Information about the used Digital Elevation Models

GeoDLL contains functions to use Digital Elevation Models (DEM) to determine ground level elevations of arbitrary points on the earth's surface. Therefore special elevator data of the Digital Elevator Models are needed. GeoDLL supports functions to use the data of the 3 seconds elevator model (90 m raster) CGIAR and of the 30 seconds elevator model (1 km raster) GLOBE, which are free available with some restrictions. Both systems were developed by the NASA.

The Digital Elevation Model GLOBE

The 30 seconds elevation model of the NASA Shuttle Radar Topographic Mission "Global Land One-Kilometer Base Elevation (GLOBE)" as data set G.O.O.D. is free available for private and commercial purposes. An other data set B.A.D, which is balanced with the national Elevation models of many countries, can be acquired for commercial use economically priced from the GLOBE Task Team. With a license for the GeoDLL function group "Elevation calculations" you only acquired the right to use the functions to compute the elevation data. The elevation data are subject to the copyright of the "GLOBE Task Team". This organization revised and supplemented the original data of the NASA. Please consider the terms of use and the copyright conditions of the GLOBE Task Team.

The 30 seconds elevation model consists of altogether 16 files with elevation data of the whole world. Each file covers a range of 90 lines of longitude and 40 or 50 lines of latitude. The ground elevations are put down in a 30 seconds raster grid. That corresponds to a resolution of maximal 925 x 925 meters in equator proximity. In direction to the poles the grid consolidates with increasing resolution over the geographical latitude. The elevation data are horizontal georeferenced with geographic coordinates in the Reference System WGS84. Vertical the ground elevation is georeferenced as the "Mean Sea Level" in integral meters. Further information you find on the Internet site of the "National Geophysical Data Center (NGDC)".

You can download the data of the 30 seconds elevation model from the Internet site of the "National Geophysical Data Center". The current Internet address is "http://www.ngdc.noaa.gov/mgg/topo/report/". Click on the link "GLOBE Data" inside the navigation column. Activate on the new opened Internet site the link "Any or all 16 tiles". Click inside the world raster map with the right mouse button on the segments, from which you need elevation data. Select in the context menu the option "Save link as..." to copy the data to your computer. Alternatively a ftp server is available under "ftp.ngdc.noaa.gov". Register "anonymous" as UserID and your email address as password. After unpacking the ZIP compressed data they are present ready for use as binary data files.

The Digital Elevation Model CGIAR

The 3 seconds elevation model of the NASA Shuttle Radar Topographic Mission "SRTM 90m Digital Elevation Data" is free available for non commercial purposes and can be acquired for commercial use economically priced from the CGIAR. With a license for the GeoDLL function group "Elevation calculations" you only acquired the right to use the functions to compute the elevation data. The elevation data are subject to the copyright of the "CGIAR Consortium for Spatial Information". This organization revised and supplemented the original data of the NASA. Please consider the terms of use and the copyright conditions of the CGIAR.

The 3 seconds elevation model consists of altogether 1728 files with elevation data between the 60. northern latitude and the 60. southern latitude. Each file covers a range of 5 lines of longitude and 5 lines of latitude. The ground elevations are put down in a 3 seconds raster grid. That corresponds to a resolution of maximal 90 x 90 meters in equator proximity. In direction to the poles the grid consolidates with increasing resolution over the geographical latitude. The elevation data are horizontal georeferenced with geographic coordinates in the Reference System WGS84. Vertical the ground elevation is georeferenced as the "Mean Sea Level" in integral meters. Further information you find on the Internet site of the CGIAR.

You can download the data of the 3 seconds elevation model from the Internet site of the "CGIAR Consortium for Spatial information". The current Internet address is "http://srtm.csi.cgiar.org/". Click in the navigation column on the link "SRTM Data Search and Download". Activate the radio buttons "Multiple Selection" and "ArcInfo ASCII". Click inside the world raster map on the segments, from which you need elevation data. Confirm your selection with the Button "Search >>". In a new window the segments you selected are arranged for the download. Click on one of the links "Data Download (FTP)" or to "Data Download (HTTP)" for each data segment you need to copy the data to your computer.

After unpacking the ZIP compressed CGIAR data they are present as ASCII data files. With the first access of a GeoDLL function to an ASCII formatted file this is automatically converted to the substantially more efficient and more compact binary data format. The conversion to the binary format can be done alternatively with the function convelev03ascii2bin().

Differences between the elevation models GLOBE and CGIAR

Elevation model:             GLOBE              CGIAR

GeoDLL function              getelevation30()   getelevation03()

Range longitude [degrees]    -180 to 180        -180 to 180

Range latitude [degrees]     -90 to 90          -60 to 60

Resolution [sec]             30 x 30            3 x 3

Resolution [m]               925 x <=925        90 x <=90

Horizontal reference         WGS84              WGS84

Vertical reference           Mean Sea Level     Mean Sea Level

Horizontal deviation [m]     <6                 <6

Vertical deviation [m]       <10                <7

Number of files              16                 1728

File size [MB]               101/126            70

Range per file [degrees]     90 x 40/50         5 x 5