DEM from DMA DCW EROS Data Centre

To meet the needs of the geospatial data user community for regional and continental scale elevation data, the U.S. Geological Survey’s EROS Data Center (EDC) started to develop DEM’s at a horizontal grid spacing of 30 arc seconds (approximately 1 km). 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, hydrologie flow paths, and watersheds. The primary collaborators and data contributors to date are the USGS, NASA, UNEP/GRID (United Nations Environment Program / Global Resource Information Database office.), USAID, DMA, the Institute Nacional de Estadistica Geografica e Informatica (INEGI) of Mexico, and the Geographical Survey Institute of Japan.

These data were made available to the public via electronic distribution and hard media. As of July, 1996 data was made available for Africa, Antarctica, Asia, Europe, and North America. Data sets for South America, Australia, New Zealand, the islands of southeast Asia, and Greenland were under development and were scheduled for release before the end of 1996. Most are now available on the World Wide Web (WWW). The Internet address for these data is edcftp.cr.usgs.gov:/pub/data/30ASDCWDEM. The description of the creation of the DEM by the USGS follows next.

Chapter 4: Geoid determination Data Sources

Zimbabwe Topography From G ravity Stations Heights. Cl: 150m

A H O V E 1500 U 5 0 - 1200- 1050

Zimbabwe Topography From Gravity Stations Heights. Cl:I5Um

d ] ABOVIÎ 1650 d ISOO- 1650 d I3S0. 1500 1200 - 1350 lOSO- 1200 ms 000 - 1050 H 750. 900 000 - 750 450- 600 300 - 450 150. 300 BELOW ISO Q) -19 mi 27 28 29 30 31 Longitude (deg. E)

Chapter 4: Geoid determination Data Sources

The Africa 30 arc second DEM extends from 40° north to 35° south latitude, and from 20° west to 60° east longitude. The horizontal grid spacing is 30 arc seconds (0.°00833), 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 metres above mean sea level. The elevation values range from -407 to 5825m. In the DEM, ocean areas have been masked as "no data" and have been assigned a value of -9999.

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_l .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)

Each DEM is accompanied by four ancillary ASCII text files (header file, world file, sta­ tistics file, and data descriptor record).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. The topographic information of interest for generating DEMs is contained in several DCW hypsography layers. This information was converted into an elevation grid through a vector-to-raster gridding approach. The hyp­ sography, drainage, stream networks, lake shorelines, lake elevations, and coastline data were input to the ANUDEM surface gridding program developed at the Australian Na­ tional University. [Hutchinson,M.F.,1989]. ANUDEM, specifically designed for creating DEMs 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. [Larson,K.S., 1996].

Chapter 4: Geoid determination Data Sources

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 de­ rived 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 refer­ enced 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, M.F, 1996].

The DTED is a raster topographic data base with a horizontal grid spacing of 3 arc seconds (approximately 90m). DTED are available for over 65% of the global land surface. Ac­ cess 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 DCW and the ONC series are products of the Defense Mapping Agency (DMA). [Danko,D.M., 1992].

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 ir­ regular boundary to minimize the differences and smooth the transition. The blending was performed in an overlap area of 1° (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,R., 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. Prior to merging with the gener­ alized DTED, several postprocessing steps were also performed on the DCW grid.

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 ±30m linear error at the 90% confidence level [Defense Mapping Agency, 1986]. with an RMSE of 18m. If the averaged DTED are thought of as the reference data set, the RMSE of the DCW grid is 95m.

Chapter 4: Geoid determination Data Sources

The Africa 30second DEM was the basis for the second Zimbabwean DEM. The ex­ tracted and reformatted data set covered the same area of 14° South to 24° South and 24° East to 34° East.