US GeoData Digital Elevation Models DEM by alicejenny


									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
                                                                                           east files.
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               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
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       
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
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                                               
                                          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                                           
                                          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,

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