkpoint surveys must be performed to establish independent QA/QC checks for points of higher 
accuracy than the geospatial datasets being evaluated.  Checkpoints are used to check the horizontal 
accuracy of base maps or the vertical accuracy of digital elevation data.  They are most commonly 
used to determine the vertical accuracy of LIDAR datasets.  Checkpoint surveys are performed to 
achieve 5-centimeter Local Network accuracy per NGS-58.  The assigned Mapping Partner shall 
select checkpoint locations on Government-owned or public areas, where possible, to avoid the 
need for obtaining individual land owner permission.
Checkpoints are normally surveyed with transections or small clusters of points, at least two of 
which have been surveyed using GPS relative to NSRS monuments that have both "Ellipsoid 
Order" and "Vertical Order" specified on their NGS data sheets. If selected monuments are farther 
than 20 kilometers from the test areas to be surveyed, the assigned Mapping Partner shall establish 
Secondary Base Stations so that final surveys of checkpoints satisfy NGS-58 requirements for Local 
Network accuracy of 5-centimeters at the 95-percent confidence level. 
Alternatively, the assigned Mapping Partner may use GPS real-time-kinematic (RTK) procedures 
provided: 
1.	The base station is an existing NGS three-dimensional mark or a new NGS-58 mark, 
2.	RTK procedures are used only in open areas and each RTK point is occupied twice with at least 
2 hours difference in time between observations, and 
3.	The difference between observations does not exceed 5 cm. 
The assigned Mapping Partner may use third-order conventional surveys to extend control from 
GPS "anchor points" to other transection or cluster points, especially when checkpoints are in 
forested areas where GPS signals are blocked.  
When checkpoints are to be used for QA/QC reviews of digital elevation data (e.g., TINs, DEMs), 
the checkpoints must be at least 5 meters away from any breakline where there is a change in slope. 
Such checkpoints must be on flat or uniformly sloping terrain.  The assigned Mapping Partner shall 
take photographs to record the location of the checkpoint relative to its surroundings, and to verify 
the vegetation category within which the checkpoint is located.  The assigned Mapping Partner 
shall mark each checkpoint with a 60d nail or larger. The assigned Mapping Partner shall write the 
station ID number on an adjacent above-ground stake within 1 foot of the referenced stake, to aid in 
subsequent recovery if required during the course of the Flood Map Project. The assigned Mapping 
Partner shall use "to reach" location descriptions and photographs to document the location, the 
land cover surrounding each stake, and the uniform slope of the terrain surrounding each stake.
[February 2002]
A.6.5	Survey Records 
Upon completion of the project, the assigned Mapping Partner shall deliver information meeting the 
requirements described below, upon request, to the FEMA Lead.  
*	Field notebooks must be carefully and neatly prepared, identified, indexed, and preserved.
*	All data regarding the establishment and extension of vertical and horizontal control, including 
descriptions of all established and recovered monuments, must be recorded. 
*	Each field notebooks must contain the name and the field address/location of the Party Chief, 
and the identity of the survey instruments.  
*	Each field notebook must be numbered and marked with a brief description of the contents on 
the cover, carefully indexed, and each page numbered.  
*	For conventional surveys, each horizontal traverse line and vertical control line must be 
identified by number and brief description in the field book.  
*	The first page used on each day of field work must be dated.  
*	Each field notebook must be free of erasures; any line of horizontal and vertical control may be 
rejected by the FEMA Lead if any erasure is made in recording the data for that line.  
*	If the field notes are electronically recorded, printouts of the electronically recorded field notes 
must be provided.  
*	For GPS surveys, the full network adjustment report must be provided.
The assigned Mapping Partner must furnish a schematic control diagram of the survey records on a 
photo index for all basic horizontal and vertical control pertinent to the project.  The schematic 
diagram must show all existing and established control points properly identified in their 
approximate location.  The schematic diagram also must show all traverse lines with their 
designations to include the beginning and ending points.
[February 2002]

A.7	Photogrammetric Surveys
The aerial photogrammetric surveys that may be performed by a Mapping Partner or a contractor to 
a Mapping Partner generally will include establishment of the following:
*	Photogrammetrically obtained stream and valley cross sections (portions above water);
*	Planimetric compilation manuscript of key hydraulic structures (bridges, culverts, dams, 
levees);
*	For rolling/hilly terrain, 4-foot contours of floodplains from the waterline to the nearest 4-foot 
contour above the 0.2-percent annual chance flood elevation line; for flat terrain, 2-foot 
contours of floodplains from the waterline to the nearest 2-foot contour above the 0.2-percent-
annual-chance flood elevation line;
*	Contours of 1-percent-annual-chance and 0.2-percent-annual-chance floodplain elevations (if 
profiles have been determined from previous studies); and 
*	Tabulations of spot heights on hydraulic structures and temporary bench marks (TBMs).
[February 2002]
A.7.1	Aerial Photography
The assigned Mapping Partner shall first determine the area to be flown, the approximate location 
and vertical ranges of the cross-sectional information to be flown, and the approximate location and 
vertical ranges of the cross-sectional information needed to represent all reaches under study. In 
planning for photogrammetry, the assigned Mapping Partner shall perform an approximate analysis 
to estimate the 1- and 0.2-percent-annual-chance flood elevations for every reach for which a 
detailed flood hazard analysis is required in order to estimate the extent of horizontal aerial photo 
coverage required. Where available, FIRMs, USGS floodprone area maps, or similar studies may be 
used for this purpose.  The area of detailed study, for which stereophotography is required, normally 
includes a 1,000-foot buffer zone outside the 0.2-percent-annual-chance flood limits.  (See Figure 
A-1). 
Schedule requirements are an important consideration in the decision regarding the applicability of 
aerial photogrammetry.  In many areas, good aerial photography can be obtained only during short 
periods of the year, when foliage does not obscure the landscape, the ground is free of snow, and 
the sky is clear.  Poor weather and difficult terrain conditions can also delay required ground 
surveys.  However, these factors have no effect on the schedule for determination of cross sections 
and contours by photogrammetry once photography and ground control have been completed.  The 
Flood Map Project schedule must reflect these considerations to avoid delays. 
The photography must be flown while the sun angle is above 30 degrees, when there is no snow 
cover, when the flooding sources are in the main channels, and when leaves are off the trees.
The assigned Mapping Partner shall perform aerial surveys under the direct supervision of a 
Registered Land Surveyor or ASPRS-Certified Photogrammetrist, consistent with State regulations. 
The aerial photography deliverables consist of the following: 
*	One photo index of all photography on a sheet that is no larger than 24 inches x 36 inches; 
*	One set of black-and-white contact prints of photographs on resin-coated, neutral-toned, 
medium-weight paper with matte surface; and
*	Two sets of black-and-white prints of alternate photographs enlarged two times on resin-coated, 
neutral-toned, medium-weight paper with matte surface (optional if requested).
All deliverables must provide complete single coverage of the flight area.  The screen must be 
appropriate so that quality bluelines of a pilot sheet can be made and accepted prior to quantity 
production.   
[February 2002]
A.7.2	Aerial Triangulation (AT)	
The assigned Mapping Partner normally shall use Fully Analytical Aerial Triangulation (FAAT) of 
the entire floodplain area and buffer zone for which stereo coverage is required.  No analog or semi-
analytical aerial triangulation procedures is to be used. The positional accuracy of horizontal and 
vertical photo control established by FAAT must meet or exceed each of the following conditions:
*	The horizontal RMSE of the final block adjustment must not exceed 1/10,000 of the flight 
height.
*	The vertical RMSE of the final block adjustment must not exceed 1/9,000 of the flight height.
*	The maximum allowable error of any vertical or horizontal point must not exceed 3 RMSE.
*	The mean of all points (taking into account positive and negative signs) must not exceed 
1/15,000 of the flight height.
Upon completion of all aerial triangulation work, the assigned Mapping Partner shall prepare an 
aerial triangulation report for submission to the FEMA Lead.  The report shall include, but not 
necessarily be limited to, flight lines; exposure stations or model layout; and control points 
appropriately labeled with station designations, computer designations (if any), and agency 
responsible for establishing the stations.  Aerial triangulation results shall include the following:
*	All residuals (misclosures) at ground control points with and without use of checkpoints;
*	Computer printout of the final adjusted aerial triangulation solution to horizontal and vertical 
ground control, including the final State Plane and/or UTM coordinates for all ground control 
points, pass points, and check points;
*	Identification of all points included in the initial solution that were subsequently discarded, with 
an explanation of the reasons for being discarded;
*	Identification of the weighting factors applied to all points used in the final solution;
*	Diskette containing the coordinate data in ASCII format; and
*	A brief narrative explaining the above solutions as well as descriptions of equipment, 
procedures, and computer programs used.  
The report will include RMS error summaries for bundle adjustment photographic measurement 
residuals or strip tie point residuals and misclosures at control/check points.  In addition, significant 
misfits encountered at control points, and steps taken to analyze such misfits and to rectify the 
discrepancies, shall be described.
[February 2002]
A.7.3	Direct Georeferencing
A primary purpose of aerial triangulation is to use ground control points, pass points, and tie points 
in a FAAT least squares block adjustment to determine the position and orientation of the mapping 
camera at the instant of each film exposure.  The position consists of the x/y/z coordinates of the 
camera's focal point in the air, and the orientation is the angular roll (?), pitch (?), and yaw (?) of 
the camera about the x, y and z axes.  Collectively, these six position and orientation terms (x, y, z, 
?, ?, ?) are called the "exterior orientation (EO) parameters" of each photograph.  These EO 
parameters for each photograph are needed to correctly establish the stereomodel needed to prepare 
accurate three-dimensional mapping from two-dimensional photographs.
"Direct Georeferencing" is the direct measurement of the position and orientation parameters of a 
remote sensor.  The state-of-the-art technology in Direct Georeferencing is the GPS-aided inertial 
navigation system (INS).  The INS consists of an inertial measurement unit (IMU) and a navigation 
processor (NP).  The IMU comprises three accelerometers, three gyroscopes, and the electronics to 
provide digitally encoded samples of the accelerometer and gyro data on a serial interface to 
measure the specific force vector experienced by the IMU.  The force vector is the vector sum of 
the local gravity vector and the acceleration vector experienced by the IMU.  The NP solves 
Newton's equations of motion of the IMU on the rotating earth, and establishes the local North, 
East, and Down directions.  The INS essentially provides direct measurements of the ?, ?, and ? 
EO parameters.  A GPS-aided INS uses a Kalman filter to regulate the INS position and velocity 
errors to be consistent with GPS position and velocity errors, regulating the INS angular errors and 
continuously improving the INS alignment   The net result is that GPS-aided INS provides direct 
recording of the six EO parameters for each photograph, providing Direct Georeferencing of those 
photographs without the normal FAAT solution.  
It is becoming common practice with direct georeferencing solutions to use only a few ground 
control points, at the outer perimeter of a mapping project area, in conjunction with GPS-aided INS 
data to perform a complete aerial triangulation, without the need for hundreds of surveyed ground 
control points. Although direct georeferencing is an acceptable alternative to FAAT, it is a new 
technology that is more commonly used in inaccessible or sensitive terrain (e.g., wetlands) where 
there is additional reason to avoid the survey of numerous ground control points.  Technically, with 
direct georeferencing solutions, no ground control points are needed; however, a few are advisable 
for overall quality control of the mapping project.
[February 2002]
A.7.4	Photogrammetric Compilation
Photogrammetric compilation for a Flood Map Project normally must include determination of 
floodplain cross-section geometry; compilation of 4-foot (or 2-foot) contours in the floodplain; and 
preparation of limited planimetric map features for registration to digital orthophoto base maps, for 
example. The compilation of such FIRM work map information requires high-precision 
photogrammetric equipment, hardware/software, and experienced operators.
[February 2002]
A.7.4.1	Cross Sections
On the enlarged photographs, the assigned Mapping Partner shall designate the position, 
approximate termini, and minimum range in elevation for each cross section to be read, and the 
position of approximately two photo-identifiable ground points near each cross section.  The range 
in elevation is the vertical distance from the water surface at the time of photography to the upper 
limit of the cross section.  If the channel is dry, the lowest point in the streambed is used to define 
the range.
The assigned Mapping Partner shall compile cross sections in accordance with requirements 
specified in Subsection A.4.6. 
[February 2002]
A.7.4.2	Contours  
The assigned Mapping Partner shall compile contours of the areas within the estimated 0.2-percent- 
annual-chance floodplain and buffer zone. The assigned Mapping Partner may use DEMs with 
uniformly spaced elevation points, non-uniformly spaced mass points and breaklines, and TINs to 
generate contours so long as the resulting contours satisfy the specified vertical accuracy 
requirements.  The Mapping Partner shall start the contours at the next even foot elevation above 
the water surface and continue at 4-foot (or 2-foot for flat terrain) intervals until the shaded area 
edge is reached.  The specified format is illustrated in Figure A-3. 
In the compilation manuscript, the assigned Mapping Partner shall include 4-foot contour lines (or 
2-foot contours for flat terrain) on each bank of each flooding source for which cross sections are 
read.  The assigned Mapping Partner shall use the contours to delineate floodplain boundaries 
between cross sections after precise flood elevations are computed.  
In situations where the 1- and 0.2-percent-annual-chance flood elevations are available, or can be 
closely approximated in advance of the photogrammetric compilation, the assigned Mapping 
Partner shall consider the use of "bracketing contours" that cover only the ranges of elevations near 
those of the floods for which floodplain boundaries are to be delineated on the work maps. 
Compilation costs can often be reduced in this manner by eliminating the plotting of contours 
above or below the expected range of these floods. The assigned Mapping Partner shall obtain 
approval from the FEMA Lead before using this approach. 
[February 2002]
A.7.4.3	Planimetric Work Maps
The assigned Mapping Partner shall compile a work map, compiled on different GIS layers, that 
shows the following:  
*	Alignment of all cross sections read, with zero stations plotted and labeled; 
*	Low water outlines of streams; 
*	Limited planimetric features of hydraulic structures, e.g., bridges, culverts, dams, dikes and 
levees; and 
*	Any contours that are specified.
Streets, highways, and railroads are not required because they should be visible on community base 
maps or DOQ base maps.
Work map specifications are provided in Volume I, Subsection 1.4.2.3.4 of these Guidelines.  
Digital data submittal requirements are provided in Appendix L of these Guidelines.

Figure A-3  Example Floodplain Contours

[February 2002]

A.7.5	Quality Control and Quality Assurance
FEMA uses qualifications-based criteria for selection of contracted photogrammetric Mapping 
Partners, recognizing that quality FIRM work maps result from professional Mapping Partners with 
superior engrained QA/QC procedures that are routinely adhered to.  Overall QA/QC is the 
responsibility of the assigned Mapping Partner and is exercised at specific stages of the map 
production process.  FEMA's role during data acquisition and map/data base compilation generally 
must be limited to performing QA/QC reviews, which may involve only cursory spot-checking of 
the FIRM work maps and supporting data, or to performing formal field map testing using FEMA 
or third-party forces. This philosophy is especially true for photogrammetric mapping which is a 
mature technology and for which procedures have been well established for many decades
The QA/QC process for photomapping work may be divided into two categories: process QC and 
product QA.
[February 2002]
A.7.5.1	Process Quality Control
Process QC is primarily the responsibility of the assigned Mapping Partner.  This includes QC 
reviews of flight alignments, photographic quality, aerial triangulation, stereocompilation, and 
completeness of supporting data (e.g., cross sections, profiles).  The degree of QC required will be 
governed by the contract specifications.
[February 2002]
A.7.5.2	Product Quality Assurance  
FEMA's primary role is that of product QA reviewer.  Product quality will be assured by FEMA 
using a variety of inspection and testing techniques on the final deliverables.  FEMA may perform 
the product QA reviews or they may task other Mapping Partners to perform the QA reviews. 
Product QA checks, tests, or field inspections likely will be called for in the contract documents for 
each Flood Map Project; however, FEMA has the option to waive any or all tests and accept the 
delivered product without formal field testing/checking.
The assigned Mapping Partner shall be responsible for internal QC functions involved with field 
surveying, photography and laboratory processing, aerial triangulation, stereocompilation and field 
checking and editing of the photogrammetrically made measurements and compiled maps to 
ascertain their completeness and accuracy.  Also, the assigned Mapping Partner shall make the 
additions and corrections that are required to complete the FIRM work maps, cross sections, and 
Flood Profiles.
[February 2002]

A.7.5.3	Quality Assurance of DEMs and/or TINs

See Subsection A.8.6 for detailed QA/QC procedures for DEMs and/or TINs produced from 
LIDAR, photogrammetry or other source.

[February 2002]

A.7.5.4	Quality Assurance of Contours
The accuracy of contouring on FIRM work maps must be tested by comparing a 
photogrammetrically derived cross section on the FIRM work map with a cross section established 
by ground survey.  The FEMA Lead shall designate the location of each test traverse.  The assigned 
Mapping Partner shall record the elevation and station for each break in the terrain and for each 
contour elevation.  Ground-surveyed cross sections must be at least 6 inches long at final map scale, 
with an elevation measured at least every 100 feet on the ground, and must cross at least 10 contour 
lines when possible.  Cross sections must start and close upon map features or previously 
established control points.  In flat areas and at principal road and rail intersections, spot elevations 
may be checked.  In general, one cross section per map sheet is sufficient.  
[February 2002]
A.7.5.5	Acceptance/Rejection
FEMA will accept a Flood Map Project when FEMA or a Mapping Partner designated by FEMA 
has performed sufficient testing to ensure that each phase of the mapping meets FEMA standards 
and specifications.  When a series of FIRM work maps are involved, the existence of errors (i.e., 
map test failure) on any individual sheet will constitute prima facie evidence of deficiencies 
throughout the project (i.e., all other sheets are assumed to have similar deficiencies); and field map 
testing will cease.  
When the first ground-surveyed test cross section indicates that a FIRM work map fails to comply 
with accuracy requirements, FEMA or an assigned Mapping Partner shall make an additional test 
cross section. This cross section generally shall be parallel to the first cross section at a distance 
from the first as specified by FEMA. No FIRM work map is to be rejected unless the sum of the 
lengths of the test cross sections completed is 12 inches or more at final map scale.  To determine 
acceptability of the contouring, the data from all the cross sections are combined and treated as a 
unit. 
[February 2002]

A.8	Airborne Light Detection and Ranging (LIDAR) Surveys
When Mapping Partners choose to use airborne LIDAR systems for gathering the data necessary to 
create digital elevation data for hydraulic modeling of floodplains, digital terrain maps, and other 
NFIP products, the guidelines and specifications presented in this section of these Guidelines must 
be used.   
LIDAR systems may not be able to gather all of the data necessary to create those products.  Data in 
areas such as bodies of water or dense forests may not meet the requirements of this Appendix.  For 
FEMA products containing such areas, the assigned Mapping Partner shall supplement LIDAR data 
with data acquired by other means, as needed for hydraulic modeling. Lesser standards may be 
applied for hydrologic modeling of watersheds or other studies.  
The guidelines and specifications herein are to be used by FEMA Mapping Partners who use 
LIDAR systems for data gathering purposes.  
[February 2002]
A.8.1	System Definitions
For the purpose of these Guidelines, LIDAR is defined as an airborne laser system, flown aboard 
rotary or fixed-wing aircraft, that is used to acquire x, y, and z coordinates of terrain and terrain 
features that are both manmade and naturally occurring.  LIDAR systems consist of an airborne 
GPS with attendant GPS base stations, Inertial Measurement Unit (IMU), and light-emitting 
scanning laser. 
The system measures ranges from the scanning laser to terrain surfaces within a scan width beneath 
the aircraft.  The time it takes for the emitted light (LIDAR return) to reach the earth's surface and 
reflect back to the onboard LIDAR detector is measured to determine the range to the ground.  Scan 
widths will vary, depending on mission purpose, weather conditions, desired point density and 
spacing, and other factors. 
The other two components of LIDAR systems are the airborne GPS, which ascertains the in-flight 
three-dimensional position of the sensor, and the IMU, which delivers precise information about the 
attitude of the sensor, i.e., the roll, pitch and heading.
[February 2002]
A.8.2	General Guidelines for Use   
Two important factors in the LIDAR system mission planning are the point density of the randomly 
spaced LIDAR points and the point spacing of the uniformly spaced DEM points derived from the 
randomly spaced LIDAR returns. The correct point density necessary to accurately represent terrain 
and terrain features will depend on flight conditions, mission purpose, and required accuracy.  As 
discussed in Sections A.3 and A.4, DEM point spacing of 5 meters or less and vertical accuracy of 
1.2 feet at the 95-percecent confidence level (Accuracyz) is required for digital elevation data 
equivalent to 2-foot contours for flat terrain, and Accuracyz of 2.4 feet is required for digital 
elevation data equivalent to 4'-foot contours for rolling to hilly terrain. 
Flight-path planning is another important factor in the LIDAR system mission.  The flight path shall 
cover the study area satisfactorily including both parallel and enough cross flight lines to eliminate 
shadowing and allow for proper quality control.
Unlike aerial photogrammetry, LIDAR missions can be flown without regard to sun angle. Flights 
may take place at night, if conditions otherwise allow. However, it is recommended that some form 
of imagery exist over the LIDAR area. Simultaneous video or digital imagery cannot be acquired if 
LIDAR data are acquired at night, but LIDAR intensity returns can be acquired day or night, and 
these help to identify features during LIDAR post-processing. Alternatively, digital orthophotos 
acquired at a different time can be used for this purpose.
Elevation and measurement information related to subsurface channel and hydraulic structure 
geometry shall be obtained through the use of other mapping technologies over deep or turbid 
water.  In some instances, shallow water and near-shore coastal surveys can be accomplished using 
airborne LIDAR bathymetric systems equipped with lasers operating in portions of the light 
spectrum that allow transmission through water.
LIDAR system tolerance for inclement weather conditions (e.g., high winds, wet snow, rain, fog, 
high humidity, low cloud cover) generally is higher than that of photogrammetric methods. 
However, such conditions have been known to degrade the accuracy of laser return data.  Therefore, 
Mapping Partners shall generally avoid missions during inclement weather.
High point densities may allow satisfactory data collection in areas of dense foliage.  Still, care shall 
be taken in planning missions with regard to both natural (vegetative) and manmade (structure) 
ground cover. Pulse width, beam divergence, first and last pulse return discrimination, and choice 
of the post-processing algorithms used for vegetation or structure removal may all affect the 
accuracy of LIDAR-derived data in areas of dense foliage.
[February 2002]
A.8.3	Performance Standards
The accuracy standards in Sections A.3 and A.4 of this Appendix also shall apply to NFIP maps 
and map products derived from LIDAR systems. LIDAR-derived data shall have the accuracy 
required to produce topographic maps and products that meet NSSDA.
FEMA is not aware of any existing LIDAR system performance standards. Current information 
about LIDAR systems is available from ASPRS (See ASPRS, 2001), National Oceanic and 
Atmospheric Administration (NOAA), National Aeronautic and Space Administration, U.S. Army 
Corps of Engineers, LIDAR system manufacturers and venders, and private firms that provide 
LIDAR system services. As professional or trade associations issue specifications and standards, 
FEMA may adopt those standards and specifications and amend this Appendix. 
[February 2002]
A.8.3.1	Overall Standards
The assigned Mapping Partner shall furnish all necessary materials and equipment. The assigned 
Mapping Partner also shall supply the supervisory, professional, and technical services personnel 
required to manage, survey, document, and process all data associated with LIDAR system 
mapping, scanning, and digital image processing. The assigned Mapping Partner shall provide all 
deliverables in accordance with the contract or other agreement with FEMA and the requirements 
in this Appendix.
DEM posting shall be the minimum allowed by the data and shall not exceed 5 meters.  As 
specified in Section A.3, Accuracyz shall normally be 1.2 foot (equivalent to 2-foot contours) in flat 
terrain and 2.4 feet (equivalent to 4-foot contours) in rolling to hilly terrain.
[February 2002]
A.8.3.1.1	Data Voids
For the purposes of this Appendix, areas not within two times the DEM posting of data points are 
data voids.  Data voids may occur from several different causes, including the following: 
*	LIDAR pulses may be naturally absorbed by water bodies or areas recently covered with 
asphalt. Such voids are normally considered to be unavoidable.
*	The LIDAR system may have malfunctioned for some reason.
*	Heavy winds, flight navigation system (FMS) error or pilot error may have caused "holidays" 
between flight lines.
*	Data points may be intentionally removed as part of the bare-earth post-processing to delete 
points that impinged on the tops of manmade structures or failed to penetrate dense vegetation.
Data voids caused by system malfunctions or flight line holidays are unacceptable, requiring new 
flights. Data voids caused by removal of LIDAR data points on manmade structures are always 
acceptable. Data voids caused by removal of LIDAR data points on dense vegetation are subject to 
additional guidelines, depending on whether or not the voids exist inside or outside the floodplain 
for which hydraulic modeling is to be performed:
When data voids exist outside the floodplain, hydrologic modeling of the watershed can still be 
performed acceptably by "filling" the voids through interpolation from surrounding points.
When data voids exist inside the floodplain, the cause of the data voids, the size of the voids, and 
the location of the voids all have a bearing on whether additional ground surveys will be required to 
"fill" the voids.  For example, if the data voids are caused by dense mangrove or sawgrass areas, 
supplemental ground surveys within such areas are not needed. These voids are normally "filled" by 
interpolation from elevation points immediately surrounding the mangrove or sawgrass areas. 
When small data voids of less than 1 acre are caused by removal of points in other types of dense 
vegetation, similar interpolation procedures may be employed at the discretion of the FEMA Lead.
The principal scenario where additional ground surveys are required involves larger void areas, 
normally larger than 1 acre, located in areas where representative cross sections must be cut. If 
equally acceptable areas exist elsewhere to cut representative cross sections, the FEMA Lead may 
decide that the additional expense is unwarranted for filling larger data voids by ground surveys. 
This "judgment call" is roughly equivalent to photogrammetric mapping where the 
photogrammetrist cannot see through the dense vegetation, determines areas to be "obscured," and 
maps the contours with dashed lines.  In such cases, the photogrammetrist compiles representative 
cross sections only where the bare earth terrain can be seen in stereo, and performs ground surveys 
of additional cross sections only if alternative photogrammetrically compiled locations for cross 
sections are determined to be unacceptable.  
[February 2002]
A.8.3.1.2	Artifacts
Artifacts are regions of anomalous elevations or oscillations and ripples within the DEM data 
resulting from systematic errors, environmental conditions, or incomplete post-processing. They 
may result from malfunctioning sensors, poorly calibrated instrumentation, adverse atmospheric 
conditions, or processing errors. When artifacts are not totally removed, the assigned Mapping 
Partner shall provide an analysis of the effects of the remaining artifacts on DEM accuracy.  The 
analysis shall include a description of the causes (contributing sources) of the artifacts and a 
description of the steps taken to eliminate them. Again, imagery is helpful in identification of 
artifacts. Figure A-4 shows examples of artifacts that remain after automated post-processing and 
prior to manual post-processing of LIDAR data.  Since a major portion of post-processing costs 
pertain to manual post-processing which could consume as much as 80 percent of the total budget 
for a LIDAR project, the FEMA Lead may decide that it is not cost effective to require the bare-
earth digital terrain data to be 100 percent clean and free of artifacts.  As with data voids, the 
severity of such artifacts depends upon their size and location.  In fact, the removal of such artifacts 
would create new data voids; therefore the guidelines are essentially identical.  Artifacts outside the 
floodplain, but inside the watershed, have no bearing on hydraulic modeling and can be neglected.  
Often, LIDAR cross sections can be "cut" in areas other than where the artifacts remain; and 
additional ground surveys are required only if necessary to "cut" cross sections through such artifact 
areas.
The exception to this policy is in the event that the Mapping Partner needs DEMs to be 100 percent 
clean of artifacts for reasons other than hydraulic modeling (e.g., for community GIS requirements). 
In such cases, other criteria must be applied to justify the additional costs for removal of all 
artifacts.  
[February 2002]
A.8.3.1.3	Outliers
During the RMSE calculation process in which survey checkpoints are compared to the interpolated 
TIN values, for example, discrepancies may exist between the two values. Some of the 
discrepancies may be larger than expected, especially in steep terrain or if a "blanket" of dense 
vegetation such as mangrove, sawgrass, or dense wheat fields exists where the LIDAR may 
accurately survey the top surfaces of such vegetation rather than penetrating to the ground; in areas 
of complex topography (e.g., levees that look like hedge rows, boulders that look like haystacks); or 
in forested areas where removal of the top canopy may yield a lower canopy of underbrush that still 
does not represent the bare-earth terrain. If there are a limited amount of larger than normal 
differences between the LIDAR interpolated elevations and the elevations of surveyed checkpoints, 
this may have a significant impact on the final RMSE calculation.  
Outliers often occur as a result of the failure to achieve a true bare-earth surface from the vegetation 
removal process. A single outlier can override dozens or hundreds of accurate checkpoints, making 
the entire RMSE value appear to be poor. When this occurs, the coefficient of skew may indicate 
that the errors do not have a normal distribution, a sign that the RMSE calculation may be 
misleading. (See Subsection A.8.6.3.4.) Having a large quantity of outliers may highlight improper 
post processing. In order to deal with outliers, a criterion may be established to remove a small 
percentage of the total number of checkpoints surveyed which exhibit the largest discrepancies.   
Statisticians almost unanimously agree that errors exceeding the "3-sigma" level are outliers; the "3-
sigma" level provides confidence at the 99.75% confidence level, enabling only the worst 0.25% of 
a dataset to be discarded as outliers.  Less stringent criteria consider the discarding of the worst 1%, 
5% or 10% of the data. Allowing 10% outliers would be in accord with the traditional National 
Map Accuracy Standard where the worst 10% of all checkpoints are treated as outliers that can be 
discarded, regardless of size.  The PO is free to consider the provision of a limited percentage of 
outliers, especially if the skew calculation indicates that the dataset lacks a normal distribution.  The 
option to collect additional check points to replace the outliers may be considered.
[February 2002]
A.8.3.2	System Calibration
LIDAR system components are most effectively tested and calibrated by the equipment 
manufacturer.  Therefore, the Mapping Partner shall provide FEMA with evidence of manufacturer 
calibration.
In addition to evidence of manufacturer calibration of system components, the Mapping Partner 
shall submit evidence that the total LIDAR system was calibrated prior to project initiation for the 
purposes of identifying and correcting systematic errors. Proper system calibration requires 
repetitive overflight of terrain features of known and documented size and elevation using flight 
paths similar to those that will be used in the study area.  For some projects, daily in-situ 
calibrations may be required.
[February 2002]
A.8.3.3	Flight Planning
Planning a flight path that considers all aspects of data collection is critical to the success of the 
mission. An analysis of the project area, project requirements, topography, proximity to restricted 
air space, and other factors will determine the flight path configuration. The mission should include 
parallel flight lines and, for quality control purposes, at least one cross flight line.  The spacing 
between the flight lines will depend on the desired amount of sidelap between swaths and the 
terrain.
The density and accuracy of data generated by different equipment vary widely. The assigned 
Mapping Partner shall have the flexibility of providing a flight path to create the necessary point 
density to meet the posting and accuracy requirements and minimize the occurrence of data voids. 
The assigned Mapping Partner shall check the Position Dilution of Precision (PDOP) in the study 
area. The PDOP is an indicator of the positional accuracy that can be derived from the current GPS 
satellite geometry, which varies continuously; the smaller the PDOP number, the higher the data 
quality.
The assigned Mapping Partner shall document mission date, time, flight altitude, airspeed, scan 
angle, scan rate, laser pulse rates, and other information deemed pertinent.  For a sample mission 
data recordation checklist, refer to Table A-4, which appears at the end of this Appendix.
[February 2002]
A.8.3.4	Global Positioning System (GPS) Base Stations
The assigned Mapping Partner shall select the GPS base stations carefully to ensure reliable 
differential processing of airborne GPS data.  NGS recommends the simultaneous use of two GPS 
base stations during the mission. Either public- or private-domain GPS base stations are suitable for 
use for this purpose, provided they are within the acceptable GPS baseline limits.  Normally 20 
kilometer baseline lengths are maximum for high-accuracy LIDAR surveys, where the airborne 
GPS error component should contribute no more than 5 centimeters to the vertical error budget. 
Where possible, GPS base stations shall have ellipsoid heights to an accuracy of 2 centimeters 
relative to the Continuously Operating Reference Stations or the High Accuracy Reference 
Network.  The assigned Mapping Partner shall use high-quality, dual-frequency GPS receivers and 
associated antennae at the GPS base stations.
[February 2002]
A.8.4	Accuracy Reporting Methodology
FGDC documented a common methodology for determining and reporting the accuracy of 
horizontal and vertical coordinates for geodetic control points (survey monuments) in FGDC-STD-
007.1 and FGDC-STD-007.2.  Additional guidance is included in NGS-58 (NOAA, 1997).
[February 2002]
A.8.5	Post-Processing of Data
For hydraulic modeling, the assigned Mapping Partner shall provide high-resolution, high-accuracy, 
"bare-earth" ground elevation data.  To restrict data to ground elevations only, the Mapping Partner 
shall remove elevation points on bridges, buildings and other structures, and on vegetation from the 
LIDAR-derived data.  In addition to randomly spaced LIDAR points, before and after removal of 
data associated with structures and vegetation, the assigned Mapping Partner shall produce a bare-
earth TIN as well as a DEM, with the minimum regular point spacing, no greater than 5 meters, 
allowed by the data in eastings and northings. The assigned Mapping Partner shall use TIN linear 
interpolation procedures when validating the vertical accuracy of the elevation dataset. Accuracy 
assessments should normally be performed on the TIN and not on the DEM which is a derived 
product of slightly lower accuracy.
Using digital orthophotos, stereo photogrammetry, or other source materials, the assigned Mapping 
Partner shall produce breaklines for stream centerlines, drainage ditches, tops and bottoms of 
streambanks, ridge lines, road crowns, levees, bulkheads, seawalls, road/highway embankments, 
and selected manmade features that constrict or control the flow of water (e.g., curb lines).  When 
merged with mass points, breaklines are used to enforce TIN triangle edges. The assigned Mapping 
Partner also shall specify the sources and accuracy of breakline data. Breaklines are not depicted for 
small culverts that pass under roads unless those culverts (or ditches leading in and out of them) are 
visible on available imagery without the use of photogrammetric stereo compilation which is 
normally too expensive for this purpose.
[February 2002]
A.8.6	Quality Assurance/Quality Control 
QA/QC of the LIDAR-derived data is primarily the responsibility of the assigned Mapping Partner. 
This QA/QC process shall include reviews of flight alignments and completeness of supporting 
data (e.g., cross sections, profiles).  Until LIDAR technology and procedures become mature, 
FEMA will normally provide funding for independent QA/QC testing. 
[February 2002]
A.8.6.1	Vertical RMSE Standards
When systematic errors have been corrected and remaining errors have a normal distribution, the 
NSSDA uses the root mean square error (RMSE) to estimate both horizontal and vertical accuracy. 
RMSE is the square root of the average of the set of squared differences between dataset coordinate 
values and coordinate values from an independent source of higher accuracy for identical points. If 
those differences are normally distributed and average zero, 95 percent of any sufficiently large 
sample should be less than 1.96 times the RMSE.  Therefore, vertical RMSE of 18.5-centimeters is 
often referred to as equivalent to 2' contours, or "37-centimeter accuracy at the 95-percent 
confidence level."  Following that convention, the vertical accuracy of any digital elevation dataset 
is defined as 1.96 times the RMSE of linearly interpolated elevations in that dataset, as compared 
with known elevations from high-accuracy test points.
For the NFIP, TINs (and DEMs derived therefrom) should normally have a maximum RMSE of 
18.5 cm, equivalent to 2' contours, in flat terrain; and a maximum RMSE of 37 cm, equivalent to 4' 
contours, in rolling to hilly terrain.  The Mapping Partner shall field verify the vertical accuracy of 
this TIN to ensure that the 18.5 cm or 37.0 cm RMSE requirement is satisfied for all major 
vegetation categories that predominate within the floodplain being studied.
[February 2002]
A.8.6.2	Ground Cover Categories
The assigned Mapping Partner shall separately evaluate and report on the TIN accuracy for the 
main categories of ground cover in the study area, including the following:
1.	Bare-earth and low grass (plowed fields, lawns, and golf courses);
2.	High grass, weeds, and crops (hay fields, corn fields, and wheat fields);
3.	Brush lands and low trees (chaparrals, mesquite);
4.	Forested, fully covered by trees (hardwoods, evergreens, and mixed forests);
5.	Urban areas (high, dense manmade structures);
6.	Sawgrass; and
7.	Mangrove.
Ground cover Categories 1 through 5 above are fairly common everywhere.  FEMA experience 
with Categories 6 and 7 is that sawgrass and mangrove each have vegetation so dense that LIDAR 
pulses do not penetrate to the ground, but instead map the top reflective surface.  The assigned 
Mapping Partner must treat such areas like "obscured terrain" with conventional photogrammetry 
whereby bare earth elevations within such vegetation category polygons can only be estimated by 
interpolating elevations from ground points surrounding such polygons.
The assigned Mapping Partner may further subdivide and expand the above definitions to better 
accommodate the predominant vegetation types in the study area.  The assigned Mapping Partner 
shall distribute sample points throughout each category area being evaluated and not group the 
sample points of the same type in a small sub-area.
The assigned Mapping Partner shall select a minimum of 20 test points for each major vegetation 
category identified.  Therefore, a minimum of 60 test points shall be selected for three (minimum) 
major land cover categories, 80 test points for four major categories, and so on.  The Mapping 
Partner shall consider establishing test points when planning field surveys to gather cross section 
data for hydraulic modeling. Confidence in the calculated RMSE value increases with the number 
of test points and is a function of sample size. By specifying a minimum of 60 checkpoints (20 each 
in three or more land cover categories), FEMA is specifying that 60 test points are the minimum 
necessary for a practical level of confidence in the calculated RMSE statistic but recognizing that a 
higher number of checkpoints will provide higher confidence that performance standards have been 
achieved. 
The test points are to be selected in terrain that is flat or uniformly sloped within 5 meters in all 
directions. The uniform slope must not exceed 20 percent. The test points must never be located 
near to breaklines, such as bridges or embankments. Test points on sloping or irregular terrain 
would be unreasonably affected by the linear interpolation of test points from surrounding TIN 
points and, therefore, shall not be selected.
[February 2002]
A.8.6.3	Analysis of Error Frequency Distribution     
The RMSE process assumes that errors have a normal distribution (bell curve) with zero mean, and 
that all systematic errors have been removed.  This is rarely the case, especially with LIDAR data. 
Some systematic errors or biases remain undetected, even after regular calibrations of LIDAR 
systems.  This Subsection addresses statistical processes for evaluating whether the errors have a 
central tendency close to zero, evaluating the measure of general variability, and evaluating the 
measure of skewness.  Unfortunately, no hard and fast rules to pinpoint problems exist, but each of 
the statistical processes mentioned have some value in detecting the potential presence of 
systematic errors, for which further investigations are warranted, especially if the RMSE calculation 
fails to pass the vertical accuracy criteria established for the project.
[February 2002]
A.8.6.3.1		Histograms
The most common form of error analysis is to develop a histogram of all errors.  If a dataset 
tabulates errors for all checkpoints, a histogram might show how many of those errors fell within 
different 5-centimeter ranges, for example, graphing the number of errors between the following 
error values: - 50 cm, -45 cm, -40 cm, -35 cm, -30 cm, -25 cm, -20 cm, -15 cm, -10 cm, -5 cm, 0, 
+5 cm, +10 cm, +15 cm, +20 cm, +25 cm, +30 cm, +35 cm, +40 cm, +45 cm, and +50 cm.  
Histograms illustrate whether the errors are unimodal, bimodal, or multimodal, have small or large 
variability, or are skewed on either the positive or negative side. Often, such histograms for LIDAR 
data appear to approximate a normal distribution, except that it does not have a zero mean; this 
would appear to indicate the presence of uncorrected systematic error. Separate histograms for 
individual land cover categories often depict significantly different characteristics that help to 
understand LIDAR system performance (including post-processing) in the different categories. 
Figure A-5 is an example of a histogram reflecting checkpoint errors from a LIDAR dataset that 
does approximate a normal error distribution.

Figure A-5  Histogram for Checkpoint Errors in LIDAR Dataset

[February 2002]

A.8.6.3.2		Measures of Central Tendency
Three common measures of the central tendency of errors in a dataset may be used to quantify the 
nearness to or departure from the expected central tendency of zero:
*	The Arithmetic Mean is the sum of all the errors, divided by the number (n) of checkpoints.  
When the mean error is large, this is an indicator that systematic errors may be present.
*	The Median is the value at which there are as many larger errors as there are smaller errors.
*	The Mode is the value at which the greatest number of errors is concentrated.  In a histogram, 
the mode is the error value or error range which has the highest ordinate value at the peak of the 
histogram.
[February 2002]

A.8.6.3.3		General Variability
Various measures of general variability for statistical data exist, including range, average deviation, 
RMSE, standard deviation, and variance.  However, the industry standard for geospatial data is the 
RMSE explained in Sections A.2 and A.3.  For the dataset used to compute the histogram in Figure 
A-5, the RMSEz = 11.7-centimeters which equates to vertical Accuracyz of approximately 22.9-
centimeters at the 95-percent confidence level.  The standard deviation (? or sigma) is primarily 
used for determination of statistical outliers, i.e., elevation errors that exceed the 3-sigma level.
[February 2002]
A.8.6.3.4		Skewness
Skewness is the condition of being asymmetrical or lopsided, and departing from the expected 
normal distribution shown by the "bell curve."  The coefficient of skewness (a unitless number) is 
used to compute the asymmetry of the errors about the arithmetic mean error. When the absolute 
value of the skew exceeds 0.5, this means that the error distribution is asymmetrical and may not 
represent a normal distribution.  The Mapping Partners should investigate all datasets with skew 
values exceeding ?0.5 to determine if there is a valid reason why the errors do not have a normal 
distribution (a basic assumption in calculation of RMSE values). For the dataset used to compute 
the histogram in Figure A-5, the skew is 0.429.  The skew is computed automatically by Microsoft 
Excel and other spreadsheets, and uses calculations of the second and third central moments of the 
dataset. 
The coefficient of skew is zero when the error distribution is perfectly symmetrical.
[February 2002]
A.8.6.4	Error Assessment 
When a LIDAR dataset does not pass the vertical accuracy standard, steps must be taken to identify 
the cause of the errors.  Systematic corrections should never be applied without first identifying the 
cause of errors.  Many factors could have contributed to errors, including the following:
*	The LIDAR aircraft may have flown at too high of an altitude, causing IMU angular errors to 
propagate to larger errors on the ground from a higher altitude.  
*	The GPS baselines may be too long, flying too far from the GPS base stations, and causing 
larger than normal errors in airborne GPS positions.  
*	The scan angle may have been too large or the airspeed too fast, causing wider point spacings 
and fewer laser pulses that were near vertical.  
*	The satellite PDOP may have been too high, reducing the accuracy of airborne GPS positions.  
*	The post-processing algorithms may have been applied incorrectly.  
*	There may have been  systematic errors with the airborne GPS, IMU, or laser scanner.  
The list of potential causes is a long one because the process is complex.
Many of the statistical tools described in Subsection A.8.6.3 may point to the apparent existence of 
systematic errors.  The following are standard procedures to be used by FEMA's Mapping Partners 
to "trouble shoot" total LIDAR systems in order to isolate systematic errors:
[February 2002]
A.8.6.4.1		Assessments by Land Cover  
If ground cover category 1 (bare earth, sand, rock, dirt, short grass) does not pass the vertical 
accuracy standard, the potential causes are different from potential causes when vegetated or urban 
areas do not pass.  Ground cover category 1 essentially should always pass accuracy standards, 
unless something systematic is wrong.  Daily calibration tests help to confirm that the system 
performed well at the calibration test site (on that same day with the same sensor), and the 
calibration site normally includes bare earth terrain and short grasses.  If ground cover categories 3 
or 4 (see Subsection A.8.6.2) alone do not pass the vertical accuracy standard, the most probable 
causes are shortcomings in the vegetation-removal procedures used in post-processing.  If ground 
cover category 5 (urban terrain) does not pass, something systemic about urban buildings may cause 
the errors.
[February 2002]
A.8.6.4.2		Assessments by Error Locations
The Mapping Partner shall assess errors relative to their location within flight lines, or near the 
beginning or end of swaths.  This may indicate IMU errors or high satellite residuals due to aircraft 
banking causing cycle slips and multipath.
[February 2002]
A.8.6.4.3		Assessments by Dates/Sensors
When LIDAR datasets are flown on different dates, or with different sensors, the Mapping Partner 
must consider these variables in the error analyses.
[February 2002]
A.8.6.4.4		Airborne GPS Verification
The Mapping Partner shall examine GPS flight trajectories, compare the forward and reverse flight 
trajectories' combined separation, check satellite health, check geo-magnetic observations, and 
verify the following:
*	PDOP; 
*	GPS satellite residuals;
*	Satellite phase RMS;
*	Weighting adjustments when two or more differential base stations are used;
*	Base station distance separation;
*	Position standard deviations;
*	Vertical datum used; and
*	Correct application of Geoid99 calculation of orthometric heights.
[February 2002]
A.8.6.4.5		Inertial Measurement Unit (IMU) Verification
The Mapping Partner normally shall do the following to verify the accuracy of the IMU:
*	Review the Kalman filter, the measurement residual ratio (MRR), and the consecutive 
measurement rejections settings; 
*	Confirm IMU was in "fine align" mode for the whole of the dataset; 
*	Check accelerometer drift and scale factor, and the gyro drift and scale factor to ensure they are 
within specifications; 
*	Compare GPS trajectory with recomputed IMU trajectory and investigate large discrepancies; 
and 
*	Review IMU to lever arm parameter measurements and ensure they are entered correctly in the 
proper reference system.
[February 2002]
A.8.6.4.6		Laser Range Verification
The Mapping Partner shall normally do the following to verify the accuracy of the laser ranges:
*	Review raw laser ranges; 
*	Identify areas of high dropouts (no returns) and correlate them to justifiable features; 
*	Review scanner mirror angles (galvanometers or micro-controller); 
*	Examine intensity images if collected; and 
*	Review system generated error log sheets.
[February 2002]

A.8.6.4.7		Total System Verification
To verify the accuracy of the whole system, the Mapping Partner shall do the following:
*	Review calibration flights and compare to system flight parameters; 
*	Use CAD software to analyze individual flightlines and verify pitch, roll and heading errors; 
*	Check overlap for roll and scaling errors; 
*	Check ground features for pitch (i.e., buildings, bridges, etc.); 
*	Check scaling errors if water bodies exist; 
*	Compare cross flight data for attitude;  and
*	Verify vegetation removal procedure and review parameters if data were "corrected" or adjusted 
for pitch, roll and heading errors; for example, was the iteration angle too high for the terrain 
type?  Is the morphological window within a reasonable size for area features (e.g., is it too 
large and removes key components such as large dunes?).  
[February 2002]
A.8.6.4.8		Systematic Error Corrections
It is relatively easy to determine the magnitude of systematic errors and adjust all data accordingly, 
however, the assigned Mapping Partner must not be "correct" such errors until the error source is 
clearly identified and documented.  The Mapping Partner must report systematic errors to the 
FEMA Lead for review before systematic reprocessing of data that initially failed to pass the 
accuracy criteria.  
[February 2002]
A.8.6.5	Cross Flight Verification
To supplement the QA/QC process for LIDAR data, the Mapping Partner may employ different 
optional techniques to check not only the accuracy but also the precision. The balance is to address 
the need for additional verification checkpoints with cost-effective methods that aid in identifying 
the internal consistency (precision) of LIDAR data. Two GPS techniques that may be utilized are 
continuous kinematic and Real Time Kinematic (RTK).  Continuous kinematic collects positional 
data every epoch while maintaining fixed ambiguities on a minimum of five satellites.  In the event 
of loss of lock on satellites, the integer ambiguities must be recomputed to their fixed integer value 
before a sub decimeter position can be calculated. Post processing is required to obtain the 
positional data.  RTK is similar to continuous kinematic except that the data do not need to be post-
processed.  The estimated ambiguities are fixed to their integer values and positional data are 
calculated and stored in real time.  Both methods are efficient for GPS surveys along highways.
With both types of GPS procedures the process is to survey checkpoints perpendicular to the 
LIDAR flight paths at different strategic locations, with these additional checkpoints along roads or 
highways that transect the survey area. The assigned Mapping Partner shall conduct surveys close 
to the center of roads and highways, but avoiding bridges, elevated roads, or roads with steep 
embankments so that LIDAR points can be interpolated accurately without concern for steep slopes. 
If the flights are flown north to south, then the checkpoints must traverse east and west across the 
project area. A minimum of three cross lines must be measured, one at each end of the flight lines 
and another through the center.  Typically the weakest data will be at the beginning of the flight line 
if the GPS and IMU have not stabilized due to the turning of the aircraft.  The center cross line will 
help identify if the airborne data are stabilized. Ideally checkpoints should have a minimum point 
spacing of one half the LIDAR swath width to ensure each swath has at least one checkpoint.  
Increasing the number of cross flight checkpoints will assist in verifying confidence within the data. 
Because such surveys are less accurate than the surveys of checkpoints to NGS-58 5-centimeter 
standards, these additional verification points are not to be mixed with the survey checkpoints used 
to compute the vertical RMSE or Accuracyz. However, this is a relatively inexpensive way to 
generate a relatively large number of somewhat less-accurate checkpoints when there is concern 
that the 60, 80 or 100 checkpoints (20 each in the major land cover categories) are insufficient for 
providing a high degree of confidence in the accuracy statistics.
[February 2002]
A.8.7	Deliverables
All data and products associated with contract deliverables shall meet or exceed relevant NSSDA 
standards and fully comply with the FGDC metadata format standard with the provisions in the 
contract. The Mapping Partner shall use Appendix I of these Guidelines as a guide for preparing 
and submitting deliverable products to FEMA in digital format.
[February 2002]
A.8.7.1	Pre-Project Deliverables
Prior to data collection, the Mapping Partner shall submit the following to the FEMA Lead or a 
Mapping Partner designated by the FEMA Lead:
A map showing the study area boundaries and flight path, at a medium scale (1:24,000) or small 
scale (1:50,000); typically, USGS maps are desirable for this purpose
*	Documentation specifying altitude, airspeed, scan angle, scan rate, LIDAR pulse rates, and 
other flight and equipment information deemed appropriate; and
*	A chart of areas of high PDOP, or a list showing the time of the beginning and end of high 
PDOP.
[February 2002]
A.8.7.2	Post-Project Deliverables
Following project completion, the Mapping Partner shall submit the following to the FEMA Lead 
or a Mapping Partner designated by the FEMA Lead:
*	A LIDAR system data report;
*	A flight report;
*	A ground control report;
*	Data processing procedures for selection of postings, and all orthometric values of x, y, and z 
coordinates for LIDAR returns.  Elevations shall be orthometric heights; and 
*	A system calibration report.
The LIDAR system data report shall include discussions of the following: 
*	Data processing methods used, including the treatment of artifacts; 
*	Final LIDAR pulse and scan rates; 
*	Scan angle; 
*	Capability for multiple returns from single pulses; 
*	Accuracy and precision of the LIDAR data acquired; 
*	Accuracy of the topographic surface products; 
*	Any other data deemed appropriate; 
*	Companion imagery, if any; and
*	A digital spatial index showing the orientation of all data tiles within the project site with tile 
labels corresponding to the CD (or other media) ID and file name on that CD.
The flight report shall document mission date, time, flight altitude, airspeed, and other information 
deemed pertinent.  The report shall include information about GPS-derived flight tracks, provide a 
detailed description of final flight line parameters and GPS controls (i.e., benchmarks), and include 
ground truth and complementary reference data.
The ground control report shall include, at a minimum, all pertinent base station information and 
mission notes, including information on GPS station monument names and stability.
[February 2002]
A.8.7.3	Delivery of Digital Data
In addition to the pre- and post-project deliverables described above, the Mapping Partner shall 
submit the following:
*	All raw datasets, dataset of survey points filling voids, dataset of transects (if generated), bare-
earth DEM data, and breaklines in separate data files; and 
*	Uniformly spaced DEM(s), on ISO 9660 standard CD-ROM (or DVD) media in a format 
specified in Appendix L.
The Mapping Partner shall deliver raw datasets and LIDAR system data, including orthometric 
heights for each point, in comma-delimited ASCII files in x, y, and z format.  The Mapping Partner 
shall deliver the DEM in the format designated on the Table A-4 checklist. The Mapping Partner 
also shall flag raw datasets from sidelap and overlap areas of separate flight lines.  Breaklines shall 
be produced, and breakline files shall contain a flag record that identifies them as breakline features 
and identifies their source and accuracy.  The Mapping Partner shall submit raw datasets in tiles or 
data models matching those of the DEM.  
 The Mapping Partner shall ensure that all deliverables conform to the projection, datum, and 
coordinate system specified in the agreement with FEMA.  File sizes cannot exceed 1 gigabyte, 
unless otherwise specified by the FEMA Lead.  Each file shall be organized to facilitate data 
manipulation and processing.
[February 2002]
A.8.8	Acceptance/Rejection
The terms of acceptance/rejection in Section A.7.5.5 of this Appendix shall apply.

Table A-4. LIDAR System Mission Data Collection Checklist
Notes
A.	Data collection  (each flight)  
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________

1.	Record flight date and time.*
2.	Record flight altitude(s).*
3.	Record LIDAR system scan angle, scan rates, and 
pulse rates.*
4.	Record time LIDAR system receiver is activated/ 
deactivated.*
5.	Record all Position Dilution of Precision values.*
6.	Record height of instrument (before and after 
flight).
7.	Record on-board antenna offsets.
8.	Note any site obstructions at GPS base station(s).
9.	Record airborne and ground-site GPS receiver 
types and serial numbers.
10.	Record ground site GPS station monument names 
and stability.*
11.	Record flight staff.

Notes
B.  Data handling (each flight)
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________

1.	Record that all files have been labeled correctly 
and cross indexed.
2.	Record analyst name(s) responsible for 
processing and product generation .
3.	List any auxiliary information used during 
processing of LIDAR to generate products 
delivered.
4.	List major data processing components used.

*	Denotes Minimum Required Information
[February 2002]

A.9	References
American Society for Photogrammetry and Remote Sensing, ASPRS 1990, "ASPRS accuracy 
standards for large-scale maps," Photogrammetric Engineering and Remote Sensing, Vol. 56, 
No. 7, pp 1068-1070.
American Society for Photogrammetry and Remote Sensing, ASPRS 1995, Draft Aerial 
Photography Standards, Photogrammetric Engineering and Remote Sensing, Vol. 61, 
Number 9, pp. 1097-1103.
American Society for Photogrammetry and Remote Sensing, ASPRS 2001, Digital Elevation 
Model Technologies and Applications: The DEM Users Manual, Chapter 2 (Vertical 
Datums), Chapter 3 (Accuracy Standards), Chapter 5 (Photogrammetry), Chapter 7 
(Topographic LIDAR), Chapter 12 (DEM Quality Assessment), Chapter 13 (DEM User 
Requirements), and Appendices A and B (Acronyms and Definitions).
Bureau of the Budget, 1947, National Map Accuracy Standards, Office of Management and 
Budget, Washington, D.C. 
Federal Geographic Data Committee, FGDC-STD-007.1-1998, Geospatial Positioning 
Accuracy Standards, Part 1: Reporting Methodology, FGDC, c/o USGS, Reston, VA, 
www.fgdc.gov/standards/documents/standards/accuracy/chapter1.pdf 
Federal Geographic Data Committee, FGDC-STD-007.2-1998, Geospatial Positioning 
Accuracy Standards, Part 2: Standards for Geodetic Networks, FGDC, c/o USGS, Reston, 
VA, www.fgdc.gov/standards/documents/standards/accuracy/chapter2.pdf 
Federal Geographic Data Committee, FGDC-STD-007.2-1998, Geospatial Positioning 
Accuracy Standards, Part 3: National Standard for Spatial Data Accuracy, FGDC, c/o 
USGS, www.fgdc.fgdc.gov/standards/documents/standards/accuracy/chapter3.pdf 
Federal Geographic Data Committee, FGDC 1999, Content Standards for Framework Land 
Elevation Data (final draft), FGDC, c/o USGS, Reston, VA, 
www.fgdc.gov/standards/documents/standards/elevation/ele_199.pdf.
National Oceanic and Atmospheric Administration, NOAA 1997, Guidelines for Establishing 
GPS-Derived Ellipsoid Heights (Standards: 2 cm and 5 cm), version 4.3, NOAA Technical 
Memorandum NOS NGS-58, www.ngs.noaa.gov/PUBS_LIB/NGS-58.html.
U.S. Army Corps of Engineers, USACE 1993, Engineer Manual 1110-1-1000, 
Photogrammetric Mapping, dated 31 March 1993.
U.S. Army Corps of Engineers, USACE 1994, Engineer Manual 1110-2-1003, Hydrographic 
Surveying, dated 31 October 1994.

Guidelines and Specifications for Flood Hazard Mapping Partners
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Section A.1		February 2002 Edition
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Section A.2		February 2002 Edition
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Section A.3		February 2002 Edition
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Section A.4		February 2002 Edition
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Section A.5		February 2002 Edition
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Section A.6		February 2002 Edition
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Section A.7		February 2002 Edition
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Section A.8		February 2002 Edition
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Section A.9		February 2002 Edition