Appendix B
Converting to the North American  
Vertical Datum of 1988
B.1		Background
Every Flood Insurance Study (FIS) Report and Flood Insurance Rate Map (FIRM) that contains 
detailed flood hazard information is prepared based on hydraulic analyses that are referenced to a 
specific vertical datum.  The two standard datums in use nationwide are the National Geodetic 
Vertical Datum of 1929 (NGVD29) and the North American Vertical Datum of 1988 
(NAVD88).  Information on these datums and on software that is available to convert NGVD29 
to or from NAVD88 is provided in Subsections B.1.1, B.1.2, and B.1.3.
[February 2002]
B.1.1	National Geodetic Vertical Datum of 1929 
Historically, the most common vertical datum used for FEMA flood hazard studies/restudies and 
map revisions has been NGVD29.  NGVD29 assumed that 26 tide gages in the United States and 
Canada all represented the same zero elevation, which was mean sea level. As survey 
technologies became more accurate, it became increasingly apparent that NGVD29 constraints 
were incorrectly forcing surveys to fit different tide stations (all zero elevation or mean sea level) 
that actually had different elevations relative to each other.  NGVD29 essentially warped the 
geoid, which represents an equipotential surface where gravity, and elevations, should be the 
same.  Fortunately, the maximum warp anywhere in the United States, caused by forced 
constraints of NGVD29 at 26 tidal stations, is no more than 1.5 meters.  Although there are 
exceptions, the warping found over smaller geographic areas, such as the area within a county, is 
small.
[February 2002]
B.1.2	North American Vertical Datum of 1988   
In the 1970s, the National Geodetic Survey (NGS), and counterpart agencies in Mexico and 
Canada, decided to adopt a vertical datum based on a surface that would closely approximate the 
Earth's geoid.  The new adjustment, NAVD88, was completed in June 1991 and is now the only 
official vertical datum in the United States.  NAVD88 was created by adding 625,000 kilometers 
of leveling, performed since NGVD29 was established, and performing a major least squares 
adjustment that constrained only a single tide station at zero elevation.  The height of the primary 
tidal bench mark at Father Point/Rimouski, Quebec, Canada, was held fixed as the constraint, 
enabling NAVD88 and the International Great Lakes Datum of 1985 (IGLD85) to be one and the 
same.


Now, other tide stations may have elevations other than zero.  Subsequent to the establishment of 
NAVD88, new flood hazard studies are preferably referenced to that datum.
[February 2002] 
B.1.3	Conversion Software
The NGS and the U.S. Army Corps of Engineers (USACE) have developed software, which may 
be obtained free of charge, for performing conversions between NGVD29 and NAVD88. 
Interested Mapping Partners may download the PC-compatible NGS VERTCON software from 
the NGS home page at www.ngs.noaa.gov, by selecting the NGS Geodetic Tool Kit option.  
Similarly, interested Mapping Partners may download the USACE CORPSCON software from 
the U.S. Army Topographic Engineering Center home page at www.tec.army.mil; by selecting 
What We Do; Products and Services; Software Available, and CORPSCON.  The CORPSCON 
software can be use to convert horizontal datums (between the North American Datum of 1927 
[NAD27] and NAD 83) as well as vertical datums (between NGVD 29 and NAVD 88) based on 
NGS NADCON and VERTCON software.  Both programs compute the modeled differences in 
orthometric heights (elevations) between NGVD29 and NAVD88 for a given location specified 
by geographic coordinates (latitude and longitude).  CORPSCON also allows Universal 
Transverse Mercator or State Plane coordinates to be entered in lieu of geographic coordinates.  
The Mapping Partner may obtain identical results using either the VERTCON or CORPSCON 
software. 
[February 2002]



B.2	Data Collection
One of the stated goals of FEMA's Map Modernization Program is to convert all flood maps 
from NGVD29 to NAVD88.  The Mapping Partner that performs a study/restudy for FEMA or 
submits a map revision request will be responsible for applying proper vertical datum protocols 
for new and/or revised flood hazard data when preparing or revising flood hazard study/restudy 
materials that have been chosen for the datum conversion. FEMA recognizes that there are, and 
will continue to be, limiting factors in achieving this conversion.  To evaluate the suitability of a 
subject jurisdiction for datum conversion, the Mapping Partner shall gather the following 
information during the initial coordination efforts for a study or restudy:
*	Datum used for the existing study, if one exists, and the extent of changes that will occur as a 
result of the restudy;
*	Number (percentage) of streams that will be revised and the number of unrevised flooding 
sources that must be converted from NGVD29 to NAVD88 if the datum conversion option is 
chosen;
*	Conversion factor from NGVD29 to NAVD88 for the subject community, whether or not the 
conversion factor for the community is constant, and maximum offset from the established 
conversion factor (see Section B.4.1);
*	Reference datum used by FEMA for adjacent communities;
*	Datum of choice for the local surveyors and any known difficulties that the community would 
have with the use of NAVD 88; and 
*	Approximate effort (man-hours) associated with conversion to NAVD 88.
[February 2002]



B.3	Conversion Criteria
To eliminate possible confusion and misuse of elevation information, the flood elevations for all 
flooding sources studied by detailed methods within a given community must be referenced to 
the same datum.  Therefore, if a Mapping Partner undertakes a restudy or map revision that does 
not include all flooding sources studied by detailed methods, the elevations for the flooding 
sources also must be converted to NAVD88.    
Therefore, it is essential for the Regional Project Officer (RPO) and the Mapping Partner to make 
an initial sound decision about which vertical datum can and should be used for a study, restudy, 
or map revision. Once the Mapping Partner has gathered the information specified in Subsection 
B.2, FEMA, in consultation with the Mapping Partner, will make the final decision regarding the 
datum to which the new, revised, and unrevised flood hazard information will be referenced.  
When a new or revised study is being processed, the decision to use NAVD88 over NGVD29 
will depend largely on the data gathered early in the process.  Criteria that facilitate a conversion 
from NGVD29 to NAVD88 are as follows:
*	All flooding sources in the community are being studied or restudied by detailed methods.
*	Less than 50 percent and fewer than 20 miles of detailed study streams that are not being 
restudied will have to be converted from NGVD29 to NAVD88.
*	No more than 5 percent of the total printed FIRM panels for the community have to be 
revised solely to convert the elevations for the flooding sources that were not restudied from 
NGVD29 to NAVD88.
*	The maximum offset from an established average conversion (from NGVD 29 to NAVD 88) 
for the subject jurisdiction does not exceed 0.25 foot.  Protocol for determining an average 
conversion factor as well as a maximum offset value is provided in Subsection B.4.1.  
*	The Mapping Partner performing the flood hazard analysis is able to use NAVD88 for the 
study/restudy or map revision.
*	The community is familiar with NAVD88.
If the mapping activity undertaken by the Mapping Partner is a comprehensive restudy of all 
flooding sources studied by detailed methods, or if the Flood Map Project is resulting in a first 
time FIRM, all flood elevations shall be referenced to NAVD88 unless otherwise specified 
during the Scoping process.
The above criteria are provided for general guidance to aid the RPO and Mapping Partner in 
making a technically sound, cost-effective, and user-friendly decision.
[February 2002]


B.4	Conversion of Unrevised Flood Elevations
There will be occasions were a Mapping Partner submits restudy or map revision information 
referenced to NAVD88 to FEMA to support preparation of a new or revised FIRM but does not 
address the remainder of the unrevised flood elevations.  In those circumstances, FEMA will 
decide whether the Mapping Partner that performed the study/restudy or the Mapping Partner that 
prepares the Preliminary FIS Report and FIRM will convert the flood hazard information for the 
entire community to NAVD88.  This decision will be made on a case-by-case cost-benefit 
assessment.   If FEMA determines that the cost to convert the entire community is reasonable 
(considering the other scope of work), the Mapping Partner that is selected to complete the 
conversion shall follow the procedures in Subsections B.4.1, B.4.1.1, B.4.1.2, and B.4.1.3 of this 
Appendix.
[February 2002]
B.4.1	Protocol for Determining Conversion Factor
The Mapping Partner responsible for conducting the new or revised flood hazard analyses shall 
establish single or multiple conversion factors to be applied to the unrevised 1-percent annual- 
chance flood elevations in the FIS Report and on the FIRM.  To determine an average conversion 
factor from NGVD29 to NAVD88, the steps below shall be followed:
Step 1 - Locating the Subject Jurisdiction 
The Mapping Partner shall locate the subject jurisdiction on U.S. Geological Survey 7.5-minute 
series topographic quadrangle maps. 
Step 2 - Determining Conversion Factors for Quadrangle Corners
The Mapping Partner shall use VERTCON or CORPSCON to determine the conversion factor 
for each quadrangle corner that falls inside the jurisdiction boundary and each quadrangle corner 
that lies within 2.5 miles outside of the jurisdiction boundary. This information is to be entered 
into a table that will be used in the FIS Report (sample table provided below.)  Details for the 
production of FIS Reports are provided in Appendix J.

Quad Name
Corner
Latitude
Longitude
Conversion from 
NGVD29 to NAVD88
Johnsonville West
SE
35.375
82.125
-0.54 ft
Johnsonville East
SE
35.375
82.250
-0.32 ft
Gilberts Corner
SW
35.250
82.000
-0.54 ft
Farmville
SW
35.250
82.125
-0.37 ft
Taylor's Grove
SW
35.250
82.250
-0.25 ft
Thompsonville
SW
35.250
82.375
-0.14 ft

Figure B.1 shows five quadrangle corners within the county, and one outside the county but 
within 2.5 miles of the county boundary. (This corner is thus to be included in the calculation of 
average conversion factors.)  The shaded circles represent the quad intersections that meet the 
selection criteria.
[February 2002]



























For quadrangles that include oceans or other major water bodies, quadrangle corners that are 
more than 2.5 miles away from land shall not be used, except in the case of small islands or 
narrow bands of land that would not be represented by quadrangle corners on either side.  When 
this occurs, the Mapping Partner's discretion may be used in selecting the nearest quadrangle 
corner to ensure that conversion factors for small islands or terrain such as the North Carolina 
Outer Banks are considered in determining the average conversion factor to be applied to a 
county.
Step 3 - Determining an Average Conversion Factor
Once conversion factors for all eligible quadrangle corners have been established, the Mapping 
Partner shall determine an average conversion factor by calculating a simple, unweighted 
arithmetic mean of all points for the entire jurisdiction.  In the example shown in Figure B.1, the 
average conversion factor is -0.36 foot.
Step 4 - Establishing a Range of Conversion Factors
The Mapping Partner shall establish a range of conversion factors from all eligible points 
(minimum and maximum values) for the subject jurisdiction from the values documented in Step 
2.  In the sample table above, the range of conversion factors is between -0.14 foot and -0.54 
foot.  The Mapping Partner shall use the range of conversion values, combined with the average 
conversion factor determined above, to determine if a conversion is needed, or if a passive-
conversion approach is feasible.  A passive conversion, discussed in more detail in Step 6 
(below), is defined as applying the average conversion factor determined above to the FIS Report 
(tables) only.  The FIRM and Flood Profiles would not be affected by a passive conversion 
decision.
Step 5 - Determining the Maximum Offset
The Mapping Partner shall determine the maximum offset from the average conversion factor, 
compared with the minimum and maximum conversion factors.  In the above example, the 
maximum offset is 0.22 foot (0.36 foot to 0.14 feet).   If the Mapping Partner determines that the 
maximum offset exceeds 0.25 foot for any of the qualifying quadrangle corners, the multiple-
conversion protocol (stream-by-stream conversion) detailed in Section B.4.1.2 shall be applied.  
The exception to this rule is situations whereby qualifying exterior quad corners are the sole 
cause for the subject jurisdiction being ineligible for the application of a standard conversion 
value.   To determine if this is the case, the Mapping Partner shall replace anomalous conversion 
values that are obtained from quadrangle corners outside the subject jurisdiction with locations 
along the jurisdiction boundary closest to the anomalous quadrangle corner (one per quadrangle 
corner).  Once this has been done, if the jurisdiction continues to be mathematically ineligible for 
conversion to NAVD88 using a standard jurisdiction-wide conversion value, the Mapping 
Partner shall use the stream-by-stream conversion approach detailed in Section B.4.1.2.
Step 6 - Establishing the Conversion Factor
Using the values documented above, the Mapping Partner shall establish a single average 
conversion factor or multiple conversion factors and apply the factor(s) to existing effective data 
to be converted to NAVD88.  The criteria for determining whether a single conversion factor, or 
multiple conversion factors will be used, are described in Subsections B.4.1.1, B.4.1.2, and 
B.4.1.3.  If the average conversion factor above is less than 0.1 foot, there may be only a passive 
datum conversion applied.  As noted in Step 4, a passive conversion would affect the values 
shown on data tables in the FIS Report, but would not affect the 1-percent-annual-chance flood 
elevations shown on the FIRM or on the Flood Profiles in the FIS Report.  The Mapping partner 
shall apply the average conversion factor determined in Step 3 above to all data tables in the FIS 
Report containing flood elevations referenced to NGVD29. The use of this option is contingent 
on the range of conversion values determined in Step 4.  An average conversion of 0.1 foot or 
less could be deceiving in areas that contain conversions to NAVD88 that show plus and minus 
values.  To illustrate this point, the following table shows a hypothetical county where the 
average conversion to NAVD88 was calculated to be -0.09 foot.  However, it is also shown that 
the county contained a large range of conversion values thereby rendering a passive-conversion 
decision inappropriate.  

Range of conversion values
- 0.38 through  + 0.24
Average conversion factor
- 0.09
Maximum variance from the average conversion
0.33
Maximum variance from a no-conversion value
0.38

This table demonstrates that, although a jurisdiction may have an insignificant average 
conversion value (0.1 foot or less), the range of conversion values indicates that there are 
quadrangle corners that are more than 0.25 foot askew of a zero conversion factor.  The situation 
shown in the above table indicates that a passive-conversion approach could yield up to a 0.38-
foot discrepancy in the jurisdiction. For this reason, a 0.25-foot tolerance was established as the 
maximum variance acceptable from a passive-conversion value.  It is also worth noting that this 
hypothetical jurisdiction also would be ineligible for the application of an average conversion 
factor.  A multiple conversion (stream-by-stream) approach as detailed in Section B.4...1.2 below 
would be required for this jurisdiction.

[February 2002]
B.4.1.1	Single Conversion Factor
A single conversion factor from NGVD29 to NAVD88 may be applied when the maximum 
offset from the average conversion factor does not exceed 0.25 foot.  When a decision has been 
made during the Project Scoping phase (discussed in detail in Volume I, Section 1.3) to apply a 
datum conversion and a single conversion factor is appropriate, the Mapping Partner responsible 
for the new or updated flood hazard analyses shall apply the following procedures.
1.	Determine an average conversion factor for the subject community following the procedures 
detailed in Subsection B.4.1 and apply the average to the dynamic flood elevations.
2.	Convert static (primarily, lacustrine) flood elevations using VERTCON or CORPSCON.
3.	Apply the conversion factor to the FIRM and to all components of the FIS Report that display 
1-percent-annual-chance flood elevations (i.e., Flood Profiles, data tables).  
4.	Document the datum conversion details as specified in Appendix J of these Guidelines.
5.	Ensure that all unrevised hydraulic models and supporting backup information are clearly 
labeled in the Technical Support Data Notebook (see Appendix M of these Guidelines) to 
indicate that the FIRM and FIS Report  reflect a datum conversion and  specify the 
conversion criteria that was applied.
[February 2002]
B.4.1.2	Multiple Conversion Factors (Stream by Stream)
In situations where the range of conversion factors across the subject community is prohibitively 
high (thereby resulting in a maximum offset from the established average conversion factor of 
greater than 0.25 foot), the Mapping Partner shall not apply a standard conversion factor for the 
entire community.  In the event that conversion to NAVD88 remains a desirable option, the 
Mapping Partner shall convert the unrevised flood elevations on a stream-by-stream basis.  
Under this approach, the Mapping Partner shall develop an average conversion factor for each 
stream or flooding source by establishing three conversion factors at three locations and 
developing an average conversion factor from those data.  If the maximum offset from the 
average conversion factor determined for a flooding source converted in this fashion exceeds 
0.25 foot, the Mapping Partner shall follow the protocol described in Subsection B.4.1.  When 
multiple conversion factors are applied, on a stream-by-stream basis, the Mapping Partner shall 
present the conversion factors in a table to be placed on the FIRM and in the FIS Report.  An 
example of the table to be used in this scenario is shown in the below as well as in Appendix J of 
these Guidelines.  As noted in Subsection B.4.1.1 when a datum conversion is conducted on 
unrevised flood elevations, the Mapping Contractor responsible for preparing the Technical 
Support Data Notebook (see Appendix M) shall ensure that all unrevised hydraulic models and 
supporting backup information are clearly labeled to indicate that the FIRM and FIS Report 
reflect a datum conversion and shall specify the conversion criteria that was applied.
[February 2002]

Stream Name
Minimum 
Conversion
Maximum 
Conversion
Average 
Conversion
Maximum 
Offset
Jones Branch
-0.74
-0.92
-0.81
0.11
Mud River
-0.59
-0.80
-0.74
0.15

B.4.1.3	Conversion of Hydrologic and Hydraulic Models
In situations where the range of conversion factors for a given flooding source is prohibitively 
high (thereby resulting in a maximum offset from the average established for the flooding source 
of greater than 0.25 foot) the Mapping Partner shall remodel the subject stream by applying 
either the VERTCON or the CORPSCON program to the effective hydrologic and hydraulic  
models.  (NOTE: To date, these details and protocols have not been finalized and FIS Report 
paragraphs have not been formulated to address this situation.)
[February 2002]



B.5	Conversion from NAVD88 to NGVD29
Although a datum selection shall normally be determined during the Project Scoping phase 
(Volume I, Section 1.3) of a study/restudy, situations may be encountered when the Mapping 
Partner responsible for the new or revised flood hazard analysis provides flood hazard data 
referenced to NAVD88, but FEMA determines that a full conversion is not an acceptable 
solution because of cost constraints or other reasons.  In this case, the Mapping Partner 
responsible for preparation of the Preliminary FIS Report and FIRM shall develop an average 
conversion factor and apply it to convert the flood elevations provided by the Mapping Partner 
responsible for the new or revised flood hazard analysis from NAVD88 to NGVD29.  For static 
flood elevations, the Mapping Partner responsible for preparation of the Preliminary FIS Report 
and FIRM may apply VERTCON or CORPSCON to convert the NAVD88 elevations to 
NGVD29.  In those situations where an average conversion factor is not practical, the Mapping 
Partner responsible for preparation of the Preliminary FIS Report and FIRM may apply the 
approaches outlined in Subsections B.4.1.2 and B.4.1.3 as warranted.
[February 2002]



B.6	Paragraphs for Flood Insurance Study Report
For all studies/restudies and map revisions, the Mapping Partner responsible for preparing the 
Preliminary copies of the FIS Report and FIRM shall follow the guidelines provided in Appendix 
J, which detail the appropriate paragraphs to address datums and datum conversions.
[February 2002]



Guidelines and Specifications for Flood Hazard Mapping Partners
B-3
Section B.1		February 2002 Edition
B-4
Section B.2		February 2002 Edition
B-5
Section B.3		February 2002 Edition
B-11
Section B.4		February 2002 Edition
B-12
Section B.5		February 2002 Edition
B-13
Section B.6		February 2002 Edition