1:250,000 USGS DEM

The following document was taken from http://edc.usgs.gov/Webglis/glisbin/guide.pl/glis/hyper/guide/usgs_dem.

For information on the dataset on this Adirondack Park CD-ROM called 'dem250k' refer to the sections of this document under the heading 'Small Scale' which discuss the 1-degree (30 arc-second), or 1:250000-scale DEM data.

The data was downloaded from USGS, merged and clipped to the vicinity of the Adirondack Park and projected in ArcInfo GRID from UTM meters, Zone 18, NAD27 to NAD83.



Background

The USGS Digital Elevation Model (DEM) data files are digital representations of cartographic information in a raster form. DEMs consist of a sampled array of elevations for a number of ground positions at regularly spaced intervals. These digital cartographic/geographic data files are produced by the U.S. Geological Survey (USGS) as part of the National Mapping Program and are sold in 7.5-minute, 15-minute, 2-arc-second (also known as 30-minute), and 1-degree units. The 7.5- and 15-minute DEMs are included in the large scale category while 2-arc-second DEMs fall within the intermediate scale category and 1-degree DEMs fall within the small scale category.

Large scale
The DEM data for 7.5-minute units correspond to the USGS 1:24,000 and 1:25,000 scale topographic quadrangle map series for all of the United States and its territories. Each 7.5-minute DEM is based on 30- by 30-meter data spacing with the Universal Transverse Mercator (UTM) projection. Each 7.5- by 7.5-minute block provides the same coverage as the standard USGS 7.5-minute map series.

The 7.5-minute Alaska DEM data correspond to the USGS 1:24,000 and 1:25,000 scale topographic quadrangle map series of Alaska by unit size. The unit sizes in Alaska vary depending on the latitudinal location of the unit. The 7.5-minute Alaska DEM data consist of a regular array of elevations referenced horizontally to the geographic (latitude/longitude) coordinate system of the North American 1927 Datum (NAD 27) or the North American 1983 Datum (NAD 83). The spacing between elevations along profiles is 1 arc second in latitude by 2 arc seconds of longitude.

IMAGE Syrup Creek, Idaho DEM (44.8KB)

The 15-minute DEM data correspond to the USGS 1:63,360 scale topographic quadrangle map series of Alaska by unit size. The unit sizes in Alaska vary depending on the latitudinal location of the unit. The 15-minute DEM data consist of a regular array of elevation referenced horizontally to the geographic (latitude/longitude) coordinate system of NAD 27. The spacing between elevations along profiles is 2 arc seconds of latitude by 3 arc seconds of longitude.

IMAGE Seward, Alaska DEM (58.8KB)

Intermediate scale
The 2-arc-second DEM data cover 30-minute by 30-minute areas which correspond to the east half or west half of the USGS 30- by 60-minute topographic quadrangle map series for the conterminous United States and Hawaii. Each 2-arc-second unit is produced and distributed as four 15- by 15-minute cells. The spacing of elevations along and between each profile is 2 arc seconds.

IMAGE Sheridan, Wyoming DEM (80.9KB)

Small scale
The 1-degree DEM (3- by 3-arc-second data spacing) provides coverage in 1- by 1-degree blocks for all of the contiguous United States, Hawaii, and most of Alaska. The basic elevation model is produced by or for the Defense Mapping Agency (DMA), but is distributed by the USGS, in DEM data record format. In reformatting the product, the USGS does not change the basic elevation information. The 1-degree DEMs are also referred to as 3-arc-second or 1:250,000 scale DEM data.

IMAGE Yellowstone DEM (31.8KB)

The EROS Data Center (EDC) also concatenated the 1- by 1-degree blocks for the contiguous United States in the Land Analysis System (LAS) environment using the elevation data from the photographic sources. This is referred to as the 1-degree DEM mosaic data set. Nine strips of concatenated imagery comprise the data set.

Extent of Coverage

Large scale
The UTM-based 7.5-minute DEM data are available for much of the contiguous United States, Hawaii, and Puerto Rico. Data availability for the contiguous United States may be viewed on a status coverage map. The geographic-based 15-minute Alaska DEM data are available for most of Alaska.

The 7.5-minute DEM data are available for portions of Alaska.

Intermediate scale
The 2-arc-second DEM data are available for portions of the contiguous United States and Hawaii.

Small scale
The 1-degree DEM data are available for all of the contiguous United States, Hawaii, and most of Alaska. The concatenated images (1-degree mosaic data) cover only the contiguous United States.

Acquisition

Processing Steps

Large scale
The 7.5-minute DEM data are produced in 7.5- by 7.5-minute blocks either from digitized cartographic map contour overlays or from scanned National Aerial Photography Program (NAPP) photographs.

Four processes have been used to generate DEM data for 7.5-minute units. The first three processes have been discontinued:

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 DEMs 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. If derived from hypsography overlays, data production for this series is classified according to Level 2 specifications.

Intermediate scale
The USGS uses two processes to collect the digital elevation data for production of 2-arc-second DEMs: (1) derivation from DLG contours of any map series 7.5 minutes to 30 by 60 minutes (1:24,000 scale to 1:100,000 scale); and (2) resampling from digital elevation models with a source spacing equal to or less than a 2-arc-second sampling interval (if the data are resampled from preexisting DEMs, they are arbitrarily archived as Level 1). The DEM data for this series are tested according to Level 2 accuracy specifications.

Small scale
The majority of the 1-degree DEMs are produced by DMA from cartographic and photographic sources. Elevation data from cartographic sources are collected from any map series 7.5 minutes through 1 degree (1:24,000 scale through 1:250,000 scale). The topographic features (e.g., contours, drain lines, ridge lines, lakes, and spot elevations) are first digitized and then processed into the required matrix form and interval spacing.

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 drainage, ridge, water, and spot heights during the interpolation process in which final elevations are determined for the required matrix form and interval spacing.

The DEMs distributed within the Department of Defense cover 1- by 1-degree blocks and are called Digital Terrain Elevation Data Level 1 (DTED-1). In reformatting the DMA product to create the DEMs, the USGS restructures the header records and data but does not change the basic elevation information.

The DMA 1-degree DTED-1 data and the USGS-distributed 1-degree DEMs are gridded by using the World Geodetic System 1972 (WGS 72) or World Geodetic System 1984 (WGS 84). All 1-degree DEMs were converted to WGS 84.

The EDC concatenated the 1- by 1-degree blocks for the contiguous United States in the LAS environment using the elevation data from the photographic sources. The starting line and sample of each 1- by 1-degree image was calculated from the projection coordinates, contained in the data descriptor record, to form nine strips of data. The coordinates of each strip, along with their number of lines (rows) and samples (columns), are: 
   strip      coordinate range               line/sample

     1       50N 125W / 31N 113W             22801/14401
     2       50N 117W / 31N 106W             22801/13201
     3       50N 110W / 28N 100W             26401/12001
     4       50N 104W / 25N  95W             30001/10801
     5       50N  99W / 25N  90W             30001/10801
     6       50N  94W / 29N  83W             25201/13201
     7       50N  87W / 24N  79W             31201/ 9601
     8       50N  83W / 24N  75W             31201/ 9601
     9       50N  79W / 33N  66W             20401/15601

Data Characteristics

Large scale
Each 7.5-minute unit of DEM coverage (based on the 7.5-minute quadrangle) consists of a regular array of elevations referenced horizontally in the UTM projection coordinate system. Elevation units are in meters or feet relative to National Geodetic Vertical Datum of 1929 (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.

These horizontally referenced data may be NAD 27, NAD 83, Old Hawaiian Datum (OHD), or Puerto Rico Datum (PRD) of 1940.

The 7.5-minute Alaska DEMs consist of a regular array of elevations referenced horizontally to the geographic (latitude/longitude) coordinate system of NAD 27 or NAD 83. The data are ordered from south to north in profiles that are ordered from west to east. The unit of coverage corresponds to four basic quadrangle sizes for 1:63,360 scale graphics (depending on latitude):

Cell size limits

7.5 by 18 minutes -- State of Alaska north of 68°N latitude
7.5 by 15 minutes -- Between 62°N and 68°N latitude
7.5 by 11.25 minutes -- Between 59°N and 62°N latitude
7.5 by 10 minutes -- State of Alaska south of 59°N latitude

The 15-minute Alaska DEMs consist of a regular array of elevations referenced horizontally to the geographic (latitude/longitude) coordinate system of NAD 27 or NAD 83. The data are ordered from south to north in profiles that are ordered from west to east. Elevation units are in meters or feet relative to NGVD 29. The unit of coverage corresponds to four basic quadrangle sizes for 1:63,360-scale graphics (depending on latitude):

Cell size limits

15 by 36 minutes -- State of Alaska north of 68°N latitude
15 by 30 minutes -- Between 62°N and 68°N latitude
15 by 2.5 minutes -- Between 59°N and 62°N latitude
15 by 20 minutes -- State of Alaska south of 59°N latitude

Intermediate scale
The 2-arc-second DEM data consist 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- by 30-minute block. Saleable units are four 15-minute DEMs covering a 30- by 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.

Small scale
The 1-degree DEM consists of a regular array of elevations referenced horizontally on the geographic (latitude/longitude) coordinate system of the WGS 72 (converted to WGS 84). The information content is approximately equivalent to that which can be derived from contour information represented on 1:250,000 scale maps.The unit of coverage is a 1- by 1-degree block. Elevation data on the integer degree lines (all four sides) overlap with the corresponding 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. DEM accuracy information is provided in the Appendix.

The 1-degree DEM mosaic data set is characteristically the same as the source 1- by 1-degree DEM unit of coverage.

Spatial Resolution

Large scale
The 7.5-minute DEM data are stored as profiles in which the spacing of the elevations along and between each profile is 30 meters. The number of elevations in a profile will vary because of the variable angle between the quadrangle's geographic boundary (neatline) and the UTM coordinate system. DEM data of low-relief terrain or generated from contour maps with intervals of 10 feet or less are recorded in feet while DEM data of moderate to high-relief terrain or generated from maps with terrain contour intervals greater than 10 feet are generally recorded in meters.

IMAGE UTM Meter Grid for 7.5 DEM

The 15-minute DEM data are collected with a 2- by 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. Elevation data on the quadrangle neatlines (all four sides) share edge profiles with the surrounding eight quadrangles.

Intermediate scale
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.

Small scale
Spacing of elevations along and between each profile of 1-degree DEMs is 3 arc seconds with 1,201 elevations per profile. The exception is DEM data in Alaska, where the profile spacing varies depending on the latitudinal location of the DEM. Latitudes between 50 and 70 degrees north have spacings at 6 arc seconds with 601 profiles per 1-degree unit and latitudes greater than 70 degrees north have spacings at 9 arc seconds with 401 profiles per 1-degree unit.

The 1-degree mosaic data set spacing of elevation and profile data is the same as the data of conterminous U.S. 1-degree DEM data set.

IMAGE Arc Second Grid for 1-degree DEM

DEM Accuracy

The method of determining DEM accuracy involves computation of the root-mean-square error (RMSE) for linearly interpolated elevations in the DEM and corresponding true elevations from the published maps. Test points are well distributed and representative of the terrain. Collection of test point data and comparison of the DEM to the quadrangle hypsography are conducted by USGS quality-control groups.

Large scale
The vertical accuracy of 7.5-minute DEMs is equal to or better than 15 meters. A minimum of 28 test points per DEM is required (20 interior points and 8 edge points). The accuracy of the 7.5-minute 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 scale.

Accuracy of the 15-minute DEMs is equal to or better than one-half of a contour interval of the 15-minute topographic quadrangle map. The accuracy of the 15-minute 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 scale. The plotting of contours from the 15-minute Alaska DEMs at scales larger than 1:63,360, or reliance on the elevation heights without incorporating the National Map Accuracy Standard (NMAS) horizontal error tolerance, will lead to less reliable results.

Intermediate scale
The 2-arc-second DEM accuracy is equal to or better than one-half of a contour interval of the 30- by 60-minute topographic quadrangle map. The accuracy of the 2-arc-second 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 scale. The plotting of contours from 2-arc-second 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.

Small scale
The 1-degree mosaic data set spacing of elevation and profile data is the same as the 1-degree DEM contiguous U.S. data. The accuracy of the 1-degree 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 1:250,000 scale. 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 NMAS horizontal error tolerance, will lead to less reliable results.

For general information on accuracy, see Horizontal Positional Accuracy and Vertical Positional Accuracy.

Data Organization

A DEM file is organized into three logical records:

Examples of a DEM record format may be found in the Appendix.


Entity and Attribute Information

A digital elevation model is composed of integer values representing a gridded form of a topographic map hypsography overlay.

Additional information on DEM specifications can be found in the USGS National Mapping Program's Standards for Digital Elevation Model (DEMs) and in the USGS publication titled "Digital Elevation Models -- Data Users Guide 5."


Classification Levels

DEM data are organized in three classification levels. Level-1 DEMs are elevation data sets in a standardized format. The intent is to reserve this level for 7.5-minute DEMs which are created by scanning National High Altitude Photography (NHAP)/NAPP photography. A vertical RMSE of 7 meters is the desired accuracy standard. A RMSE of 15 meters is the maximum permitted.

Level-2 DEMs are elevation data sets that have been processed or smoothed for consistency and edited to remove identifiable systematic errors. DEM data derived from hyposographic and hydrographic data digitizing, either photogrammetrically or from existing maps, are entered into the Level-2 category. A RMSE of one-half contour interval is the maximum permitted.

Level-3 DEMs are derived from DLG data by incorporating selected elements from both hypsography (contours, spot elevations) and hydrography (lakes, shorelines, drainage). A RMSE of one-third of the contour interval is the maximum permitted.


Digital Elevation Model Caveats

Large scale
The majority of the 7.5-minute DEMs produced to date are categorized as Level-1 DEMs.

Intermediate scale
All 2-arc-second DEMs derived from contours are Level 2. All 2-arc-second DEMs derived for 7.5-minute DEMs are Level 1.

Small scale
All 1-degree DMA DTED-1 data have been classified as Level 3.

A more detailed description of the DEM accuracy and classification levels may be found in the USGS publication titled "Digital Elevation Models -- Data Users Guide 5."

Data Availability

Procedures for Obtaining Data

To place orders and to obtain additional information regarding technical details and pricing schedules, contact:

Customer Services, EROS Data Center
Earth Science Information Centers (ESICs)

A Data Users Guide is provided with each mailed order. Data Users Guides and Technical Instructions also may be ordered separately for a nominal fee.

Large and Intermediate scale
The data are available now (at the cost of reproduction) through a semi-anonymous FTP account at the EDC, on 3480 magnetic tape, 8mm cartridges, or CD-Recordable. The data also are available in SDTS format through anonymous FTP at no cost. (only SDTS format)

SDTS FTP Instructions

To access the account for the 7.5 MIN DEM data:

Small scale
The data are available at no charge through an anonymous File Transfer Protocol (FTP) account at the EDC.

FTP Instructions

To access the account:

The files are also available sorted by state, or can be selected from an index map. The data are available on 3480 magnetic tape, 8mm cartridges, or CD-Recordable. The mosaic data set is available as a set of three 8mm cartridges in raster, compressed LAS format.

Online requests for these data can be placed via the USGS Global Land Information System (GLIS) interactive query system. The GLIS system contains metadata and online samples of Earth science data. With GLIS, you may review metadata, determine product availability, and place online requests for products.

The 1-degree DEM files are written as ANSI-standard ASCII characters in fixed-block format. The 1-degree DEM mosaic data set is in the LAS format. (See Data Availability).

Applications and Related Data Sets

The DEM files may be used in the generation of graphics such as isometric projections displaying slope, direction of slope (aspect), and terrain profiles between designated points. They may also be combined with other data types such as stream location data and weather data to assist in forest fire control or they may be combined with remote sensing data to aid in the classification of vegetation. Applications include: (1) modeling terrain gravity data for use in locating energy resources, (2) determining the volume of proposed reservoirs, (3) calculating the amount of material removed during strip mining, (4) determining landslide probability, and (5) developing parameters for hydrologic models.

Further information regarding additional data sets may be obtained by contacting:

ESIC

References

Large scale
U.S. Geological Survey, 1992, US GeoData, [brochure]: [Reston, Virginia], U.S. Geological Survey, 2 p. [fold out, oversized]

U.S. Geological Survey, 1992, US GeoData data users guides and associated documentation, order form: [Reston, Virginia], U.S. Geological Survey, 2 p.

U.S. Geological Survey, 1993, Digital elevation models -- data users guide 5: Reston, Virginia, U.S. Geological Survey, 48 p.

U.S. Geological Survey, 1993, US GeoData digital elevation model, factsheet: [Reston, Virginia], U.S. Geological Survey, 2 p.

U.S. Geological Survey, 1994, Cartographic technical instructions, price list: [Reston, Virginia], U.S. Geological Survey, 2 p.

U.S. Geological Survey, 1994, US GeoData available through the Internet, factsheet: [Reston, Virginia], U.S. Geological Survey, 2 p.

Intermediate scale
U.S. Geological Survey, 1993, Digital elevation models --data users guide 5: Reston, Virginia, U.S. Geological Survey, 48 p.

Small scale
U.S. Geological Survey, 1993, Digital elevation models --data users guide 5: Reston, Virginia, U.S. Geological Survey, 48 p.

Appendix

A soft copy in ASCII text format of the individual records is available at:

<URL:ftp://www-nmd.er.usgs.gov/pub/ti/DEM/demstnds/pt2_apps_dem.txt>

DEM Accuracy

General Information

The accuracy of a DEM is dependent upon its source and the spatial resolution, that is grid spacing, of the data profiles. One factor influencing DEM accuracy is source data scale and resolution. A dependency exists between the scale of the source materials and the level of grid refinement possible. The source resolution is also a factor in determining the level of content that may be extracted during digitization. For example, 1:250,000-scale topographic maps are the primary source of 1-degree DEMs.

Another factor is the horizontal and vertical dimension of the DEM. Horizontal accuracy of DEM data is dependent upon 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. 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 is dependent upon 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 have the effect of compounding for each step of the process. Production personnel are directed to account for each production step leading to the final DEM.

SDTS Format

The Spatial Data Transfer Standard (SDTS) distribution format is designed as a mechanism for the transfer of spatial data between various computer systems. The SDTS format is designed to transfer data with complete content transfer (no loss of information). Data quality reports provide complete metadata and documentation of processing. The SDTS is a FIPS standard intended to work between various platforms and various data models. There is limited software available at this time which can accommodate the SDTS-formatted data. For more in-depth information on SDTS, refer to the SDTS technical documentation (available online).