30 arc-second DEM for Asia
Entry ID:
GNVd0182_104
|
Summary
PLEASE NOTE: This is a preliminary release of the Asian 30 arcsecond
DEM. Constructive comments from users of this data set are greatly
appreciated. Please contact Kris Verdin
(kverdin@edcserver1.cr.usgs.gov) or Sue Jenson
(jenson@edcserver1.cr.usgs.gov) with your comments.
Data Set Assembly
A Digital Elevation Model (DEM) consists ... of a sampled array of
elevations for ground positions that are normally spaced at regular
intervals. The EROS Data Center's (EDC) 30 arcsecond DEM project
includes generation of 30 arcsecond DEM data for the entire world.
These data are being made available to the public via anonymous ftp as
they become available. As major geographic regions are completed, the
data, along with various derivative data sets, will be published on
CD-ROM. As of December 1995, Asia, Europe, North America, and Africa
are complete and available for distribution. The South American data
set is under development, as are Australia, Antarctica and Indonesia.
The Asian 30 arcsecond DEM was compiled from several data sources.
The primary source was a generalization of the Level 1 Digital Terrain
Elevation Data. Digital Terrain Elevation Data (DTED) is a 1 degree
by 1 degree dataset produced by the US Defense Mapping Agency (DMA)
that contains digital data in the form of a uniform matrix of terrain
elevation values for most parts of the world. It was originally
designed to provide basic quantitative data for military training,
planning and operating systems that require terrain elevation, slope
and related information. This includes applications such as modeling
the influence of terrain on radar line-of-sight, automatic height
determination, terrain modeling etc.
The EROS Data Center's Digital Elevation Modeling project, that
includes generation of 30 arc second data for the entire world,
employed the DTED-Level 1 data in developing a 30 arc second DEM of
Asia. The DTED data cover over 90% of the landmass of Asia. The
elevation data for the areas of Asia which are not covered by the DTED
data were developed using the 1:1,000,000 scale Digital Chart of the
World (DCW) mapping.
The point and contour hypsography coverages were processed into a 30
arcsecond DEM using the ANUDEM gridding software. This software
utilitized the DCW hydrography information along with the hypsograpy
to generate a hydrologically realistic DEM. This technique was used
to produce the 30 arcsecond DEM in all areas not covered by existing,
distributable DEMs.
The data generated in this fashion were merged with the existing
resampled data using various mosaicing techniques to minimize the
discontinuities between the data sources.
Spatial extent of coverage
The 30 arcsecond Asian DEM complements the previously released 30
arcsecond DEM of Europe to provide 30 arcsecond cover of the entire
Eurasian landmass. The largest part of the Asian DEM extends from 70E
to 180E longitude and 0N to 85N latitude. A small portion covering
the Chukchi Peninsula which extends across the international date line
extends from 180W to 168W longitude and 56N to 72N latitude. To fill
out the Eurasian continent, additional coverage extending from the
Middle East to India extends from 32E to 70E longitude and 12N to 35N
latitude.
Data sources and characteristics
DTED-1 has been derived from a number of cartographic and photographic
sources, and by utilizing various techniques. Level 1 within DTED
refers to the specific spatial resolution model used, which in this
case, posts every 3 arcsecond in the original data.
DTED-1 is set to the Geographic Coordinate Reference System, and
the following datum specifications:
Horizontal: World Geodetic System (WGS 84)
Vertical : Mean Sea Level (MSL)
The standard file size is a 1 degree by 1 degree geographic cell
referenced by its south-west corner coordinates. Each of the
elevation data records contain 1201 elevation values in meters along a
single meridian, and a cell can have 201 to 1201 elevation data
records depending on the latitudinal location. The spacing of
elevations is according to the following:
Latitude Spacing - Latitude/Longitude
0 - 50 N-S 3 by 3 arcseconds
50 - 70 N-S 3 by 6 arcseconds
70 - 75 N-S 3 by 9 arcseconds
75 - 80 N-S 3 by 12 arcseconds
80 - 90 N-S 3 by 18 arcseconds
The accuracy objectives for DTED-1 according to the DMA Product
Specifications are as follows:
Absolute Horizontal 130 meters at 90% circular error
Absolute Vertical +/- 30 meters at 90% linear error
Mechanics of the DEM generation
The original 3 arcsecond DTED-1 data for Asia exists on Compact Discs
(CD-ROM) that use the standard volume and file structure - ISO 9660.
These data were converted to a Grid format using Arc/Info's Grid
module. These full resolution 3 arcsecond grids were resampled to 30
arcsecond using Grid's "aggregate" function that generates a reduced
resolution version of a grid:
Grid: aggregate (, , {aggregation_type},
{expand | truncate}, {data | nodata})
The cell size of 3.000 in the original grids was aggregated using a
cell factor of 10 and median as the aggregation type. The raster
image contains representative elevation values for each pixel in
meters above MSL.
Data Distribution
In order to facilitate distribution of the data, the Asian 30
arcsecond DEM has been divided into six pieces as follows:
EXTENT
DEM Name Longitudinal Latitudinal
asia_30_dem1 70E - 125E 0N - 42.5N
asia_30_dem2 70E - 125E 42.5N - 85N
asia_30_dem3 125E - 180E 0N - 42.5N
asia_30_dem4 125E - 180E 42.5N - 85N
asia_30_dem5 180W - 168W 56N - 72N
asia_30_dem6 32E - 70E 12N - 35N
Data are distributed as compressed BIL images, 16-bit straight raster
image files in a latitude/longitude system. The image files were
compressed using the GNU 'gzip' utility and can be identified by the
.bil.gz extensions. These files must be uncompressed before use.
They can be uncompressed with the gzip utilities or if you do not have
access to gzip, the FTP server can uncompress the files as you
retrieve them. To do this, simply leave off the ".gz" extension when
retrieving the file (NOTE: This option is not available through
MOSAIC). For example, to retrieve the file "asia_30_dem1.bil.gz"
without compression just use "get asia_30_dem1.bil". Please note that
the uncompressed files are typically many times larger than the
compressed versions and therefore will take many times longer to
transmit. If you would like to obtain the gzip program, it is
available via anonymous FTP at the following sites:
prep.ai.mit.edu:/pub/gnu
wuarchive.wustl.edu:/systems/gnu
Each image file is accompaned by five ancillary files (header file,
world file, statistics file, coordinate file, and data descriptor
record). The DEM Name makes up the prefix of each file. The three
digit suffix varies depending on the file type. The formats of the
ancillary files are described below:
Header File (.hdr)
The header file can be used as input to the ARC/INFO Image Integration
application. Information contained within this file can also be used
with other image display packages such as IMDISP. The following list
identifies the keywords used in the header file (ARC/INFO User's
Guide, Image Integration, 5-4 to 5-10):
byteorder: byte order in which image pixel values are stored
M = Motorola byte order (high order byte first--Sun, HP,
etc.)
layout: organization of the bands in the file
bil = band interleaved by line (note: single band image.
This field is required for image integration in
ARC/INFO.
nrows: number of rows in the image
ncols: number of columns in the image
nbands: number of spectral bands in the image (1)
nbits: number of bits per pixel (16)
bandrowbytes: number of bytes per band per row (twice the number of columns)
totalrowbytes: total number of bytes of data per row (twice the number of
columns)
bandgapbytes the number of bytes between bands in a BSQ format image (0)
nodata value used for masking purposes (will be ignored by
the Image Integration application; it is not a standard
ARC/INFO keyword)
ulxmap: longitude of the center of the upper-left pixel
ulymap latitude of the center of the upper-left pixel
xdim x dimension of a pixel in geographic units (decimal degrees)
ydim y dimension of a pixel in geographic units (decimal degrees)
Example Header file:
BYTEORDER M
LAYOUT BIL
NROWS 5100
NCOLS 6600
NBANDS 1
NBITS 16
BANDROWBYTES 13200
TOTALROWBYTES 13200
BANDGAPBYTES 0
NODATA -9999
ULXMAP 70.00416666666667
ULYMAP 42.49583333333334
XDIM 0.0083333333334
YDIM 0.0083333333334
World File (.blw)
The world file can be used for image-to-world transformation when
displaying an image using the ARC/INFO Geographic Information System
(GIS) Image Integration routine. The following is an example of a
world file with a description of each record (ARC/INFO User's Guide,
Image Integration, 2-3 to 2-5):
Example World file: Description
0.0083333333333333 x dimension of a pixel (decimal degrees)
0.00000000000000 rotation term (will always be zero)
0.00000000000000 rotation term (will always be zero)
-0.0083333333333333 negative y-dimension of a pixel (in
decimal degrees)
70.00416666666666 longitude of the center of the upper-left
pixel
42.49583333333333 latitude of the center of the upper-left
pixel
The Statistics File (.stx)
The statistics file contains the band number, the minimum pixel value
(elevation expressed in meters above mean sea level), the maximum
pixel value, the mean, and the standard deviation.
Example Statistics File:
1 -9999 9715 -3482.2 5841.1
The Coordinate File (.tik)
The coordinate file is named after the image with a .tik extension. The
following list identifies the keywords used in the coordinate file.
ulxmap: longitude of the center of the upper-left pixel
ulymap: latitude of the center of the upper-left pixel
urxmap: longitude of the center of the upper-right-pixel
urymap: latitude of the center of the upper-right-pixel
llxmap: longitude of the center of the lower-left-pixel
llymap: latitude of the center of the lower-left-pixel
lrxmap: longitude of the center of the lower-right-pixel
lrymap: latitude of the center of the lower-right-pixel
xdim: x dimension of a pixel in geographic units (decimal degrees)
ydim: y dimension of a pixel in geographic units (decimal degrees)
Example Coordinate File:
XDIM 0.00833333333333333
YDIM 0.00833333333333333
ULXMAP 70.00416666666666666
ULYMAP 42.49583333333333333
URXMAP 124.99583333333333333
URYMAP 42.49583333333333333
LLXMAP 70.00416666666666666
LLYMAP 0.00416666666666666
LRXMAP 124.99583333333333333
LRYMAP 0.00416666666666666
The Data Descriptor Record (.ddr)
This file has been included to allow the image to be read directly by
EDC's LAS software. To take advantage of this, the image file
extension will have to be renamed from bil to img. This file contains
the information contained in the other ancillary data files.
Example Data Descriptor Record:
IMAGE NAME:asia.30.dem1;img
NL:5100 NS:6600 NB:1 DTYPE:INTEGER*2
LAST MODIFIED: DATE: TIME: SYSTEM:ieee-std
PROJ. CODE:(0)GEOGRAPHIC Valid:INVALID
ZONE CODE:0 Valid:INVALID
DATUM CODE:0 Valid:INVALID
PROJ. PARM: Valid:INVALID
A: 0.00000000000000E+00 0.00000000000000E+00 0.00000000000000E+00
B: 0.00000000000000E+00 0.00000000000000E+00 0.00000000000000E+00
C: 0.00000000000000E+00 0.00000000000000E+00 0.00000000000000E+00
D: 0.00000000000000E+00 0.00000000000000E+00 0.00000000000000E+00
E: 0.00000000000000E+00 0.00000000000000E+00 0.00000000000000E+00
CORNER COOR: Valid:VALID
ULcorner:4.24958333333333E+01 7.00041666666667E+01
URcorner:4.24958333333333E+01 1.24995833333311E+02
LLcorner:4.16666668365906E-03 7.00041666666667E+01
LRcorner:4.16666668365906E-03 1.24995833333311E+02
PROJ. DIST:8.33333333333000E-03 8.33333333333000E-03 Valid:VALID
PROJ. UNITS: Valid:INVALID
INCREMENT:0.00000000000000E+00 0.00000000000000E+00 Valid:INVALID
MASTER COOR:0 0
BAND NO:1
MINIMUM:0.00000000000000E+00 Valid:INVALID
MAXIMUM:0.00000000000000E+00 Valid:INVALID
DATA SOURCE:
SENSOR TYPE:
CAPT. DIRECTION:
DATE:
TIME:
Tape Organization
Along with access via ftp transfer, customers can receive these data
sets through a tape order. If the data were obtained through a tape
order, this README.txt file will be the first record on the tape,
followed by the five ancillary files and corresponding images. Each
ancillary file contains one record, while for the image, each row of
data is one record long.
The file list for the tape is:
README.txt File: 1 Record: 1 Record Size in Bytes: 23120
asia_30_dem1.hdr 2 2 326
.blw 3 3 222
.stx 4 4 28
.tik 5 5 390
.ddr 6 6 1849
asia_30_dem1.bil 7 7 - 5106 13200
asia_30_dem2.hdr 8 5107 325
.blw 9 5108 222
.stx 10 5109 28
.tik 11 5110 390
.ddr 12 5111 1849
asia_30_dem2.bil 13 5112 - 10211 13200
asia_30_dem3.hdr 14 10212 327
.blw 15 10213 222
.stx 16 10214 28
.tik 17 10215 390
.ddr 18 10216 1849
asia_30_dem3.bil 19 10217 - 15316 13200
asia_30_dem4.hdr 20 15317 327
.blw 21 15318 222
.stx 22 15319 28
.tik 23 15320 390
.ddr 24 15321 1849
asia_30_dem4.bil 25 15322 - 20421 13200
asia_30_dem5.hdr 26 20422 326
.blw 27 20423 222
.stx 28 20424 28
.tik 29 20425 390
.ddr 30 20426 1853
asia_30_dem5.bil 31 20427 - 22346 2880
asia_30_dem6.hdr 32 22347 326
.blw 33 22348 222
.stx 34 22349 28
.tik 35 22350 390
.ddr 36 22351 1849
asia_30_dem6.bil 37 22352 - 25111 9120
DCW Gridding Techniques
For the areas of the Asian landmass lacking DTED data, the ANUDEM
software was used to grid a DEM from the 1:1,000,000 scale DCW data
set. The map source for the DCW database is the U.S. Defense Mapping
Agency (DMA)'s Operational Navigation Chart (ONC) series. This is the
largest scale map series that provides consistent, continuous global
coverage of essential basemap features.
A brief description of the techniques used to produce a DEM from the
DCW data set is included here.
Drainage Orientation:
Initially, DCW files are stored as vector contours (hypsography) and
hydrography. Using automated stream orientation procedures, contour
line and point data are compared to the hydrographic network to
determine drainage direction. Drainage lines are oriented in the
direction of flow, so that they are ordered from the highest point to
the lowest. After the stream orientation process is complete, the
resulting output is verified and edited to ensure that all streams are
represented as flowing downhill.
DEM Generation:
Once data are pre-processed, they are used as input into the
Australian National University Digital Elevation Model (ANUDEM)
generation program developed by Michael Hutchinson, Centre for
Resources and Environmental Studies, Australian National University.
This program first reads input elevations, windows the data to the
specified map limits, then generates a grid at 30 arcsecond intervals.
Elevation data are generalized by accepting a maximum of 4 data points
per grid cell and discarding any remaining points. Contour and
hydrography line data are generalized by accepting a maximum of one
line per grid cell. The program then employs a multi-grid method that
calculates grids at successively finer resolutions until the specified
grid resolution is achieved. During this process, drainage conditions
are imposed to remove sinks where possible. Values at grid points not
occupied by data points are calculated by Gauss-Seidel iteration with
over relaxation (SOR method) subject to an appropriate roughness
penalty and ordered chain constraints. The ordered chain constraints
are obtained from the stream line, sink point, and contour line data
and through automatic drainage enforcement as calculated by the
program. Starting values for the first coarse grid resolution are
calculated from a least squares plane fit to the data points. Values
for each succeeding grid are linearly interpolated from the preceding
grid (M.F. Hutchinson, unpub. data, 1991).
Data Characteristics
Spatial Resolution
Spacing of the elevations along and between each sample is 30
arcseconds (approximately 1 km). The horizontal datum is WGS84.
Elevation values are expressed in meters above mean sea level.
Accuracy
The absolute accuracy of the DCW vector information is 2000 meters
circular error (horizontal) and + or - 650 meters linear error
(vertical) at 90-percent confidence as defined by the Defense Mapping
Agency (DMA). The grid generated from these data will be no more
accurate than this source. The accuracy for the grid has not been
measured or calculated.
Data Availability
Procedures for Obtaining Data
Data sets for Asia, Europe, North America, and Africa are available
through an Internet anonymous File Transfer Protocol (FTP) account at
the EDC. These 30 arcsecond DEMs are available at no charge.
To access this account:
1. FTP to 152.61.128.6 (edcftp.cr.usgs.gov)
2. Enter "anonymous" at the Name prompt.
3. Enter your email address at the Password prompt.
4. Change (cd) to the "pub/data/30ASDCWDEM" subdirectory.
5. The files are located under the subdirectories:
EUROPE
NORTH_AMERICA
AFRICA
ASIA
6. Files are named after the image with the following extensions:
*.bil.gz = compressed image file
*.blw = world file
*.hdr = header file
*.stx = statistics file
*.tik = coordinate file
*.ddr = data descriptor record
7. Enter "binary" to set the transfer type
8. Use get or mget to retrieve the desired files.
Orders for tape copies can be placed through the GLIS Inventory
SUMMARY RESULTS screen or by contacting the EROS Data Center,
Customer Services section, at the following address:
Customer Services
EROS Data Center
Mundt Federal Building
Sioux Falls, SD 57198
TEL: (605) 594-6151
FAX: (605) 594-6589
Customer Services can also be contacted for information about DCW DEM
data that are in progress. As additional geographic areas become
available they will be announced in the GLIS News.
Applications and Related Data Sets
Moderate resolution (100 meter to 1 kilometer) topographic data have
applications in many diverse land science fields such as geology and
geophysics, ecology, soil science, botany, and glaciology.
Topographic data are also critical to procedures used for correcting
and/or presenting remotely sensed satellite and other global data.
The 30 arc-second DEM data, produced for use in conducting large-area
studies, have been generated at a resolution which is compatible with
the Advanced Very High Resolution Radiometer (AVHRR) sensor.
References
Defense Mapping Agency, Digitizing the Future, DMA Stock No. DDIPDIGITALPAC
Defense Mapping Agency, 1992, Development of the Digital Chart of the
World: Washington, D.C., U.S. Government Printing Office.
Environmental Systems Research Institute, 1991, Image Integration ARC/INFO
User's Guide: Redlands, California, Environmental Systems Research
Institute, Inc.
Hutchinson, M.F., 1988, Calculation of hydrologically sound digital
elevation models: Proc. Third Inter. Symp. Spatial Data Handling,
Columbus, Ohio, August 17-19.
Hutchinson, M.F., 1989, A new method for gridding elevation and stream
line data with automatic removal of pits: J. Hydrol, 106, 211-232 p.
Hutchinson, M.F., 1991, A continental hydrological assessment of a new grid-
based digital elevation model of Australia: Hydrological Processes 5, 45-58 p.
Attached Raster(s):
Member_ID: 2
Raster Name: COVERS AN AREA OF LATITUDE 42.5N-85N AND LONGITUDE 70E-125E
Raster Projection: GEOGRAPHIC
Raster Resolution: 92
Number of Rows: 5100
Number of Columns: 6600
Number of Bits: 16
Raster Summary:
30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA
Attached Raster(s):
Member_ID: 3
Raster Name: COVERS AN AREA OF LATITUDE 0-42.5N AND LONGITUDE 125E-180E
Raster Projection: GEOGRAPHIC
Raster Resolution: 92
Number of Rows: 5100
Number of Columns: 6600
Number of Bits: 16
Raster Summary:
30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA
Attached Raster(s):
Member_ID: 4
Raster Name: COVERS AN AREA OF LATITUDE 42.5N-85N AND LONGITUDE 125E-180E
Raster Projection: GEOGRAPHIC
Raster Resolution: 92
Number of Rows: 5100
Number of Columns: 6600
Number of Bits: 16
Raster Summary:
30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA
Attached Raster(s):
Member_ID: 5
Raster Name: COVERS AN AREA OF LATITUDE 56N-72N AND LONGITUDE 180W-168W
Raster Projection: GEOGRAPHIC
Raster Resolution: 92
Number of Rows: 1920
Number of Columns: 1440
Number of Bits: 16
Raster Summary:
30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA
Attached Raster(s):
Member_ID: 6
Raster Name: COVERS AN AREA OF LATITUDE 12N-35N AND LONGITUDE 32E-70E
Raster Projection: GEOGRAPHIC
Raster Resolution: 92
Number of Rows: 2760
Number of Columns: 4560
Number of Bits: 16
Raster Summary:
30 ARC-SECONDDEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA
Attached Raster(s):
Member_ID: 1
Raster Name: COVERS AN AREA OF LATITUDE 0-42.5N AND LONGITUDE 70E-125E
Raster Projection: GEOGRAPHIC
Raster Resolution: 92
Number of Rows: 5100
Number of Columns: 6600
Number of Bits: 16
Raster Summary:
30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA
Geographic Coverage
Spatial coordinates
| N: 85.0 |
|
S: 0.0 |
|
E: 180.0 |
|
W: 32.0 |
Data Set Citation
Temporal Coverage
Start Date:
1996-12-01
Stop Date:
1996-12-01
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Location Keywords
Science Keywords
ISO Topic Category
Platform
Access Constraints
Public
Ancillary Keywords
Data Center
Personnel
RONALD
G.
WITT
Role:
TECHNICAL CONTACT
Phone:
(+41-22) 917-8294/5
Fax:
(+41-22) 917-8029
Email:
Ron.Witt at grid.unep.ch
Contact Address:
International Environment House
City:
Geneva
Postal Code:
CH-1219
Country:
SWITZERLAND
|
Creation and Review Dates
DIF Creation Date:
1996-12-12
Last DIF Revision Date:
2006-06-01
|
|
Links
