30 arc-second DEM for Africa
Entry ID:
GNVd0188_104
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Summary
Abstract:
PLEASE NOTE: This is an updated release of the Africa 30 arc second DEM. Comments from users of this data set are welcome. Please contact Dean Gesch (gesch@dg1.cr.usgs.gov) or Sue Jenson (jenson@dg1.cr.usgs.gov). A digital elevation model (DEM) consists of a sampled array of elevations for ground positions that are normally ... spaced at regular
intervals. To meet the needs of the geospatial data user community
for regional and continental scale elevation data, the staff at the
U.S. Geological Survey's EROS Data Center (EDC) are developing DEM's
at a horizontal grid spacing of 30 arc seconds (approximately 1
kilometer). These data are being made available to the public via
electronic distribution and hard media. As of July, 1996 data are
available for Africa, Antarctica, Asia, Europe, and North America.
Data sets for South America, Australia, New Zealand, the islands of
southeast Asia, and Greenland are under development and are scheduled
for release before the end of 1996.
DEM Coverage and Characteristics
The Africa 30 arc second DEM extends from 40 degrees north to 35
degrees south latitude, and from 20 degrees west to 60 degrees east
longitude. The horizontal grid spacing is 30 arc seconds
(0.00833333333333 degrees), resulting in a DEM having dimensions of
9000 rows by 9600 columns. The horizontal coordinate system is
decimal degrees of latitude and longitude referenced to WGS84. The
vertical units represent elevation in meters above mean sea level.
The elevation values range from -407 to 5825 meters. In the DEM,
ocean areas have been masked as "no data" and have been assigned a
value of -9999.
Data Format
To facilitate electronic distribution, the DEM has been divided into
three smaller pieces that together cover the African continent. There
is no overlap among the three pieces, so the continental DEM may be
assembled by simply abutting the three files. The geographic coverage
and dimensions of the DEM files are as follows:
DEM file name Latitude extent Longitude extent
af30as_1.dem 40 N - 0 20 W - 20 E (4800 rows x 4800 columns)
af30as_2.dem 40 N - 0 20 E - 60 E (4800 rows x 4800 columns)
af30as_3.dem 0 - 35 S 5 E - 60 E (4200 rows x 6600 columns)
The data are distributed as 16-bit simple binary raster image files
(no header or trailer bytes or records). The DEM files have been
compressed using the GNU 'gzip' utility, and they 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 a file (NOTE: This option is not available
through MOSAIC). For example, to retrieve the file "af30as_1.dem.gz"
without compression just use "get af30as_1.dem". Please note that an
uncompressed file is typically many times larger than the compressed
version and therefore will take much 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 DEM is accompanied by four ancillary ASCII text files (header
file, world file, statistics file, and data descriptor record). The
ancillary files are named the same as the DEM with the file name
extension indicating the file type. The formats of the ancillary
files are described below:
Header File (.hdr)
The header file can be used for image integration in ARC/INFO,
although to do so the image file extension will have to be renamed
from .dem to .bil. Information contained within this header file can
also be used with other image display packages such as IMDISP. The
following list identifies the keywords used in the header file:
BYTEORDER byte order in which image pixel values are stored
M = Motorola byte order (high order byte first)
LAYOUT organization of the bands in the file
BIL = band interleaved by line (note: the DEM is a 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 for a DEM)
NBITS number of bits per pixel (16 for a DEM)
BANDROWBYTES number of bytes per band per row (twice the number of columns
for a 16-bit DEM)
TOTALROWBYTES total number of bytes of data per row (twice the number
of
columns for a single band 16-bit DEM)
BANDGAPBYTES the number of bytes between bands in a BSQ format image (0 for
a DEM)
NODATA value used for masking purposes (will be ignored by the ARC/INFO
image integration application)
ULXMAP longitude of the center of the upper-left pixel (decimal degrees)
ULYMAP latitude of the center of the upper-left pixel (decimal degrees)
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 4800
NCOLS 4800
NBANDS 1
NBITS 16
BANDROWBYTES 9600
TOTALROWBYTES 9600
BANDGAPBYTES 0
NODATA -9999
ULXMAP -19.99583333333333
ULYMAP 39.99583333333333
XDIM 0.00833333333333
YDIM 0.00833333333333
World File (.dmw)
The world file can be used for image-to-world transformation when
displaying an image using the ARC/INFO image integration application.
When displaying an image with the .bil file name extension in
ARC/INFO, the world file must be named with a .blw extension. The
following is an example of a world file with a description of each
record:
Example world file: Description
0.00833333333333 x dimension of a pixel (decimal degrees)
0.00000000000000 rotation term (will always be zero)
0.00000000000000 rotation term (will always be zero)
-0.00833333333333 negative y dimension of a pixel (decimal degrees)
-19.99583333333333 longitude of the center of the upper-left pixel
39.99583333333333 latitude of the center of the upper-left pixel
The Statistics File (.stx)
The statistics file contains the band number, the minimum pixel value,
the maximum pixel value, the mean, and the standard deviation. The
"no data" pixels with a value of -9999 were excluded when the
statistics were calculated.
Example statistics file:
1 -103 4059 439.0 299.3
Data Descriptor Record (.ddr)
This file has been included to allow the image to be read directly by
EDC's Land Analysis System (LAS) software package. To take advantage
of this, the image file extension will have to be renamed from .dem to
.img. This file includes most of the information contained in the
other ancillary files.
Example data descriptor record:
IMAGE NAME:af30as_1.dem
NL:4800 NS:4800 NB:1 DTYPE:INTEGER*2
LAST MODIFIED: DATE:23-Jul-96 TIME:1559:14 SYSTEM:ieee-std
PROJ. CODE:(0)GEOGRAPHIC Valid:VALID
ZONE CODE:62 Valid:VALID
DATUM CODE:8 Valid:VALID
PROJ. PARM: Valid:VALID
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:3.99958333333333E+01 -1.99958333333333E+01
URcorner:3.99958333333333E+01 1.99958333333333E+01
LLcorner:4.16666666666666E-03 -1.99958333333333E+01
LRcorner:4.16666666666666E-03 1.99958333333333E+01
PROJ. DIST:8.33333333333333E-03 8.33333333333333E-03 Valid:VALID
PROJ. UNITS:degrees Valid:VALID
INCREMENT:0.00000000000000E+00 0.00000000000000E+00 Valid:INVALID
MASTER COOR:0 0
BAND NO:1
MINIMUM:-9.99900000000000E+03 Valid:VALID
MAXIMUM:4.05900000000000E+03 Valid:VALID
DATA SOURCE:
SENSOR TYPE:
CAPT. DIRECTION:
DATE:
TIME:
Procedures for Obtaining Data
The DEM and ancillary files are available from the EROS Data Center
at: http://lpdaac.usgs.gov/gtopo30/gtopo30.html
Orders for the data on hard media can be submitted to:
LP DAAC User Services
U.S. Geological Survey
EROS Data Center
47914 252nd Street
Sioux Falls, SD 57198-0001
Tel: 605-594-6116 (7:30 am to 4:00 pm CT)
Tel: 866-573-3222 (toll free)
Fax: 605-594-6593 (24 hours)
Email: edc@eos.nasa.gov (24 hours)
DEM Development
The Africa 30 arc second DEM was developed using commercially
available geographic information system software, vector-to-raster
gridding software, and utilities developed specifically for this
project.
The DEM is based on data derived from two sources: Digital Chart of
the World (DCW) and Digital Terrain Elevation Data (DTED).
Approximately 56% of the Africa DEM is based on DTED, while the
remaining 44% is based on DCW.
DCW is a vector cartographic data set based on the 1:1,000,000-scale
Operational Navigation Chart (ONC) series, which is the largest scale
base map source with global coverage (Danko, 1992). The DCW and the
ONC series are products of the Defense Mapping Agency (DMA).
The topographic information of interest for generating DEM's is
contained in several DCW hypsography layers. The primary contour
interval on the source ONC's is 1000 feet (305 meters), and
supplemental contours with an interval of 250 feet (76 meters) are
found in areas below 1000 feet in elevation. Information from the DCW
drainage layers was also used as input to the DEM generation process;
this information included stream networks, lake shorelines, lake
elevations, and ocean coastlines.
DTED, also a DMA product, is a raster topographic data base with a
horizontal grid spacing of 3 arc seconds (approximately 90 meters).
DTED are available for over 65% of the global land surface. Access to
full resolution DTED for areas outside the United States is limited to
agencies of the U.S. government, but permission has been granted by
DMA to the EROS Data Center to use and distribute a generalized 30 arc
second version.
The 3 arc second DTED were generalized to 30 arc seconds by selecting
one elevation value to represent the area covered by 100 full
resolution grid cells (a 10 by 10 matrix). The generalization
approach emphasizes the topographic breaklines (ridges and stream
channels) as found in the full resolution data. The goal is to retain
as much of the significant topographic information as possible in the
generalized DEM.
The topographic information from the DCW was converted into an
elevation grid through a vector-to-raster gridding approach. The
hypsography, drainage, and coastline data were input to the ANUDEM
surface gridding program developed at the Australian National
University (Hutchinson, 1989). ANUDEM, specifically designed for
creating DEM's from digital contour, spot height, and stream line
data, employs an approach known as drainage enforcement to produce
raster elevation models that represent more closely the actual terrain
surface and contain fewer artifacts than those produced with more
general purpose surface interpolation routines. However, in order to
prepare and format the DCW for input to ANUDEM a significant amount of
preprocessing was required. This processing included editing and
updating the vector stream lines so that the direction of each was
oriented downstream, a requirement of ANUDEM. Further preprocessing
involved detection and correction of erroneous contour and point
elevations (Larson, 1996).
Ocean coastlines were assigned an elevation of zero for input as
contours. Also, shorelines of lakes for which the DCW included
elevations were tagged and used as contour input. Additional point
control was input into the DCW gridding process in an effort to
minimize the discrepancies along the border between the DCW and DTED.
Points derived from generalized DTED within a 1-degree buffer
surrounding the DCW areas were input to ANUDEM so that the 30 arc
second grids from the two sources would match better in the overlap
area.
The output from ANUDEM was an elevation model grid referenced in the
same horizontal coordinate system as the generalized DTED. The output
grid spacing of 30 arc seconds has been shown to be appropriate for
the information content present in the DCW hypsography layers
(Hutchinson, 1996; Shih and Chiu, 1996).
Prior to merging with the generalized DTED, several postprocessing
steps were performed on the DCW grid. Lakes for which the DCW did not
indicate an elevation were updated on the grid with the lowest grid
cell elevation found along the shoreline. Also, an artifact of the
surface interpolation process in areas just inland from the ocean
coastline was detected and corrected. In these areas ANUDEM produced
anomalous below sea level elevation values. Any grid cell that did
not coincide with the DCW polygons that denote real below sea level
areas was remapped to an elevation above sea level.
Merging of the generalized DTED and the DCW grid to produce the Africa
DEM was accomplished by mosaicing the two data sets. The generalized
DTED had the highest priority so that coverage of the source with the
greater topographic detail and accuracy was maximized. The DCW grid
filled in the areas of the African continent not covered by DTED. The
merging procedure included blending of the two data sources along the
irregular boundary to minimize the differences and smooth the
transition. The blending was performed in an overlap area of 1 degree
(120 grid cells) surrounding the DTED areas. The blending algorithm
computes a weighted average with the weights for each data source
determined on a cell-by-cell basis according to the cell's proximity
to the edges of the overlap area (Franke, 1982.) The final processing
step performed on the mosaiced and blended product involved "clipping
out" the land (as defined by the DCW coastline) and setting the ocean
areas to a constant background value.
DEM Accuracy
The absolute vertical accuracy of the Africa DEM varies by location
according to the source data. Generally, the areas derived from DTED
have a higher accuracy than those derived from the DCW. The full
resolution 3 arc second DTED have a vertical accuracy of + or - 30
meters linear error at the 90% confidence level (Defense Mapping
Agency, 1986). If the error distribution is assumed to be Gaussian
with a mean of zero, the statistical standard deviation of the errors
is equivalent to the root mean square error (RMSE). Under those
assumptions, vertical accuracy expressed as + or - 30 meters linear
error at 90% can also be described as an RMSE of 18 meters. The areas
of the Africa DEM derived from DTED retain that same level of accuracy
because through generalization an actual elevation value from one full
resolution DTED cell is chosen to represent the area of the reduced
resolution cell (although the area on the ground represented by that
one elevation value is now much larger than the area covered by the
full resolution cell).
To characterize the accuracy of the areas of the Africa DEM derived
from the DCW, the DCW grid was compared to 30 arc second DTED which
had been aggregated by averaging. By aggregating, the comparison
could be done at the 30 arc second cell size of the DCW grid.
Eliminated from the comparison were those areas of the DCW grid for
which supplemental DTED point control had been included in the
gridding process. If the averaged DTED are thought of as the
reference data set, the RMSE of the DCW grid is 95 meters. To get an
idea of the overall absolute accuracy of the DCW grid, the relative
error between the DCW and DTED can be combined with the known error of
the DTED itself in a sum of squares. The root of that sum of squares
(RSS) is 97 meters. Using the assumptions about the error
distribution cited above, an RMSE of 97 meters can be expressed as +
or - 162 meters linear error at 90% confidence. This number compares
favorably with an expected vertical accuracy (linear error at 90%) of
one-half of the primary contour interval of 1000 feet (305 meters) for
the topographic maps on which the DCW is based.
The elevation data available here are a result of the global 30
arc-second elevation data project being carried out by staff of the
USGS' EROS Data Center (EDC) in Sioux Falls, South Dakota and the
co-located United Nations Environment Program / Global Resource
Information Database office (UNEP/GRID). The broad goal of this
project is the completion of global 1 km elevation data for the land
surface and the systematic extraction of derivative information to
assemble a global data base of topographic elevation, slope, aspect,
hydrologic flow paths, and watersheds. The primary collaborators and
data contributors to date are the USGS, NASA, UNEP/GRID, USAID, DMA,
the Instituto Nacional de Estatistica Geografica e Informatica (INEGI)
of Mexico, and the Geographical Survey Institute of Japan.
References
Danko, D.M., 1992. The digital chart of the world. GeoInfo Systems,
2:29- 36.
Defense Mapping Agency, 1986. Defense Mapping Agency product
specifications for digital terrain elevation data (DTED) (2d
ed.). Defense Mapping Agency Aerospace Center, St. Louis, Missouri, 26
p.
Franke, R., 1982. Smooth interpolation of scattered data by local thin
plate splines. Computing & Mathematics with Applications, 8:273-281.
Hutchinson, M.F., 1989. A new procedure for gridding elevation and
stream line data with automatic removal of spurious pits. Journal of
Hydrology, 106:211-232.
Hutchinson, M.F., 1996. A locally adaptive approach to the
interpolation of digital elevation models. In: Proceedings, Third
International Conference/Workshop on Integrating GIS and Environmental
Modeling, Santa Fe, New Mexico, January 21-26, 1996. National Center
for Geographic Information and Analysis, Santa Barbara, California.
Larson, K.S., 1996. Error detection and correction of hypsography layers.
In: Proceedings, Sixteenth Annual ESRI User Conference, May 20-24, 1996.
Environmental Systems Research Institute, Inc., Redlands, California.
Shih, T.Y., and Chiu, Y.C., 1996. On the quality of DCW hypsographic
data, a study for Taiwan. In: Technical Papers, ASPRS/ACSM Annual
Convention and Exhibition, April 22-25, 1996, Baltimore,
Maryland. American Society for Photogrammetry and Remote Sensing,
Bethesda, Maryland, Volume III, p. 248-257.
NOTE: Any use of trade, product, or firm names is for descriptive
purposes only and does not imply endorsement by the U.S. Government.
Attached Raster(s):
Member_ID: 1
Raster Name: COVERS AN AREA OF LATITUDE 40N-0 AND LONGITUDE 20W-20E
Raster Projection: GEOGRAPHIC
Raster Resolution: 92
Number of Rows: 4800
Number of Columns: 4800
Number of Bits: 16
Raster
30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA
Attached Raster(s):
Member_ID: 2
Raster Name: COVERS AN AREA OF LATITUDE 40N-0 AND LONGITUDE 20E-60E
Raster Projection: GEOGRAPHIC
Raster Resolution: 92
Number of Rows: 4800
Number of Columns: 4800
Number of Bits: 16
Raster
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 0-35S AND LONGITUDE 5E-60E
Raster Projection: GEOGRAPHIC
Raster Resolution: 92
Number of Rows: 4200
Number of Columns: 6600
Number of Bits: 16
Raster
30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA 
Geographic Coverage
(Click for Interactive Map)
Spatial coordinates
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N: 40.0
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S: -35.0
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E: 60.0
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W: -20.0
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Temporal Coverage
Start Date:
1996-07-23
Stop Date:
1996-07-23
Access Constraints
Public
Personnel
Role:
TECHNICAL CONTACT
Phone:
(+254-20) 7624214
Fax:
(+254-20) 7624315
Email:
Johannes.Akiwumi at unep.org
Contact Address:
Head, Data and Information Management Section
Division of Early Warning and Assessment (DEWA)
United Nations Environment Programme
P. O. Box 30552
City:
Nairobi
Postal Code:
00100
Country:
Kenya
Creation and Review Dates
DIF Creation Date:
1996-11-18
Last DIF Revision Date:
2012-06-11
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