US GeoData Digital Elevation Models
Digital Elevation Models in Alaska. The unit sizes in Alaska
vary depending on the latitude. Units
Digital elevation model (DEM) data south of 59° N. cover 15- by 20-
are arrays of regularly spaced minute areas, those between 59° and
elevation values referenced 62° N. cover 15-by 22.5-minute
horizontally either to a Universal areas, those between 62°
Transverse Mercator (UTM) and 68° N. cover 15- by 30-minute
projection or to a geographic areas, and those north of 68° N.
coordinate system. The grid cells are cover 15- by 36-minute areas. (All
spaced at regular intervals along values are latitude-longitude,
south to north profiles that are respectively.)
ordered from west to east. The U.S.
Geological Survey (USGS) produces DEM data are produced by the
five primary types of elevation data: National Imagery and Mapping
7.5-minute DEM, 30-minute DEM, 1- Agency (NIMA) in l- by 1-degree
degree DEM, 7.5-minute Alaska units that correspond to the east or
DEM, and 15-minute Alaska DEM. west half of the USGS 1- by 2-
degree topographic quadrangle map
Similar digital elevation data, Part of a 7.5-minute DEM plot of Tumwater, Wash.
series (1:250,000-scale) for all of the
arranged not by quadrangles but as United States and its territories. In
Alaska these are west, central, and
a huge, single, seamless file, are graphic data. DEM data for 1-degree
known as National Elevation Data, or units are collected from topographic
NED, and are treated in a separate map sources, ranging from the 7.5- All nonstandard quadrangles with
fact sheet available at minute map series to the 1- by 2- neat-lines that extend beyond the
mapping.usgs.gov/mac/isb/ degree map series, or from standard unit size to accommodate
pubs/factsheets/fs14899.html. photographic sources by using image overedge boundaries are collected
correlation systems. and sold as multiples of the standard
Data Collection unit sizes.
Unit Size and File Extent
The USGS has used four methods to Data Characteristics
collect DEM data. Of these methods, DEM data for 7.5-minute units corre-
only one, interpolation from vectors spond to the USGS 7.5-minute topo- Vertical elevations are in decimal and
or digital line graph (DLG) graphic quadrangle map series for all whole units of meters and feet except
hypsographic and hydrographic data, of the United States. for the 1-degree DEM, which is
is currently used for 7.5-minute distributed in whole meters only.
DEM’s and other series DEM’s. The Data for 2-arc-second (30-minute) Elevation values for the continental
following three methods (now DEM’s provide coverage for the United States and Alaska are in
discontinued or deactivated) were conterminous United States. This reference to the National Geodetic
used primarily for the production of series is distributed as four units of Vertical Datum of 1929 (NGVD 29),
7.5-minute DEM data: (1) the Gestalt 15-minute DEM data covering full 30- with values for the islands of Hawaii,
Photo Mapper II, (2) manual profiling by 30-minute areas that correspond Puerto Rico, and Guam refer-enced
from photogrammetric stereomodels, to the east or west half of the USGS to local mean sea level. Additional
and 30- by 60-minute topographic characteristics by data type are as
(3) interpolation of the elevations quadrangle map series (1:100,000- follows:
from stereomodel digitized contours. scale).
The 7.5-minute DEM’s cast on the
DEM data for 15-minute and 2-arc- DEM data for 15-minute units UTM projection system are
second (30-minute) units are derived correspond to the USGS 15-minute referenced to either the North
from DLG hypsographic and hydro- topographic quadrangle map series American Datum of 1927 (NAD 27)
U.S. Department of the Interior USGS Fact Sheet 040-00
U.S. Geological Survey April 2000
or NAD 83. These data are stored as Data Records and identify the sales unit by
profiles with a 10- or 30-meter square topographic quadrangle name or by
grid spacing along and between each A DEM file is organized into three the southeast latitude and longitude
profile. Approximate file sizes are 9.9 logical record types: A, B, and C. The corner coordinates.
megabytes for a 10-meter resolution type A record contains information
and 1.1 megabytes for a 30-meter defining the general characteristics of The Earth Science Information Center
hori-zontal resolution. Data profiles the DEM, including its name, can furnish indexes, price lists, and
for this product do not always have boundaries, units of measurement, order forms. Data users guides are
the same number of elevations minimum and maximum elevations, included with each order.
because of the variable angle number of type B records, and
between true north and grid north in projection parameters. There is only *Except the 1- by 1-degree DEM’s
the UTM coordinate system. one type A record per DEM file. The that are available at no charge using
type B record contains profiles of FTP from
The 7.5-minute Alaska DEM data are elevation data and associated header edcwww.cr.usgs.gov/doc/edchome/
referenced horizontally to the information. There is a type B record ndcdb/ndcdb.html.
geographic (latitude/longitude) for each profile. The type C record
coordinate system of horizontal contains statistics on the accuracy of For More Information
datums NAD 27 or NAD 83. The the data.
spacing between elevations along EROS Data Center
profiles is 1 arc-second of latitude by Data Accuracy Customer Services
2 arc-seconds of longitude. Sioux Falls, SD 57198
The accuracy of DEM data depends Telephone: 605-594-6151
The 30-minute DEM data consist of on the source and resolution of the Fax: 605-594-6589
four 15-minute units of coverage that data samples. DEM data accuracy is E-mail:
are referenced horizontally to either derived by comparing linear email@example.com
NAD 27 or NAD 83. The spacing of interpolation elevations in the DEM
elevations along and between each with corresponding map location For information on these and other
profile is 2 arc-seconds. elevations and computing the USGS products and services, call 1-
statistical standard deviation or root- 888-ASK-USGS, use the ASK.USGS
The 15-minute Alaska DEM data are mean-square error (RMSE). The fax service, which is available 24
referenced horizontally to NAD 27 or RMSE is used to describe the DEM hours a day at 703-648-4888, or visit
NAD 83. The spacing between accuracy. For 7.5-minute DEM’s the general interest publications Web
elevations along profiles is 2 arc- derived from a photo-grammetric site on mapping, geography, and
seconds of latitude by 3 arc-seconds source, 90 percent have a vertical related topics at
of longitude. Each profile has 451 accuracy of 7-meter RMSE or better mapping.usgs.gov/mac/isb/pubs/
elevations. and 10 percent are in the 8- to 15- pubslists/index.html.
meter range. For 7.5- and 15-minute
The 1-degree DEM data are DEM’s derived from vector or DLG
referenced geographically with the Please visit the USGS home page at
hypsographic and hydrographic
horizontal datum of the World www.usgs.gov.
source data, an RMSE of one-half of a
Geodetic Survey (WGS) system of contour interval or better is required.
1972. A few units are also available The address for the WebGLIS is
The 1-degree DEM data have an
using the WGS 1984 Datum. Spacing edcwww.cr.usgs.gov/webglis.
absolute accuracy of 130 meters
of the elevations along and between horizontally and 30 meters vertically.
each profile is 3 arc-seconds with
1,201 elevations per profile. The only Ordering Instructions
exception is DEM data in Alaska,
where the spacing and number of All DEM’s are available on the
elevations per profile vary depending following media: CD-Recordable,
on the latitude. Latitudes between 50° Internet “at cost” File Transfer Protocol
and 70° N. have spacings at 6 arc- (FTP) delivery,* 8- mm tape, and
seconds with 601 elevations per 3,480-cartridge tape. DEM data are
profile, and latitudes greater than 70° written as ANSI-standard ASCII
N. have spacings at 9 arc-seconds characters in fixed-block format. To
with 401 elevations per profile. order DEM data, specify the unit size,
maximum block size, and tape label,