Prototype and description of the function getelevation30() (Function of the unlock requiring group "Elevation calculations") |

getelevation30() Elevation from the 30 sec. elevation model GLOBE for a point. Prototype of the DLL function in C++ syntax (attend lower case!): extern "C" __declspec(dllimport) unsigned long __stdcall getelevation30( double nGeoLon, double nGeoLat, unsigned long lInterpol, short *nElevation); Prototype of the DLL function in Visual Objects syntax: _DLL function getelevation30(; nGeoLon as real8,; // 8 Byte nGeoLat as real8,; // 8 Byte lInterpol as logic,; // 4 Byte nElevation ref short); // 4 Byte as logic pascal:geodll32.getelevation30 // 4 Byte The function determines the ground level elevation from the 30 seconds elevation model (1 km raster) GLOBE at a point defined by geographical coordinates. 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 website of the "National Geophysical Data Center (NGDC)". You can download the data of the 30 seconds elevation model from the website 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 website 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. To be able to use the data with GeoDLL functions, first the data path to the elevation data must be make known to GeoDLL by calling the function setelev03datapath(). 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 The parameters are passed and/or returned as follows: nGeoLon Geographic longitude of the point to get the ground elevation from in [degrees] notation vggg.ggg in the Reference System WGS84. nGeoLat Geographic latitude of the point to get the ground elevation from in [degrees] notation vgg.ggg in the Reference System WGS84. lInterPol TRUE: The ground elevation is interpolated from the 4 corner points of the 30 x 30 seconds raster field containing the coordinate. The computation is exact, but relative slow. FALSE: The ground elevation corresponds to the value of the 30 x 30 seconds raster field containing the coordinate. The computation is less exact, but relative fast. nElevation Ground elevation over the Mean Sea Level in meters as result of (ref) the computation. In the presence of a water cover the value 0 is returned. If an error occurs the function returns the value 9999. returnVal In case of an error the function returns FALSE, otherwise TRUE. For geographic coordinates in [degrees] notation applies: v Optional sign for longitudes west of Greenwich and latitudes south of the equator. g degrees and decimal fraction of a degree. Unlocking: This function is a component of the unlock requiring function group "Elevation calculations". It is unlocked for unrestricted use together with the other functions of the group by passing the unlock parameters, acquired from the software distribution company, trough the function setunlockcode(). Without unlocking only a few function calls for test purposes (shareware principle) are possible. |