DATA USERS GUIDES 

 1: Digital Line Graphs from 1:24,000-Scale Maps 

 2: Digital Line Graphs from 1:100,000-Scale Maps 

 3: Digital Line Graphs from 1:2,000,000-Scale Maps 

 4: Land Use and Land Cover from 1:250,000- and 1:100,000-Scale Maps 

 5: Digital Elevation Models 

 6: Geographic Names Information System 

 7: Alaska Interim Land Cover Mapping Program 

Data Users Guides 1-7 generally replace the Geological Survey Circular 895. 

Questions regarding availability and ordering of US GeoData (all types of digital cartographic and geographic 

data produced and distributed by the U.S. Geological Survey) should be addressed to: 

 

 Earth Science Information Center 

 U.S. Geological Survey 

 507 National Center 

 Reston, Virginia 22092 

 (703) 648-6045 

Technical questions and comments should be addressed to: 

 Branch of Technical Standards 

 and Product Development 

 U.S. Geological Survey 

 510 National Center 

 Reston, Virginia 22092 
 UNITED STATES 

DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY 

DIGITAL ELEVATION MODELS 

__________________________________________________________________________________________ 

Data Users Guide 5 

 Reston, Virginia 

 1993 
First printing, 1987 

Second printing (revised), 1990 

Third printing (revised), 1993 
 CONTENTS 

 ___________ 

 Page

Introduction................................................................................................................................................ 1 

7.5-minute Digital Elevation Models......................................................................................................... 2 

 Characteristics ....................................................................................................................................... 2 

 Data production...................................................................................................................................... 3 

1-degree Digital Elevation Models ............................................................................................................ 5 

 Characteristics........................................................................................................................................ 5 

 Data production .................................................................................................................................... 6 

30-minute Digital Elevation Models.......................................................................................................... 7 

 Characteristics........................................................................................................................................ 7 

 Data production...................................................................................................................................... 7 

15-minute Alaska Digital Elevation Models.............................................................................................. 8 

 Characteristics........................................................................................................................................ 8 

 Data production...................................................................................................................................... 8 

7.5-minute Alaska Digital Elevation Models............................................................................................. 9 

 Characteristics........................................................................................................................................ 9 

 Data production...................................................................................................................................... 9 

Geometry ................................................................................................................................................. 10 

Accuracy .................................................................................................................................................. 10 

Classification levels ................................................................................................................................. 14 

 Level 1 ................................................................................................................................................. 14 

 Level 2 ................................................................................................................................................. 14 

 Level 3 ................................................................................................................................................. 14 

Digital Elevation Model caveats.............................................................................................................. 15 

Data records ............................................................................................................................................. 15 

 Sample data records ............................................................................................................................. 16 

Sample applications ................................................................................................................................. 16 

Appendix A. Digital Elevation Model data elements-- 

 logical record type A................................................................................................................. 29 

Appendix B. Digital Elevation Model data elements-- 

 logical record type B................................................................................................................. 35 

Appendix C. Digital Elevation Model data elements-- 

 logical record type C................................................................................................................. 37 

Appendix D. Sample quadrilateral coordinates ...................................................................................... 38 

Appendix E. Codes for State plane coordinate zones ............................................................................. 39 

Appendix F. Universal Transverse Mercator zone locations and 

 central meridians....................................................................................................................... 41 

Appendix G. Parameters required for definition of map projections...................................................... 42 

Appendix H. National and international datums used for 

 digital elevation data................................................................................................................. 47 

 iii 
 ILLUSTRATIONS 

 __________ 

 

 Page 

Figure 1. Structure of a 7.5-minute Digital Elevation Model, 

 UTM meter grid ............................................................................................................................ 4 

 2. Structure of a 1-degree Digital Elevation Model, arc-second grid .............................................. 6 

 3. Computation of a first data point in a 7.5-minute Digital 

 Elevation Model west of the central meridian ............................................................................ 11 

 4. Computation of first data point in a 7.5-minute Digital 

 Elevation Models east of the central meridian............................................................................ 12 

 5. Geometry and nomenclature of the Digital Elevation Models file ........................................... 13 

 6. Preeruption isometric plot of Mount St. Helens Digital Elevation Models .............................. 25 

 7. Posteruption isometric plot of Mount St. Helens Digital Elevation Models ............................. 25 

 _________________ 

 TABLES 

 __________ 

 Page

 Table 1. Sample Digital Elevation Model type A logical records--Mannboro, 

 Virginia, quadrangle (7.5 minute)............................................................................................... 17 

 2. Sample Digital Elevation Model type B logical records--Mannboro, Virginia, 

 quadrangle (7.5 minute).............................................................................................................. 20 

 3. Sample Digital Elevation Model type C logical records--Mannboro, Virginia, 

 quadrangle (7.5 minute).............................................................................................................. 21 

 4. Sample Digital Elevation Model type A logical records--Reno, 

 Nevada-California,quadrangle (west half) (1 degree) 22

 

 5. Sample Digital Elevation Model type B logical records--Reno, 

 Nevada-California, quadrangle (west half) (1 degree)................................................................ 24 

 iv 
[blank page] 

 
DIGITAL ELEVATION MODELS 

__________________________________________________________________________________________

____ 

 INTRODUCTION 

The Earth Science Information Center (ESIC) distributes digital cartographic/geographic data files produced by 

the U.S. Geological Survey (USGS) as part of the National Mapping Program. Digital cartographic data files 

may be grouped into four basic types. The first of these, called a Digital Line Graph (DLG), is the line map 

information in digital form. These data files include information on base data categories, such as transportation, 

hypsography, hydrography, and boundaries. The second type, called a Digital Elevation Model (DEM), consists 

of a sampled array of elevations for a number of ground positions at regularly spaced intervals. The third type is 

Land Use and Land Cover digital data which provides information on nine major classes of land use such as 

urban, agricultural, or forest as well as associated map data such as political units and Federal land ownership. 

The fourth type, the Geographic Names Information System, provides primary information for all known places, 

features, and areas in the United States identified by a proper name. 

The digital cartographic data files from selected quadrangles currently available from ESIC include the 

following: 

 • Digital Line Graphs (DLG's) 

 - 1:24,000-scale 

 - 1:62,500-scale 

 - 1:63,360-scale 

 - 1:100,000-scale 

 - 1:2,000,000-scale 

 • Digital Elevation Models (DEM's) 

 - 7.5-minute 

 - 15-minute 

 - 30-minute 

 - 1-degree 

 • Land Use and Land Cover digital data 

 - 1:250,000- and 1:100,000-scale land use and land cover and 

 associated maps 

 - 1:250,000-scale Alaska Interim Land Cover Maps 

 • Geographic Names 

The digital data are useful for the production of cartographic products such as plotting base maps and for various 

kinds of spatial analysis. A major use of these digital cartographic/geographic data is to combine them with 

other geographically referenced data, enabling scientists to conduct automated analyses in support of various 

decision making processes. 

______________________ 

 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. Manuscript approved for publication August, 1993. 
This document describes five distinct digital elevation products that are distributed by the USGS in the standard 

DEM format: 

 1. 7.5-Minute DEM (30- x 30-m data spacing, cast on Universal Transverse Mercator (UTM) 

projection). Provides coverage in 7.5- x 7.5-minute blocks. Each product provides the same

coverage as a standard USGS 7.5-minute map series quadrangle. 

 2. 1-Degree DEM (3- x 3-arc-second data spacing). Provides coverage in 1- x 1-degree blocks. Two 

products (three in some regions of Alaska) provide the same coverage as a standard USGS 1- x 2-

degree map series quadrangle. The basic elevation model is produced by or for the National

Imagery and Mapping Agency (NIMA), but is distributed by USGS in the DEM data record format. 

3. 30-Minute DEM (2- x 2-arc-second data spacing). Consists of four 15- x 15-minute DEM blocks. 

Two 30-minute DEM's provide the same coverage as a standard USGS 30- x 60-minute map series 

quadrangle. Saleable units will be 30- x 30-minute blocks, that is, four 15- x 15-minute DEM's 

representing one half of a 1:100,000-scale map (30- x 60-minute coverage). 

4. 15-minute Alaska DEM (2- x 3-arc-second data spacing, latitude by longitude). Provides coverage 

similar to a 15-minute DEM, except that the longitudinal cell limits vary from 20 minutes at the 

southernmost latitude of Alaska to 36 minutes at the northernmost latitude limits of Alaska. 

Coverage of one DEM will generally correspond to a l:63,360-scale quadrangle. 

5. 7.5-minute Alaska DEM (1- x 2-arc-second data spacing, latitude by longitude). Provides coverage 

similar to a 7.5-minute DEM, except that the longitudinal cell limits vary from 10 minutes at the 

southernmost latitude of Alaska to 18-minutes at the northernmost latitude limits of Alaska. 

The UTM-based DEM's (7.5-minute DEM) and the geographic-based DEM's (30-minute, Alaska, and 1-degree 

DEM's) are identical in logical data structure but differ in sampling interval, geographic reference system, areas 

covered, and horizontal and vertical accuracy. Knowledge of all of these properties is essential to ensure that the 

user does not exceed the useful limits of the data for required applications. The 7.5-minute UTM DEM's are 

available for selected quadrangles, which are indicated on a status graphic published biannually by USGS. The 

1-degree DEM's are available for all of the contiguous United States, Hawaii, and portions of Alaska, Puerto

Rico, and the Virgin Islands. Many of the original 1-degree DEM's are being replaced with more accurate digital 

models through a cooperative regridding project with NIMA, scheduled for completion in 1995. As they become

available, these 1-degree DEM's will replace their corresponding product. The 30-minute and Alaska DEM's are 

new DEM series and are available on a limited basis as projects are completed. 

7.5-MINUTE DIGITAL ELEVATION MODELS 

Characteristics

A 7.5-minute DEM has the following characteristics: 

• The data consist of a regular array of elevations referenced horizontally in the UTM coordinate

system. The reference datum may be North American Datum of 1927 

2
(NAD 27), North American Datum of 1983 (NAD 83), Old Hawaiian Datum (OHD), or Puerto

Rico Datum of 1940 (PRD). 

• The unit of coverage is the 7.5-minute quadrangle. Overedge coverage is not provided. 

• The data are ordered from south to north in profiles that are ordered from west to east.

• The data are stored as profiles in which the spacing of the elevations along and between each 

profile is 30 m.

• The profiles do not always have the same number of elevations because of the variable angle 

between the quadrangle's true north and the grid north of the UTM coordinate system. 

• Elevations for the continental U.S. are either meters or feet referenced to the National Geodetic 

Vertical Datum of 1929 (NGVD 29). Elevations for Hawaii and Puerto Rico are either in meters or 

feet referenced to local mean sea level. DEM's of low-relief terrain or generated from contour maps 

with intervals of 10 ft (3 m) or less are generally recorded in feet. DEM's of moderate to high-relief 

terrain or generated from maps with terrain contour intervals greater than 10 ft are generally 

recorded in meters. 

Profiles for 7.5-minute DEM's are generated by using a UTM cartesian coordinate system as a base. The profiles 

are clipped to the straight-line intercept between the four geographic corners of the quadrangle--an 

approximation of the geographic map boundary (neatline) as shown in figure 1. The resulting area of coverage 

for the DEM is a quadrilateral, the opposite sides of which are not parallel. (See appendix D for an example of 

UTM coordinates describing a 7.5-minute quadrilateral figure.) 

The UTM coordinates of the four corners (bounds) of the DEM's are listed in the type A record, as shown in

table 1, data element 11; the UTM coordinates of the starting points of each profile are listed in the type B record 

(profiles), table 2, data element 3. These coordinates describe the shape of the quadrilateral and the variable x, y

starting position of each profile. Because of the variable orientation of the quadrilateral in relation to the UTM 

coordinate system, profiles intersect the east and west neatlines as well as the north and south neatlines as shown

in figure 1. In addition, DEM's have profile easting values that are continuous from one DEM to the adjoining 

DEM only if the adjoining DEM is contained within the same UTM zone. 

Data Production

The 7.5-minute DEM data are produced in 7.5- x 7.5-minute blocks either from map contour overlays that have 

been digitized, or from automated or manual scanning of National Aerial Photography Program (NAPP) quarter 

quad-centered photographs or from the National High-Altitude Photography Program (NHAP) quad-centered 

photographs. The NHAP program was formally discontinued in 1988, however limited production using this

scale source is permitted. The data are processed to produce a DEM having a 30-m sampling interval. The 

structure of a 7.5-minute DEM data file is shown in figure 1. (See tables for sample data records.) 

3
Figure 1.--Structure of a 

The USGS has 

used four processes 

to collect the 

digital elevation

data for production 

of 7.5-minute 

DEM's: (1) the 

Gestalt Photo 

Mapper II (GPM2), 

(2) manual 

profiling from

photogrammetric

stereomodels, (3) 

stereomodel 

digitizing of

contours, and (4) 

derivation from

DLG hypsography

and hydrography

categories or 

pseudo-DLG's 

(tagged vector 

contours) . 

The GPM2 (now 

discontinued) was 

an automated photogrammetric system designed to produce orthophotographs, digital terrain data, and contours. 

An electronic image correlation component of the GPM2 measured the parallax of 2,444 points within each 9- x 

8-mm area of a photogrammetric stereomodel. Of these 2,444 correlated points, subunits of 576, 1,024, or 1,600 

points were collected for inclusion in the elevation model. These subunits were called patches, and the patch size 

was selected to accommodate various terrain conditions. The horizontal (x and y) spacing of the elevation points 

within each patch was approximately 182 mm at photographic scale (equivalent to a ground distance of 

approximately 47 ft when using photographs at 1:80,000 scale). Each of the two NHAP stereomodels used to

cover a standard 7.5-minute quadrangle contained over 500,000 correlated points; these were regridded to form a 

DEM in the standard format. Before discontinuance, approximately 15,000 DEMs were added to the NDCDB

using this autocorrelation system. 

The manual profiling process uses stereoplotters, equipped with three-axis electronic digital profile recording 

modules, for scanning of stereomodels along successive terrain profiles. High-altitude aerial photographs are 

used as source material. The scan speed and distance between profiles are selected by the operator to 

accommodate steepness in topographic slope. The most commonly used profile separation is approximately 

90-m, with elevations normally recorded every 30 m along each profile. The profiled elevation data are 

reformatted and regridded using a weighted four-nearest-neighbor interpolation to a regular 30-m UTM spacing, 

written in standard DEM format, and tested for vertical accuracy before placement in the National Digital 

Cartographic Data Base. Digital profile data of this type are collected as companion products during the 

production of orthophotographs. 

4
For stereo model digitizing of contours (now deactivated), digital contours were acquired in digital form on 

stereoplotters equipped with three-axis digital recording modules. Digital data were acquired as the contours 

were stereocompiled for 7.5-minute quadrangle maps. The contours were assigned elevation values (attributes) 

during the acquisition phase. The contour data were processed into profile lines, and the elevation matrix was 

computed at a 30-m spacing using a bilinear interpolation.

Derivation of DEM's from DLG's or pseudo-DLG's is a process that involves the use of hardware such as scanners, manual digitizers, and (or) semiautomated line followers. The hypsography and hydrography categories of 

DLG or pseudo-DLG data are required as input to DEM processing. Contours and spot elevations in

three-dimensional coordinates are required from the hypsographic category. Lake and shoreline data are 

required from the hydrographic category for water body flattening. Drainage data are also extracted from the 

hydrography file. Elevations are computed where contours merge with the hydrographic data. Ridge lines, the 

opposite of drainage lines, are normally interpolated automatically by computer programs using trends within the 

hypsographic overlay. If necessary, the delineation of ridge lines may be accomplished by manual inputs or 

sophisticated medial axis computation algorithms. The DLG or pseudo-DLG are reformatted as tagged vector 

line strings and are input to gridding software to interpolate gridded elevations for the DEM. A subsequent

processing step involves trimming the grid along the quadrangle perimeter and formatting the data into a DEM 

file structure. The DEM is finally processed by the DEM Editing System to validate and test the accuracy before 

entry into the National Digital Cartographic Data Base. 

1-DEGREE DIGITAL ELEVATION MODELS 

Characteristics

A 1-degree DEM has the following characteristics: 

• The product consists of a regular array of elevations referenced horizontally on the geographic 

(latitude/longitude) coordinate system of the World Geodetic System 1972 Datum (WGS 72) or the 

World Geodetic System of 1984 (WGS 84). 

• The unit of coverage is a 1- x 1-degree block. Elevation data on the integer degree lines (all four 

sides) correspond with the same profiles on the surrounding eight blocks. 

• Elevations are in meters relative to NGVD 29 in the continental U.S. and local mean sea level in 

Hawaii and Puerto Rico. 

• The data are ordered from south to north in profiles that are ordered from west to east.

• Spacing of the elevations along each profile is 3 arc-seconds. The first and last data points are at 

the integer degrees of latitude. A profile will therefore contain 1,201 elevations. 

• Spacing between profiles varies by latitude; however, the first and last data points are at the integer 

degrees of longitude. North of 50° degrees N and south of 70° N, the spacing is 6 arc-seconds with

601 profiles per product. For the remainder of Alaska north of 70° N the spacing is 9 arc-seconds 

with 401 profiles per product.

5
Data Production

The majority of the 1-degree Digital Elevation Models are produced by NIMA from cartographic and 

photographic sources. Under a cooperative agreement, selected 1-degree DEM's are being regridded by USGS 

from 7.5-minute DEM's and 30-minute DEM's. These data sets will be available upon completion of quality 

control and exchange between NIMA and USGS. 

Elevation data from cartographic sources are collected from any map series 7.5 minute through 1 degree 

(1:24,000 scale through 1:250,000 scale). The hypsographic features (contours, drain lines, ridge lines, lakes, 

and spot elevations) are first digitized and then processed into the required matrix form and interval spacing. 

The structure of a 1-degree DEM data file is shown in figure 2 (see tables for sample data records). 

Figure 2.--Structure of a

Elevation data

from photographic 

sources are 

collected by using 

manual and 

automated correlation techniques. 

Elevations along a 

profile are 

collected at 80 to 

100 percent of the 

eventual point 

spacing. The raw 

elevations are 

weighted with

additional

information such as 

drain, ridge, water, 

and spot heights 

during the resampling process in which final elevations are determined for the required matrix form and interval 

spacing. 

The digital elevation models distributed within the Department of Defense cover 1- x 1-degree blocks but are 

called Digital Terrain Elevation Data Level 1 (DTED-1). These blocks are referenced by their southwest corner 

coordinates rather than by the name of a corresponding map sheet. The header records and profile records are 

considerably different in their structure than those in the corresponding USGS-distributed version; however, in

reformatting the product, the USGS does not change the basic elevation information. 

6
The accuracy of the NIMA product, together with the data spacing, adequately support computer applications 

that analyze hypsographic features to a level of detail similar to manual interpretations of information as printed

at map scales not larger than 1:250,000. The plotting of contours from the 1-degree DEM at scales larger than 

1:250,000, or reliance on the elevation heights without incorporating the National Map Accuracy Standard 

(NMAS) horizontal error tolerance, will lead to less reliable results. 

Note that NIMA 1-degree DTED-l data and USGS-distributed 1-degree DEM's are gridded by using the WGS 72 

or WGS 84 datum, which is significantly different than the NAD 27 datum used by the USGS for gridding 

7.5-minute DEM's. At the present time, policy issues concerning the transformation algorithms and procedures 

to convert to or from NAD 27 and WGS 72 to WGS 84 and (or) NAD 83 are being developed. See appendix H 

for additional information related to these datums. 

30-MINUTE DIGITAL ELEVATION MODELS 

Characteristics

A 30-minute DEM has the following characteristics: 

• The product consists of a regular array of elevations referenced horizontally to the geographic 

(latitude/longitude) coordinate system of NAD 27 or NAD 83. 

• The unit of coverage is a 30- x 30-minute block. Saleable units are four 15-minute DEM's covering 

a 30- x 30-minute area. Elevation data on the integer minute lines (all four sides) correspond to the 

same profiles on the surrounding eight blocks. 

• Elevations are in meters or feet relative to NGVD 29 in the continental U.S. and local mean sea 

level in Hawaii and Puerto Rico. 

• The data are ordered from south to north in profiles that are ordered from west to east.

• Spacing of the elevations along each profile is 2 arc-seconds. The first and last data points are at 

the integer 15 minutes of latitude. A 15-minute profile will therefore contain 451 elevations. 

Data Production

The USGS uses two processes to collect the digital elevation data for production of 30-minute DEM's: (1) 

derivation from DLG contours of any map series 7.5 minute to 30 x 60 minute (1:24,000 scale to 1:100,000 

scale), (2) resampling from digital elevation models with a source spacing equal to or less than 2-arc second 

sampling interval (if the data is resampled from preexisting DEMs, it is arbitrarily archived as level 1). DEM 

data for this series are tested according to level 2 (see Level 2, p. 14) accuracy specifications. 

The accuracy of the DEM data, together with the data spacing, adequately support computer applications that 

analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at 

map scales not larger than 1:100,000. The plotting of contours from 30-minute DEM data at scales larger than 

1:100,000, or reliance on the elevation heights without incorporating the NMAS horizontal error tolerance, will 

lead to less reliable results. 

7
15-MINUTE ALASKA DIGITAL ELEVATION MODELS 

Characteristics

15-minute Alaska DEM's have the following characteristics:

• The product consists of a regular array of elevations referenced horizontally on to the geographic 

(latitude/longitude) coordinate system of NAD 27 or NAD 83. 

• The unit of coverage corresponds to four basic quadrangle sizes for 1:63,360-scale graphics 

(depending on latitude):

Cell size limits 

15 x 36 minutes State of Alaska north of 68° N latitude

15 x 30 minutes Between 62° N and 68° N latitude

15 x 22.5 minutes Between 59° N and 62° N latitude

15 x 20 minutes State of Alaska south of 59° N latitude

• The longitudinal limits of these cells are computed east and west of the -150 degree meridian. The 

north-south cell limits conform to even multiples of 15 minutes of latitude. 

• The data are collected with a 2- x 3-arc-second spacing in latitude, and longitude, respectively. The 

first and last data points along a profile are at the integer degrees of latitude. A profile will 

therefore contain 451 elevations. 

• Elevation data on the quadrangle neatlines (all four sides) share edge profiles with the surrounding 

eight quadrangles. 

• Elevations are in meters or feet relative to NGVD 29. 

• The data are ordered from south to north in profiles that are ordered from west to east.

Data Production

The 15-minute Alaska DEM data are produced to match the spatial format of the 1:63,360-scale source contours. 

 The primary process used for production of Alaska DEM's is to combine digitized hypsographic and 

hydrographic data from 1:63,360-scale graphics. Processing can include all of the scanning, resampling, and 

contour interpolation programs previously mentioned. Data production for this series, if derived from

hypsography overlays, is classified according to level 2 specifications. 

The accuracy of the DEM data, together with the data spacing, adequately support computer applications that 

analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at 

map scales not larger than 1:63,360. The plotting of contours from the 15-minute Alaska DEM's at scales larger 

than 1:63,360, or reliance on the elevation heights without incorporating the NMAS horizontal error tolerance, 

will lead to less reliable results. 

8
7.5-MINUTE ALASKA DIGITAL ELEVATION MODELS 

 

Characteristics

7.5-minute Alaska DEM's have the following characteristics:

• The product consists of a regular array of elevations referenced horizontally to the geographic 

(latitude/longitude) coordinate system of NAD 27 or NAD 83. 

• The unit of coverage corresponds to four basic quadrangle sizes for 1:24,000- and 1:25,000-scale 

graphics (depending on latitude):

Cell size limits 

7.5 x 18 minutes State of Alaska north of 68° N latitude

7.5 x 15 minutes Between 62° N and 68° N latitude

7.5 x 11.25 minute Between 59° N and 62° N latitude

7.5 x 10 minutes State of Alaska south of 59° N latitude

• The longitudinal limits of these cells are computed east and west of the -150 degree meridian. The 

north-south cell limits conform to even multiples of 7.5 minutes of latitude. 

• The data are collected with a 1- x 2-arc-second spacing in latitude and longitude, respectively. The 

first and last data points along a profile are at the integer degrees of latitude. A profile will 

therefore contain 451 elevations. 

• Elevation data on the quadrangle neatlines (all four sides) share edge profiles with the surrounding 

eight quadrangles. 

• Elevations are in meters or feet relative to NGVD 29. 

• The data are ordered from south to north in profiles that are ordered from west to east.

Data Production

The 7.5-minute Alaska DEM data are produced to match the spatial format of the 1:24,000- and 1:25,000-scale 

source contours. The collection of elevation data is primarily by raster-to-vector digitizing of map separates and

gridding the resultant vectors. Data production for this series, if derived from hypsography overlays, is classified 

according to level 2 specifications. 

The accuracy of the DEM data, together with the data spacing, adequately support computer applications that 

analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at 

map scales not larger than 1:24,000. The plotting of the contours from the Alaska DEM's at scales larger than 

1:24,000, or reliance on the elevation heights without incorporating the NMAS horizontal error tolerance will 

lead to less reliable results. 

9
 GEOMETRY 

Profiles are the basic building blocks of DEM's and are defined as one-dimensional arrays, that is, arrays of 

dimension in m rows x 1 column, where m is the length of the profile (variable length for 7.5-minute DEM's or 

fixed length for 30-minute, Alaska, and 1-degree DEM's). 

Figure 3 provides an example of the computation for the first data point inside the quadrilateral representing a 

7.5-minute DEM west of the UTM central meridian. Figure 4 provides a similar example for a quadrangle east

of the central meridian. 

Figure 5, formula 1, illustrates the internal horizontal relationship (xp, yp) of elevations ordered as profiles in

which the spacing of the elevations along each profile is ∆y and the spacing between profiles is ∆x. Figure 5, 

formula 2, relates the internal array structure to actual ground coordinates (xgp, ygp) based on an origin of the 

DEM at the lower left corner (xgo, ygo), and a rotation angle measured from quadrangle north, if any. The 

rotation angle of 7.5-minute DEM's is normally set to zero (see record A, element 13 in table 1). The rotation 

angle for a 1-degree DEM is always set to zero (see record A, element 13, in table 4). In general the mathematics 

is simpler for a 1-degree DEM. Each 1-degree DEM profile is composed of the same number of elevations per 

profile and the array is a square or rectangle. Therefore, the equations of figure 5 are greatly simplified. 

 ACCURACY 

The accuracy of a DEM is dependent upon the level of detail of the source and the grid spacing used to sample 

that source. The primary limiting factor for the level of detail of the source is the scale of the source materials. 

The proper selection of grid spacing determines the level of content that may be extracted from a given source 

during digitization. For example, 1:250,000-scale topographic maps are the primary source of 1-degree DEM's. 

Larger scale maps, such as 1:100,000 and 1:24,000, are used to generate higher accuracy DEM's. Scales smaller 

than 1:250,000 have not been used as a DEM source. 

Another factor is the horizontal and vertical dimension of the DEM. Horizontal accuracy of DEM data depends 

on the horizontal spacing of the elevation matrix. Within a standard DEM, most terrain features are generalized 

by being reduced to grid nodes spaced at regular intersections in the horizontal plane (7.5-minute DEM interval 

is 30 m; 1-degree DEM interval is 3 arc-seconds). This generalization reduces the ability to recover positions of 

specific features less than the internal spacing during testing and results in a de facto filtering or smoothing of the 

surface during gridding. 

Vertical accuracy of DEM data depends on the spatial resolution (horizontal grid spacing), quality of the source 

data, collection and processing procedures, and digitizing systems. As with horizontal accuracy, the entire 

process, beginning with project authorization, compilation of the source data sets, and the final gridding process, 

must satisfy accuracy criteria customarily applied to each system. Each source data set must qualify to be used in 

the next step of the process. Errors are compounded in each step of the process. For this reason, significant 

effort is expended in each phase of the production process to minimize errors. 

10
Fig ure 3.--Computation of first data point in a 7.5

Elevation Model west of the  central meridian. 
Figure 4.-- Computation of first data point in a 7.5-minute 

Model east of the  central meridian. 
Figure 5.--Geometry and nom

The method of 

determining

7.5-minute DEM

accuracy involves 

computation of the 

root-meansquare error 

(RMSE) for 

linearly interpolated elevations in

the DEM and 

corresponding 

"true" elevations 

from the published 

maps. Test points 

are well 

distributed, are 

representative of 

the terrain, and

have "true" 

elevations well 

within the DEM 

accuracy criteria. 

Test points are 

located on contour 

lines, bench marks, 

or spot elevations. 

A minimum of 28 

test points per 

DEM is required 

(20 interior points 

and 8 edge points). 

 Collection of test 

point data and 

comparison of the 

DEM to the 

quadrangle hypsography are conducted by the quality-control units within the USGS. 

The broad NIMA production objective for a 1-degree DTED-1 is to satisfy an absolute horizontal accuracy 

(feature to datum) of 130 m, circular error at 90-percent probability; and an absolute vertical accuracy (feature to 

mean sea level) of ±30 m, linear error at 90-percent probability. The relative horizontal and vertical accuracy 

(feature to feature on the surface of the elevation 

13
model), although not specified, will in many cases conform to the actual hypsographic features with higher 

integrity than indicated by the absolute accuracy. 

 CLASSIFICATION LEVELS 

The following is a description of the general data characteristics used to classify DEM's into one of three levels 

of quality. There are varying methods of data collection and degrees of editing available for DEM data. 

Classification levels are indicated in the DEM record A (appendix A). 

Level 1

Level 1 DEM's are elevation data sets in a standardized format. The intent is to reserve this level for 7.5-minute 

DEM's or equivalent that are derived from scanning National High-Altitude Photography Program, National 

Aerial Photography Program, or equivalent photography. 

A vertical RMSE of 7 m is the desired accuracy standard. A RMSE of 15 m is the maximum permitted. The 

intent for 7.5-minute DEM data at this level is that an absolute elevation error tolerance of 50 m (approximately 

three times the 15-meter RMSE) be set for blunders for any grid node when compared to the true elevation, or 

that an array of points not encompass more than 49 contiguous elevations and be in error by more than 21 m

(three times the 7-m RMSE). Systematic errors within the stated accuracy standards are tolerated at this level. 

DEM data acquired photogrammetrically by using manual profiling or image correlation techniques are restricted 

to the level 1 category. DEM's with a RMSE of from 7 to 15 meters in elevation are being retained as an

intermediate product and eventually will be replaced by higher accuracy DEM's. The DEM record C (appendix 

C) contains the RMSE accuracy statistics acquired during quality control. 

A 30-minute DEM product has been produced by regridding level 1 or level 2 source 7.5-minute DEM data. 

These grid derivative DEM's are arbitrarily labeled as level 1 DEM's and carry a measured RMSE in record C. A 

tolerance has not been set for this RMSE, as a minimum level of accuracy has been satisfied previously with the 

data sets origin in the 7.5-minute DEM program. Alternatively, 30-minute DEM's are labeled as level 2 if

gridded from 1:100,000-scale hypsography and hydrography stable base separates. The tolerance for level 2, 30-

minute DEM's, is one-half contour interval (see level 2 specifications, below). 

Level 2

Level 2 DEM's are elevation data sets that have been processed or smoothed for consistency and edited to

remove identifiable systematic errors. DEM data derived from hypsographic and hydrographic data digitizing,

either photogrammetrically or from existing maps, are entered into the level 2 category after review on a DEM

Editing System. An RMSE of one-half contour interval is the maximum permitted. There are no errors greater 

than one contour interval in magnitude. The DEM record C contains the accuracy statistics acquired during 

quality control. 

Level 3

Level 3 DEM's are derived from DLG data by using selected elements from both hypsography (contours, spot

elevations) and hydrography (lakes, shorelines, drainage). If necessary, ridge lines and hypsographic effects of 

major transportation features are also included in the derivation. A RMSE of one-third of the 

14
contour interval is the maximum permitted. There are no errors greater than two-thirds contour interval in

magnitude. The DEM record C contains the accuracy statistics acquired during quality control. 

DIGITAL ELEVATION MODEL CAVEATS 

The majority of the 7.5-minute DEM's produced to date are level 1. The current priority for authorization of 

7.5-minute DEM production is now oriented to level 2, interpolation from digital contours. The USGS does not

currently produce level-3 DEM data. 

All 30-minute DEM's derived from contours are level 2. All 30-minute DEM's derived from 7.5-minute DEM's 

are level 1. 

All 1-degree NIMA DTED-l's have been classified as level 3 because the hypsographic information, when 

plotted at 1:250,000 scale is consistent with the planimetric features normally found on 1:250,000-scale 

topographic maps. Inconsistencies may exist, but these are regarded as isolated cases to be tempered by the 

90-percent confidence level for the overall product. NOTE: The USGS classification of "level 3" for 1-degree 

DEM's is not to be confused with the NIMA'S "DTED level 1." In the NIMA, the term "level" is related to the 

spatial resolution of the data and not to the source of the data. For example, DTED level 2 (which the USGS 

does not distribute) would have an array spacing of 1 x 1 arc-second for latitudes south of 50° N. 

 DATA RECORDS 

A DEM file is organized into a series of three logical records are formatted as shown in appendixes A, B, and C.

 Because of the restructuring of new elements within what was formerly defined as element 1, record type A, the 

element counts of old record type A elements 2-15 (see table 1) have been incremented by one to elements 3-16 

(see appendix A). A one-to-one correspondence still exists between the informational content of byte positions 

of elements in the old and new records. Therefore, DEM programs that expected elements 2-15 in specific byte 

positions in the DEM file will find the same information in those byte positions in the new files. In essence, this 

change is transparent to old DEM applications programs. Also, new data elements 17-29 have been appended to

the end of the record A (see appendix A). These elements are contained in the end of the previously blank filled

portion of the 1,024 byte record. This change is also expected to be transparent to existing DEM programs. 

Appendixes D-G consist of code definitions that are needed to interpret various data elements in the three 

records. The type A record contains information defining the general characteristics of the DEM, including 

descriptive header information relating to the DEM's name, boundaries, units of measurement, minimum and 

maximum data values, number of type B records, and projection parameters. There is only one type A record for 

each DEM file, and it appears as the first record in the data file. The type B record contains elevation data and 

associated header information. All type B records of the DEM files are made up of data from one-dimensional

arrays called profiles. The number of complete profiles covering the DEM area is synonymous with the number 

of type B records in the DEM. In a UTM structured DEM, an occasional profile that exists within the bounds of 

the DEM quadrilateral but is void of elevation grid points is not represented in the DEM. (This is called the 

"missing profile condition" and occurs occasionally as the first or last hypothetical profile of the DEM.) The 

type C record contains statistics on the accuracy of the data in the file. 

15
The physical structure of the DEM distributed to the user is as follows: 

• Data recorded in fixed-block format on unlabeled or ANSI-labeled 9-track magnetic tape at 1,600 

or 6,250 bpi density. 

• Logical record size of 1,024 bytes. No more than one logical record type (A, B, or C) recorded in

any 1,024-byte record. However, more than one 1,024-byte record is usually required to store a 

single record type B. The logical record is padded with blanks if necessary to fill to the end of the 

logical record. Bytes 1,021-1,024 of each logical record are padded with blanks. 

• Physical record size of 4,096 bytes; that is, 4 logical records per physical record. 

• Data written as ANSI-standard ASCII characters. 

Sample Data Records

Following are sample sets of A, B, and C records, corresponding to a typical 7.5-minute DEM (tables 1, 2, and 

3), and sample sets of A and B records for a typical 1-degree DEM (tables 4 and 5). Included in these samples 

are literal ASCII listings of records directly from DEM distribution tapes. Following the literal listings are 

tabular explanations of each element in the type A, B, and C records. The tabular explanations may be used as 

direct references between the literal listings and the logical record type formats shown in appendixes A-C. 

Appendixes D-G consist of code definitions that are needed to interpret various data elements in the three 

records. 

 SAMPLE APPLICATIONS 

The DEM files may be used in the generation of graphics such as isometric projections displaying slope, 

direction of slope, and terrain profiles between designated points. They may also be combined with other data

types such as stream locations and weather data to plan forest fire control, or with remote sensing data to classify

vegetation. Many nongraphic applications such as modeling terrain gravity data for use in locating energy

resources, determining the volume of proposed reservoirs, calculating the amount of material removed during 

strip mining, and determining landslide probability have also been developed. Figures 6 and 7 show two graphic 

applications of DEM's. 

16
Table 1.--Sample DEM Type A Logical Record--Mannboro, Virginia, 

 Quadrangle (7.5 minute) 

MANNBORO,VA Free form information field ------------|Blank Filler field -----------------------------------| 

3 EMC 2 1 1 18 0.0 0.0 0.0 0.0 0.0  0.0 0.0 0.0 0.0 0.0  0.0 0.0 0.0 0.0 0.0 2  2 4 0.244998676000000D+06 0.412627656700000D+07 0.245420930000000D+06 0.414014832600000D+07 0.2564918630013981850700000D+07 0.256087907000000D+06 0.412594681300000D+07 0.470000000000000D+02 0.114000000000000D+03  10.300000E+020.300000E+020.100000E+01 1 383 00 10163008908I5 0 2 1 1 Data Element Content Explanation  1 MANNBORO, VA Quadrangle name field. TEXT Bytes 41-80, free format textual information FILLER Bytes 81-135 reserved for future use. PROCESS 3 Indicates DEM was made from a DLG hypso overlayFILLER Byte 137, blank fill SECTIONAL INDICATOR Bytes 138 -140 specific to 30-minute DEM, left blank for 7.5-minute DEM 2 EMC Mapping Center origin code indicating that DEM was created in Eastern Mapping Cen 3 2 DEM level code indicating DEM level 2, synonymous with file created from DLG to D

software. 

 4 1 Pattern code indicating either a regular or random elevation pattern; 1 indicates a regul 5 1 Planimetric reference system code; 1 indicates UTM coordinate system.  6 18 Code defining the zone in the ground planimetric reference system (i.e., UTM zone 18)

17
Table 1.--Sample DEM Type A Logical Record--Mannboro, Virginia

 Quadrangle (7.5 minute)--continued 

Data Element Content Explanation  7 0.0 Map projection parameters; all 15 fields are set to zero for the UTM

(15 sets of 0.0) coordinate system and should be ignored. 

 8 2 Units code; 2 represents meters as the unit of measure for ground planimetric coordinat

file. 

 9 2 Units code; 2 represents meters as the unit of measure for elevation coordinates through 10 4 Number (n) of sides in the polygon which defines the coverage of the DEM file (usuall 11 0.2449986...D+06 A 4,2 array containing the ground coordinates of the four corners of the DEM.

................ Translation to decimal format yields: 244998.7, 4126276.6; 245420.9, ................ 4140148.3; 256491.9, 4139818.5; 256087.9 4125946.8. 

 12 0.47.....D+02 Minimum and maximum elevations for the DEM.  0.114....D+03  13 0.0 Counterclockwise angle (in radians) from the primary axis of ground planimetric refere

axis of the DEM local reference system. 

 14 1 Accuracy code; 1 indicates that a record of accuracy exists and is contained in record ty 15 0.3000000E+02 A three-element array containing DEM spatial resolution (x, y, z); usually set

0.3000000E+02 to 30, 30, 1. 

 0.1000000E+01  16 1 383 A two-element array containing the number of rows and columns of profiles in the DEM

is usually set to 1 as an indication that the arrays are actually one-dimensional profiles. equaling 383 indicates that the DEM file consists of a total of 383 profiles. 

18
Table 1.--Sample DEM Type A Logical Record--Mannboro, Virginia

 Quadrangle (7.5 minute)--continued 

Data Element Content Explanation 17 0 Indicates no largest contour interval. 18 0 Contour interval (set to 0 if previous data field is 0). 19 10 Smallest contour interval; indicates a standard 10-foot contour interval for the source q20 1 Contour units; 1 represents foot contours in multiples specified in the previous data fiel21 63 00 Data source date; this DEM was generated from contours which were compiled photog

 Numbers 00 indicate month is not specified. The quadrangle was photorevised in 198significant changes were made to the contours, therefore the date of 1963 takes precede

22 89 08 Data inspection date; this DEM was tested and inspected on the DES on August 1989. 23 I Inspection/revision flag; in this case, this is a new data set which has been inspected bu24 5 Data validation flag; qualitative and quantitative tests appropriate to DEM level 2 data 25 0 Suspect/void flag; 0 indicates none. 26 2 Vertical datum; 2 indicates DEM vertical datum is National Geodetic Vertical Datum 127 1 Horizontal Datum; 1 indicates DEM horizontal datum is North American Datum of 19228 1 Data edition; 1 indicates this is the first edition.29 Percent void, element 25 is 0, therefore this field is blank. 

19
Table 2.--Sample DEM Type B Logical Record--Mannboro, Virginia

 Quadrangle (7.5 minute) 

 1 4 111 1 0.245100000000000D+06 0.412629000000000D+07 0.0 0.540000000000000D+02 0.870000000000000D+02 78 79 79 80 80 79 80 7 73 77 80 80 81 81 81 81 82 83 83 83 83 83 82  84 84 84 84 85 86 86 87 87 87 87 87 87 87 86  83 82 82 82 82 82 83 84 85 85 84 83 83 82 82 75 74 75 74 74 73 71 68 68 65 64 61 60 60 61 6 57 57 57 56 56 55 55 55 55 55 54 55 55 55 55  56 57 57 57 58 58 60 61 61 61 61 61Data Element Content Explanation  1 1 4 Row and column identification number of the profile contained in this record; 1, 4 repr

4, in the DEM data set. 

 2 111 1 Number of rows (elevations) and column in this profile; 111, 1 indicates there are 111 

column in this profile. 

 3 0.245100...D+06 Translated to the decimal, 245100.0 and 4126290.0 are the ground planimetric 

0.4126290...D+07 coordinates (UTM) of the first elevation in the profile. 

 4 0.0 Elevation of local datum for the profile.  5 0.54....D+02 Minimum and maximum elevations for the profile.  0.87....D+02  6 78 79... An array of m x n elevations (m=111, n=1) for the profile expressed in units of 

............. measure of meters (record A, element 9, indicates meters as units of measure) above or 

datum (record A, element 26). 

.....61 61 

20
Table 3.--Sample DEM Type C Logical Record--Mannboro, Virginia

 Quadrangle (7.5 minute) 

 1 0 0 3 0 1 0 0 1 23 Data Element Content Explanation  1 1 Availability code; 1 indicates that an absolute accuracy estimate does exist for this file. 

 2 0 0 3 Absolute accuracy RMSE estimate (in x, y, and z); the first two zeros indicate that the x 

statistics are not available. A 3 in this field indicates that a vertical solution of 3 metercomputed for the source data control network. 

 3 0 A 0 implies that this test occurred prior to and is independent of the DEM collection pr

number of test points is irrelevant. 

 4 1 Availability code; 1 indicates that statistics are available in the next element.  5 0 0 1 RMSE of DEM data relative to datum (in x, y, and z); the first two zeros indicate that t

statistics are not available. The value 1 meaning 1-meter RMSE indicates that the DEM (z) accuracy criteria for the 7-meter data base. 

 6 23 Sample size code; 23 indicates that the relative statistic of element 5 is based on 23 ver

21
Table 4.--Sample DEM Type A Logical Records--Reno, Nevada-California, 

 Quadrangle (West Half) (1 degree) 

NJ11-01W 3 1 0 0 0.0 0.0 0.0 0.0 0.0  0.0 0.0 0.0 0.0 0.0  0.0 0.0 0.0 0.0 0.0 3  2 4 -0.428400000000000D+06 0.140400000000000D+06 -0.428400000000000D+06 0.144000000000000D+06 -0.424800000044000000000000D+06 -0.424800000000000D+06 0.140400000000000D+06 0.999000000000000D+03 0.264100000000000D+04  00.300000E+010.300000E+010.100000E+01 1 1201 Data Element Content Explanation  1 NJ11-01W Quadrangle name field (144 characters); NJ11-01W is the designation for the DEM cov

the Reno, Nevada-California, sheet. 

 2 3 DEM level code; 3 reflects processing by NIMA which includes registration to planime

appearing on the source 1-degree topographic map. 

 3 1 Pattern code; 1 indicates a regular elevation pattern.  4 0 Planimetric reference system code; 0 indicates geographic coordinate system.  5 0 Zone code; there are no zones in the geographic system. Therefore, the zone code is se 6 0.0 Map projection parameters; all 15 fields are set to zero for the geographic  (15 sets of 0.0) system and should be ignored. Presence of non-zero parameters are not related to the g

system and should also be ignored. 

 7 3 Units code; 3 represents arc-seconds as the unit of measure for ground planimetric coor

the file. 

 8 2 Units code; 2 represents meters as the unit of measure for elevation coordinates through 9 4 Number (n) of sides in the polygon which defines the coverage of the DEM file (usuall

22
Table 4.--Sample DEM Type A Logical Records--Reno, Nevada-California, 

 Quadrangle (West Half) (1 degree)--continued 

Data Element Content Explanation 10 -0.4284...D+06, A 4,2 array containing the ground coordinates of the four corners of the DEM.

0.1404...D+06 In this case translation from arc-seconds to degrees, minutes, and seconds 

............. yields: -119 00 00, 39 00 00; -119 00 00, 40 00 00; -118 00 00, 40 00 00; 

............. -118 00 00, 39 00 00. 

 -0.4248...D+06  0.1404...D+06 11 0.9990...D+03 Minimum and maximum elevations for the DEM. 0.2641...D+04 12 0.0 Counterclockwise angle from the primary axis of ground planimetric reference to the p

DEM local reference system. 

13 0 Accuracy code; 0 indicates that a record of accuracy does not exist and that no record t

14 0.3000000E+01 A three-element array containing DEM spatial resolution (x, y, z); set to

0.3000000E+01 3, 3, 1; or 3, 6, 1; or 3, 9, 1 depending on the latitude of the product. 

 0.1000000E+01 15 1 1201 A two-element array containing the number of rows and columns of profiles in the DEM

is usually set to 1 as an indication that the arrays are actually one-dimensional profiles. set to 1,201 indicates that the DEM file consists of a total of 1,201 profiles. 

23
Table 5.--Sample DEM Type B Logical Record--Reno, Nevada-California, 

 Quadrangle (West Half) (1 degree) 

 1 1 1201 1 -0.428400000000000D+06 0.140400000000000D+06 0.0 0.121200000000000D+04 0.177200000000000D+04 1538 1539 1539 1534 1529 1520 1513 1501488 1483 1479 1475 1474 1473 1472 1469 1469 1467 1466 1464 

(As per 7.5-minute DEM's, element 6 contains elevations for the NIMA DEM.

 This field always contains 1,201 data elements per record. See Data Element 6.) 

 1422 1417 1412 1405 1401 1 1394 1397 1398 1397 1398 1398 1397 1394 1390 1386 1383 1380 1377 1374 1372 1366 1367 1368 1368 1369 1369 1370 1371 1371 1371 1371 1370 1371 1372 1370 1 1376 1375 1374 1374 1372 1372 1374 1374 1374 1374 1376 1378 1381Data Element Content Explanation  1 1 1 Row and column identification number of the profile contained in this record; 1, 1 repr

1, in the DEM data set. 

 2 1201 1 Number of rows (elevations) and columns in this profile; 1201, 1 indicates there are 1,2

column in this profile. 

 3 -0.4284...D+06 Translated to the decimal, -428400.0 and 1404000.0 are the ground planimetric  0.1404...D+06 coordinates (arc-seconds) of the first elevation in the profile, thus computed equal to -1

 4 0.0 Elevation of local datum for the profile. Always zero for 1-degree DEM, the reference  5 0.1212...D+04 Minimum and maximum elevations for the profile.  0.1772...D+04  6 1538 1539... An array of m x n elevations (m=1201, n=1) for the profile expressed in units 

............. of measure of meters (record A, element 8, indicates meters as units of 

..1378 1381 measure) above or below the local datum (record a, element 4). 

24
 Figure 

6.--Preeruption 

isometric plot of 

Mount St. Helens 

7.5-minute Digital

Elevation 

 

Models generated 

from data obtained 

from July 15, 1979, 

photographs. View 

 is 

from the northeast 

at a 45° altitude angle; vertical exaggeration is 3:1.

 Figure 

7.--Posteruption 

isometric plot of 

Mount St. Helens 

7.5-minute Digital

Elevation 

 

Models generated 

from data obtained 

from September 6, 

1980, photographs. 

 

View is from the 

northeast at a 45° altitude angle; vertical exaggeration is 3:1.

25
26
 APPENDIXES 
APPENDIX A.--Digital Elevation Model Data Elements 

 Logical Record Type A 

___________________________________________________________________________________________________________________________________________ 

 Type Physical Record Format 

 Data (FORTRAN ASCII Starting Ending CommentElement Notation) Format byte byte ___________________________________________________________________________________________________________________________________________  1 File name ALPHA A40 1 40 Authorized DEM quadrangle nam

Free Format Text ALPHA A40 41 80 Free format descriptor field, conta

digital process such as digitizing iwidths, etc.

Filler --- --- 81 135 

Process Code ALPHA A1 136 Code 1=GPM

 2=Manual Profile 

 3=DLG2DEM (includes any D

CTOG or LINETRACE) 

 4=DCASS 

 Filler --- --- 137 

Sectional Indicator ALPHA A3 138 140 This code is specific to 30-minute 

sections. Formatted as XNN, wheminute, and NN is a two-digit sequ

 2 MC origin code ALPHA A4 141 144 Mapping Center origin Code. Val

MCMC, RMMC, FS, GPM2. 

 3 DEM level code INTEGER*2 I6 145 150 Code 1=DEM-1 

 2=DEM-2 3=DEM-3

 4 Code defining INTEGER*2 I6 151 156 Code 1=regular

elevation pattern 2=random is reserved for

(regular or random) future use.

28
APPENDIX A.--Digital Elevation Model Data Elements 

 Logical Record Type A--continued

___________________________________________________________________________________________________________________________________________ 

 Type Physical Record Format 

 Data (FORTRAN ASCII Starting Ending CommentElement Notation) Format byte byte ___________________________________________________________________________________________________________________________________________  5 Code defining INTEGER*2 I6 157 162 Code 0=Geographic 

ground planimetric 1=UTM

reference system 2=State plane 

For codes 3-20, see appendix F.

Code 0 represents the geographic 

30-minute, 1-degree and Alaska Dcurrent use of the UTM coordinate 

 6 Code defining INTEGER*2 I6 163 168 Codes for State plane and UTM co

zone in ground dinate zones are given in appendix

planimetric D and E for 7.5-minute DEM's. Creference system is set to zero if element 5 is also se

geographic.

 7 Map projection REAL*8 15D24.15 169 528 Definition of parameters for vario

parameters (see is given in appendix F. All 15 fieappendix F) element are set to zero and should 

UTM, or State plane coordinates a

 8 Code defining unit INTEGER*2 I6 529 534 Code 0=radians 

of measure for 1=feet ground planimetric 2=meters

coordinates through- 3=arc-seconds 

out the file Normally set to code 2 for 7.5-min

for 30-minute, 1-degree, and Alas

29
APPENDIX A.--Digital Elevation Model Data Elements 

 Logical Record Type A--continued

___________________________________________________________________________________________________________________________________________ 

 Type Physical Record Format 

 Data (FORTRAN ASCII Starting Ending CommentElement Notation) Format byte byte ___________________________________________________________________________________________________________________________________________  9 Code defining unit INTEGER*2 I6 535 540 Code 1=feet 

of measure for 2=meters elevation coordinates Normally code 2, meters, for throughout the file 7.5-minute, 30-minute, 1-degree, a

 10 Number (n) of sides INTEGER*2 I6 541 546 n=4.

in the polygon which defines the coverage 

of the DEM file 

 11 A 4,2 array con- REAL*8 4(2D24.15) 547 738 The coordinates of the quadrangle 

taining the ground corners are ordered in a clockwise coordinates of the direction beginning with the four corners for southwest corner. The array is the DEM stored row-wise as pairs of easting

 12 A two-element array REAL*8 2D24.15 739 786 The values are in the unit of 

containing minimum measure given by data element 9 i

and maximum eleva- this record. tions for the DEM 

 13 Counterclockwise REAL*8 D24.15 787 810 See figure 5. Set to zero to align 

angle (in radians) with the coordinate system specififrom the primary in element 5. 

axis of ground 

 planimetric reference to the primary axis of the DEM  local reference  system

30
APPENDIX A.--Digital Elevation Model Data Elements 

 Logical Record Type A--continued

___________________________________________________________________________________________________________________________________________ 

 Type Physical Record Format 

 Data (FORTRAN ASCII Starting Ending CommentElement Notation) Format byte byte ___________________________________________________________________________________________________________________________________________  14 Accuracy code for INTEGER*2 I6 811 816 Code 0=unknown accuracy

elevations 1=accuracy information is giv

 in logical record type C. 

 15 A three-element array REAL*4 3E12.6 817 852 These elements are usua

of DEM spatial resolu- 7.5-minute DEM's; 2, 2, 1 for 30-mtion for x, y, z. Units of 3, 3, 1 for 1-degree DEM's.measure are consistent 2, 1, 1 for high resolution DEM's with those indicated by data 3, 2, 1 for low resolution DEM's in 

elements 8 and 9 in this 7.5-minute DEM'S will e

record to geographics, i.e., 1,1,1.

 16 A two-element array INTEGER*2 2I6 853 864 When the row value m is set to 1 t

containing the num- n value describes the number of ber of rows and columns in the DEM file. Raw GPcolumns (m,n) of data files are set to m=16, n=16.

profiles in the DEM 

Note: Old format stops here--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 17 Largest primary INTEGER*2 I5 865 869 Present only if two or more

contour interval primary intervals exist. 

 18 Source contour INTEGER*1 I1 870 Corresponds to the units of the ma

interval units largest primary contour interval 

 0=N.A., 1=feet, 2=meters. 

 19 Smallest primary INTEGER*2 I5 871 875 Smallest or only primary contour i 20 Source contour INTEGER*1 I1 876 Corresponds to the units of the ma

interval units smallest primary contour interval. 

31
 1=feet, 2=meters.

32
APPENDIX A.--Digital Elevation Model Data Elements 

 Logical Record Type A--continued

___________________________________________________________________________________________________________________________________________ 

 Type Physical Record Format 

 Data (FORTRAN ASCII Starting Ending CommentElement Notation) Format byte byte ___________________________________________________________________________________________________________________________________________  21 Data source date INTEGER*2 I4 877 880 YYMM two-digit year and two-di

MM = 00 for source having year o

 22 Data inspection/ INTEGER*2 I4 881 884 YYMM two-digit year and two-di revision date  23 Inspection/ ALPHA*1 A1 885 "I" or "R"  revision flag  24 Data validation INTEGER*1 I1 886 0= No validation performed. flag

1=TESDEM (record C added) no 

 tative test (no DEM Edit System [DES] review). 

2=Water body edit and TESDEM 

3=DES (includes water edit) no  qualitative test (no TESDEM). 4=DES with record C added, qual tive and quantitative tests for  level 1 DEM. 5=DES and TESDEM qualitative 

 quantitative tests for levels 2 

 and 3 DEM's.

 25 Suspect and void INTEGER*1 I2 887 888 0=none  area flag 

 1=suspect areas 

33
 2=void areas 

3=suspect and void areas 

34
APPENDIX A.--Digital Elevation Model Data Elements 

 Logical Record Type A--continued

___________________________________________________________________________________________________________________________________________ 

 Type Physical Record Format 

 Data (FORTRAN ASCII Starting Ending CommentElement Notation) Format byte byte ___________________________________________________________________________________________________________________________________________  26 Vertical datum INTEGER*1 I2 889 890 1=local mean sea level 

 2=National Geodetic Vertical 

Datum 1929 (NGVD 29) 

 3=North American Vertical 

Datum 1988 (NAVD 88) 

 27 Horizontal datum INTEGER*1 I2 891 892 1=North American Datum 1927 (N

2=World Geodetic System 1972 (W

 3=WGS 84  4=NAD 83  5=Old Hawaii Datum 6=Puerto Rico Datum

7=NAD 83 Provisional (shifts in  horizontal coordinates are 

 computed, but old DEM nodes  are not resampled)

 28 Data Edition INTEGER*2 I4 893 896 01-99 Primarily a NIMA specific  29 Percent Void INTEGER*2 I4 897 900 If element 25 indicates a void, this  field (right justified) contains the  percentage of nodes in the file set  to void (-32,767).___________________________________________________________________________________________________________________________________________ 

35
APPENDIX B.--Digital Elevation Model Data Elements 

 Logical Record Type B 

___________________________________________________________________________________________________________________________________________ 

 Type Physical Record Format 

 Data (FORTRAN ASCII Starting Ending CommentElement Notation) Format byte byte ___________________________________________________________________________________________________________________________________________  1 A two-element array INTEGER*2 2I6 1 12 See figure 5. The row/column

containing the row numbers may range from 1 to m anand column identifi- 1 to n. The row number is normal

cation number of the set to 1. The column identificatio

DEM profile con- is the profile sequence number.tained in this 

 record  2 A two-element array INTEGER*2 2I6 13 24 See figure 5. The first element in 

containing the number corresponds to the number of row

(m, n,) of elevations of nodes in this profile. The secon

in the DEM profile element is set to 1, 

specifying 

 3 A two-element array REAL*8 2D24.15 25 72 See figure 5. containing the  ground planimetric  coordinates (X

go

,Y

go

) of

the first elevation in 

 the profile  4 Elevation of local REAL*8 D24.15 73 96 The values are in the units of 

datum for the measure given by data element 9,

profile logical record type A.

36
APPENDIX B.--Digital Elevation Model Data Elements 

 Logical Record Type B--continued

___________________________________________________________________________________________________________________________________________ 

 Type Physical Record Format 

 Data (FORTRAN ASCII Starting Ending CommentElement Notation) Format byte byte ___________________________________________________________________________________________________________________________________________  5 A two-element array REAL*8 2D24.15 97 144 The values are in the unit of 

of minimum and measure given by data element 9 i

maximum elevations logical record type A. for the profile 

 6 A m,n array of INTEGER*2 mn(I6) 6x(146 or 170) See data element 15 in appendix A

elevations for the 146 = max for A value in this array would be 

profile. Elevations first block. 170 = max multiplied by the spatial are expressed in for subsequent blocks value and added to the elunits of resolution local elevation datum for the elem

record) to obtain the elevation for coordinates of the point X

gp,

 Y

gp

the formulas in figure 5.

___________________________________________________________________________________________________________________________________________ 

37
APPENDIX C.--Digital Elevation Model Data Elements 

 Logical Record Type C 

___________________________________________________________________________________________________________________________________________ 

 Type Physical Record Format 

 Data (FORTRAN ASCII Starting Ending CommentElement Notation) Format byte byte ___________________________________________________________________________________________________________________________________________  1 Code indicating INTEGER*2 I6 1 6 Code 1=available 

availability of 0=unavailable 

statistics in data 

 element 2  2 RMSE of file's datum INTEGER*2 3I6 7 24 In same units as indicated by

relative to absolute elements 8 and 9 of logical record

datum (x, y, z) type A. 

 3 Sample size on which INTEGER*3 I6 25 30 If 0, then accuracy will be assume

statistics in data to be estimated rather than 

element 2 are based computed. 

 4 Code indicating INTEGER*2 I6 31 36 Code 1=available 

availability of 0=unavailable statistics in data 

 element 5  5 RMSE of DEM data INTEGER*2 3I6 37 54 In same units as indicated by

relative to file's elements 8 and 9 of logical record datum (x, y, z) type A. 

 6 Sample size on INTEGER*2 I6 55 60 If 0, then accuracy will be assume

which statistics in to be estimated rather than 

element 5 are based computed. 

___________________________________________________________________________________________________________________________________________ 

38
APPENDIX D.--Sample Quadrilateral Coordinates 

Geographic Coordinates UTM Coordinates

Quad Corner 

 No. Latitude Longitude Easting Northing

 1 35°30' 107°37’30" 261897 3931463 

 2 35°37’30" 107°37’30" 262267 3945330 

 3 35°37’30" 107°30’ 273590 3945036 

 4 35°30’ 107°30’ 273238 3931169 

38
APPENDIX E.--Codes for State Plane Coordinate Zones 

Alabama, East (AL) 0101 Michigan, South (Lambert) 2113 

Alabama, West 0102 Michigan, West 2103 

Alaska (AK) 5001 Minnesota, Central (MN) 2202 

thru 5010 Minnesota, North 2201 

Arizona, Central (AZ) 0203 Minnesota, South 2203 

Arizona, East 0201 Mississippi, East (MS) 2301 

Arizona, West 0202 Mississippi, West 2302 

Arkansas, North (AR) 0301 Missouri, Central (MO) 2402 

Arkansas, South 0302 Missouri, East 2401 

California (CA) 0401 Missouri, West 2403 

thru 0407 Montana, Central (MT) 2502 

Colorado, Central (CO) 0502 Montana, North 2501 

Colorado, North 0501 Montana, South 2503 

Colorado, South 0503 Nebraska, North (NE) 2601 

Connecticut (CT) 0600 Nebraska, South 2602 

Delaware (DE) 0700 Nevada, Central (NV) 2702 

District of Columbia (DC) 1900 Nevada, East 2701 

Florida, East (FL) 0901 Nevada, West 2703 

Florida, North 0903 New Hampshire (NH) 2800 

Florida, West 0902 New Jersey (NJ) 2900 

Georgia, East (GA) 1001 New Mexico, Central (NM) 3002 

Georgia, West 1002 New Mexico, East 3001 

Hawaii (HI) 5101 New Mexico, West 3003 

thru 5105 New York, Central (NY) 3102 

Idaho, Central (ID) 1102 New York, East 3101 

Idaho, East 1101 New York, Long Island 3104 

Idaho, West 1103 New York, West 3103 

Illinois, East (IL) 1201 North Carolina (NC) 3200 

Illinois, West 1202 North Dakota, North (ND) 3301 

Indiana, East (IN) 1301 North Dakota, South 3302 

Indiana, West 1303 Ohio, North (OH) 3401 

Iowa, North (IA) 1401 Ohio, South 3402 

Iowa, South 1402 Oklahoma, North (OK) 3501 

Kansas, North (KS) 1501 Oklahoma, South 3502 

Kansas, South 1502 Oregon, North (OR) 3601 

Kentucky, North (KY) 1601 Oregon, South 3602 

Kentucky, South 1602 Pennsylvania, North (PA) 3701 

Louisiana, North (LA) 1701 Pennsylvania, South 3702 

Louisiana, Off Shore 1703 Rhode Island (RI) 3800 

Louisiana, South 1702 South Carolina, North (SC) 3901 

Maine, East (ME) 1801 South Carolina, South 3902 

Maine, West 1802 South Dakota, North (SD) 4001 

Maryland (MD) 1900 South Dakota, South 4002 

Massachusetts, Island (MA) 2002 Tennessee (TN) 4100 

Massachusetts, Mainland 2001 Texas, Central (TX) 4203 

Michigan, Central (MI) 2102 Texas, North 4201 

Michigan, Central (Lambert) 2112 Texas, North Central 4202 

Michigan, East 2101 Texas, South 4205 

Michigan, North (Lambert) 2111 Texas, South Central 4204 

39
APPENDIX E.--Codes for State Plane Coordinate Zones--continued 

Utah, Central (UT) 4302 Wisconsin, South 4803 

Utah, North 4301 Wyoming, Zone I, East (WY) 4901 

Utah, South 4303 Wyoming, Zone II, East

Vermont (VT) 4400 Central 4902 

Virginia, North (VA) 4501 Wyoming, Zone III, West

Virginia, South 4502 Central 4903 

Washington, North (WA) 4601 Puerto Rico (PR) 5301 

Washington, South 4602 Virgin Islands, St. Croix (VI) 5202 

West Virginia, North (WV) 4701 Virgin Islands, St. John, 

West Virginia, South 4702 St. Thomas 5201 

Wisconsin, Central (WI) 4802 American Samoa (AS) 5300 

Wisconsin, North 4702 Guam (GU) 5400 

________________ 

source: Federal Information Processing Standard (FIPS) Publication 70-1, 1986 

40
APPENDIX F.--Universal Transverse Mercator Zone Locations 

 and Central Meridians 

Zone C.M. Range Zone C.M. Range 

01 177W 180W-174W 31 003E 000E-006E 

02 171W 174W-168W 32 009E 006E-012E 

03 165W 168W-162W 33 015E 012E-018E 

04 159W 162W-156W 34 021E 018E-024E 

05 153W 156W-150W 35 027E 024E-030E 

06 147W 150W-144W 36 033E 030E-036E 

07 141W 144W-138W 37 039E 036E-042E 

08 135W 138W-132W 38 045E 042E-048E 

09 129W 132W-126W 39 051E 048E-054E 

10 123W 126W-120W 40 057E 054E-060E 

11 117W 120W-114W 41 063E 060E-066E 

12 111W 114W-108W 42 069E 066E-072E 

13 105W 108W-102W 43 075E 072E-078E 

14 099W 102W-096W 44 081E 078E-084E 

15 093W 096W-090W 45 087E 084E-090E 

16 087W 090W-084W 46 093E 090E-096E 

17 081W 084W-078W 47 099E 096E-102E 

18 075W 078W-072W 48 105E 102E-108E 

19 069W 072W-066W 49 111E 108E-114E 

20 063W 066W-060W 50 117E 114E-120E 

21 057W 060W-054W 51 123E 120E-126E 

22 051W 054W-048W 52 129E 126E-132E 

23 045W 048W-042W 53 135E 132E-138E 

24 039W 042W-036W 54 138E 138E-144E 

25 033W 036W-030W 55 147E 144E-150E 

26 027W 030W-024W 56 153E 150E-162E 

27 021W 024W-018W 57 159E 156E-162E 

28 015W 018W-012W 58 165E 162E-168E 

29 009W 012W-006W 59 171E 168E-174E 

30 003W 006W-000E 60 177E 174E-180W

41
APPENDIX G.--Parameters Required for Definition of Map Projections 

___________________________________________________________________________________________________________________________________________ 

 (00)* (01)** (02) (03) (04)

Parameter Geographic Universal State Albers Lambert 

 Transverse Plane Conical Conformal 

Mercator (UTM) Equal Area ___________________________________________________________________________________________________________________________________________ 1 *** Longitude of any point *** Semimajor axis of ellipsoid. If this field 

within the zone is left blank (=0), the value for Clarke's 

1866 spheroid in meters will be assumed. 

2 *** Latitude of any point *** Eccentricity squared of ellipsoid (e2). 

within the UTM zone If field is zero, this will indicate a sphere. If the field 

as containing the semiminor axis of the ellipsoid. 

3 *** *** *** Latitude of 1st standard parallel 4 *** *** *** Latitude of 2d standard parallel 5 *** *** *** Longitude of central meridian 6 *** *** *** Latitude of projection's origin 7 *** *** *** False easting in the same units of measure as the sem8 *** *** *** False northing in the same units of measure as the se9-15 (not used for projections on this page) ___________________________________________________________________________________________________________________________________________ * Projection code number.** For the Northern Hemisphere, supplying UTM zone will result in ignoring any given projection parameters. *** Parameter is not applicable to projection.

42
Note: All angles (latitudes, longitudes, or azimuth) are required in degrees, minutes, and arc-seconds in the packed real number format 

 +DDDMMSS.SSSSS. 

43
APPENDIX G.--Parameters Required for Definition of Map Projections--continued 

___________________________________________________________________________________________________________________________________________ 

 (05) (06) (07) (08)

Parameter Mercator Polar Polyconic Equidistant Conic 

Stereographic Type A 

___________________________________________________________________________________________________________________________________________ 1 Semimajor axis of ellipsoid. 

If this field is left blank (=0), the value for Clarke's 1866 spheroid in meters will be assumed. 

2 Eccentricity squared of ellipsoid (e). 

If field this is left blank (=0), this will indicate a sphere. 

If the field is 1, this field will be interpreted as containing the semiminor axis of the ellipsoid. 

3 *** *** *** Latitude of 

standard parallel 

4 *** *** *** *** 5 Longitude Longitude directed . . . . . . Longitude of central meridian . . . . . .  of Central straight down below  Meridian pole of map6 *** Latitude of true scale . . . . . . Latitude of projection's origin . . . . . 7 . . . . . False easting in the same units of measure as the semimajor axis of ellipsoid . . . . . . 8 . . . . . False northing in the same units of measure as the semimajor axis of ellipsoid . . . . . 

9 *** *** *** Zero 10-15 (not used for projections on this page)________________________________________________________________________________________________________________________________________

44
___ 

45
APPENDIX G.--Parameters Required for Definition of Map Projections--continued 

 (09) (10) (11) (12) (13) (14)

Parameter Transverse Stereographic Lambert Azimuthal Azimuthal Gnomonic Orthographic 

 Mercator Equal-Area Equidistant 

1 Same as . . . . . . . . . . . . . . . .Radius of the sphere of reference . . . . . . . . . . 

projections If this field is left blank, the value 03 thru 08 6370997.0 meters will be assumed 

2 Same as *** *** ***  projections 

03 thru 08 

3 Scale factor *** *** ***  at central  meridian4 *** *** *** *** 5 Longitude of . . . . . . . . . . . . . . . . . . . . . .Longitude of center of projection . . . . . . . . . . . .  central  meridian6 Latitude . . . . . . . . . . . . . . . . . . . . . Latitude of center of projection . . . . . . . . . . .  of origin 7 False easting in the same units of measure as the semimajor axis or radius of the sphere8 False northing in the same units of measure as the semimajor axis or radius of the sphere9-15 (not used for projections on this page)

46
APPENDIX G.--Parameters Required for Definition of Map Projections--continued 

___________________________________________________________________________________________________________________________________________ 

 (15) (16) (17) (18) (19)

Parameter General Vertical Sinusoidal Equirectangular Miller Cylindrical Van Der Grint

Near-Side Perspective (Plate Caree) 

___________________________________________________________________________________________________________________________________________ 1 . . . . . . . . . . . . . . . . . . . . . Radius of the sphere of reference . . . . . . . . . . . . . . . 

 If this field is left blank, the value 

 6370997.0 meters will be assumed. 

2 *** *** *** *** 3 Height of perspective *** *** *** 

point above sphere

4 *** *** *** *** 5 Longitude of center of . . . . . . . . . Longitude of central meridian . . . . . . . . . . .  projection 6 Latitude of center of *** *** ***  projection 7 . . . . . . . . False easting in the same units of measure as radius of the sphere . . . . . . . 8 . . . . . . . . False northing in the same units of measure as radius of the sphere . . . . . . 9-15 (not used for projections on this page)___________________________________________________________________________________________________________________________________________ 

47
APPENDIX G.--Parameters Required for Definition of Map Projections--continued 

 (20) (20)Parameter Oblique Mercator Parameter Oblique Mercator

 (Definition Format A) (Definition Format B) (Definition Format A) (Definit1 Same as for projections 03 thru 09 9 Longitude of first *** 

point defining central 

2 Same as for projections 03 thru 09 geodetic line of 

projection 

3 Scale factor at the center of projection 

10 Latitude of first 

4 *** Angle of azimuth east point defining central 

of north for central geodetic line of 

line of projection projection 

5 *** Longitude of point 11 Longitude of second 

along central line of point defining central projection at which geodetic line of angle of azimuth is projection 

measured 

6 Latitude of origin of projection 12 Latitude of second 

point defining central 

7 Same as for projections 03 thru 19 geodetic line of 

projection 

8 Same as for projections 03 thru 19 13 Zero Any non-zero 

14 and 15 (not used for this 

projection)

48
49
APPENDIX H.--National and International Datums Used for 

Digital Elevation Data 

Two types of horizontal datums are presently in use for DEM data distributed by the USGS, the civilian North

American Datum (NAD) and military World Geodetic System (WGS). The NAD 27 datum is currently used to define 

positions on USGS topographic maps and 7.5-minute DEM's. Plans are to convert to the new NAD 83 for these 

applications. The WGS 72 is currently used to define positions for 1-degree NIMA DEM's and DTED's. NIMA is

converting these data to the new WGS 84. The NAD 83 and WGS 84 datums are being phased into the mapping 

community at different rates or where resources are available. For the conterminous United States these new datums

are considered to be functionally the same; however, the two have been defined separately since they were designed to

serve different segments of the mapping community, primarily civilian and military. The following information will 

help clarify the relationship between these datums. 

The Role of the Ellipsoid in Defining Datums

Unlike local surveys, which treat the Earth as a plane, the precise determination of the latitude and longitude of points 

over a broad area must take into account the actual shape of the Earth. To achieve the precision necessary for very 

accurate location, the Earth cannot simply be assumed to be a sphere. Rather, the Earth's shape more closely 

approximates an ellipsoid (oblate spheroid): flattened at the poles and bulging at the Equator. Thus the Earth's shape, 

when cut through its polar axis, approximates an ellipse. 

Geodetic surveying, which takes into account variations in the shape of the Earth, is based on a reference ellipsoid to 

the geoid, the actual shape of the Earth, that is selected as a best fit over a limited area. The ellipsoid used to define a 

datum is a mathematical surface upon which computation of position can be based, as opposed to the actual surface of 

the Earth on which surveys are conducted. The geoid, which approximates the sea level surface, is an equipotential 

surface of the Earth's gravity field. It can be thought of as a continuous sea-level surface extended beneath the 

continents. It is the "level" surface of reference for astronomic observations and geodetic leveling, but because of 

undulations that respond to the Earth's mass distributions, it is not a useful computational surface for horizontal 

surveys. 

Horizontal Surveys -- Conversions

 NAD 27 

The NAD 27 is defined with an initial point at Meades Ranch, Kansas, and by the parameters of the Clarke 1866 

ellipsoid. The location of features on USGS topographic maps, including the definition of 7.5-minute quadrangle 

corners, are referenced to the NAD 27. 

 NAD 83 

Using recent measurements with modern geodetic, gravimetric, astrodynamic, and astronomic instruments, the 

Geodetic Reference System 1980 (GRS 80) ellipsoid has been defined as a best fit to the worldwide geoid. Unlike 

NAD 27, which is based on an initial point (Meades Ranch, Kansas), NAD 83 is an Earth-centered datum and uses the 

GRS 80 ellipsoid. Because the NAD 83 surface deviates from the NAD 27 surface, the position of a point based on the 

two reference datums will be different. 

50
APPENDIX H.--National and International Datums Used for 

Digital Elevation Data--continued

 WGS 72 

The definition of NIMA DEM's, as presently stored in the USGS data base, references the WGS 72 datum. Like NAD 

83, WGS 72 is an Earth-centered datum. The WGS 72 datum was the result of an extensive effort extending over 

approximately three years to collect selected satellite, surface gravity, and astrogeodetic data available through 1972. 

The combination of the data was performed using a unified WGS solution (a large-scale least squares adjustment). 

Such an adjustment was made possible in part because of the availability of adequate computers and software. 

 WGS 84 

The WGS 84 datum was developed as a replacement for WGS 72 by the military mapping community as a result of 

newer, more accurate instrumentation and more comprehensive control networks. It is an improvement over WGS 72 

in several respects. New and more extensive data sets and improved software were used in the development. A more 

extensive file of Doppler-derived station coordinates was available and for many more local geodetic systems;

improved sets of ground-based Doppler and laser satellite-tracking data and surface gravity were available; and geoid 

heights were deduced from satellite radar altimetry (a new data type) for oceanic regions between 70° north and south 

latitude (approximately). This system is described in "World Geodetic System 1984," Department of Defense NIMA

TR 8350.2, September 1987. 

NIMA has recomputed the 1-degree DTED's for the contiguous United States and has made a copy of the data set

available to the USGS. 

NAD 27 - NAD 83 

The methods available for transformation from NAD 27 to NAD 83 can result in inconsistencies. Therefore, a single 

method of conversion has been adopted by the USGS. The method involves the use of 7.5-minute grid intersection 

tables developed by the National Ocean Service and the software program NADCON, which is available as an 

interactive PC-based program or adapted to batch processing on PC or mainframe computers. Bilinear interpolation of

the shifts derived for the four quadrangle corners results in a uniform horizontal translation of values which are applied 

to all points interior to and including the edges of the quadrangle. 

WGS 72 - WGS 84 

Table H-1 contains information on converting WGS 72 coordinates to WGS 84. There are no NIMA plans to develop 

WGS 72 coordinates of improved accuracy. However, if WGS 84 coordinates have been determined, the WGS 

72-to-WGS 84 formulation in table H-1 can be reversed and used with the WGS 84 coordinates to obtain improved 

WGS 72 coordinates. 

51
APPENDIX H.--National and International Datums Used for 

Digital Elevation Data--continued

DEM Datum Identification

The datum applicable to a given DEM data set can generally be determined by the following criteria: 

All USGS DEM's lacking the new record A, elements 16-29, are NAD 27. 

All NIMA DEM's and DTED's lacking datum descriptors are WGS 72. 

All DEM's having the new record A elements have datums as indicated in record A, 

 element 28. 

52
APPENDIX H.--National and International Datums Used for 

Digital Elevation Data--continued

Table H1.--Difference Between WGS 84 and WGS 72 Geodetic Coordinates* 

_______________________________________________________________________________________________

_ 

 Difference (Meters) 

 Degrees Latitude Longitude Height 

_______________________________________________________________________________________________

_ 

 90 N 0.0 0.0 4.1 

 85 0.4 1.5 4.1 

 80 0.8 3.0 4.0 

 75 1.3 4.4 3.9 

 70 1.7 5.9 3.8 

 65 2.1 7.2 3.6 

 60 2.4 8.6 3.4 

 55 2.8 9.8 3.2 

 50 3.1 11.0 3.0 

 45 3.4 12.1 2.7 

 40 3.6 13.1 2.4 

 35 3.9 14.0 2.0 

 30 4.1 14.8 1.7 

 25 4.2 15.5 1.3 

 20 4.4 16.1 1.0 

 15 4.4 16.5 0.6 

 10 4.5 16.5 0.2 

 5 N 4.5 17.1 -0.2 

 0 4.5 17.1 -0.6 

 5 S 4.4 17.1 -1.0 

 10 4.4 16.9 -1.4 

 15 4.2 16.5 -1.8 

 20 4.1 16.1 -2.1 

 25 3.9 15.5 -2.5 

 30 3.7 14.8 -2.8 

 35 3.5 14.0 -3.1 

 40 3.3 13.1 -3.4 

 45 3.0 12.1 -3.7 

 50 2.7 11.0 -3.9 

 55 2.4 9.8 -4.2 

 60 2.1 8.6 -4.3 

 65 1.7 7.2 -4.5 

 70 1.4 5.9 -4.7 

 75 1.1 4.4 -4.8 

 80 0.7 3.0 -4.8 

 85 0.4 1.5 -4.9 

 90 S 0.0 0.0 -4.9 

*Applies only when proceeding directly from WGS 72 coordinates to WGS 84 coordinates; does not contain the effect

of the WGS 84 Earth gravitational model and geoid, nor the effect of local geodetic system-to-WGS 84 datum shifts

being better than local geodetic system-to-WGS 72 datum shifts. 

53