Prototype and description of the function getelevation03() (Function of the unlock requiring group "Elevation calculations") |

getelevation03() Elevation from the 3 sec. elevation model CGIAR for a point. Prototype of the DLL function in C++ syntax (attend lower case!): extern "C" __declspec(dllimport) unsigned long __stdcall getelevation03( double nGeoLon, double nGeoLat, unsigned long lInterpol, short *nElevation); Prototype of the DLL function in Visual Objects syntax: _DLL function getelevation03(; nGeoLon AS REAL8,; // 8 Byte nGeoLat AS REAL8,; // 8 Byte lInterpol AS LOGIC,; // 4 Byte nElevation REF SHORTINT); // 4 Byte as logic pascal:geodll32.getelevation03 // 4 Byte The function determines the ground level elevation from the 3 seconds elevation model (90 m raster) CGIAR at a point defined by geographical coordinates. 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 data they are present as ASCII 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(). 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 can take a long time up to a half minute. To go on working while the conversion is in process, the event-processing can be switched on (see below). Future accesses directly use the new produced binary formatted file. The ASCII file then can be deleted. The conversion to the binary format can be done alternatively with the function convelev03ascii2bin(). Since the function due to the extensive calculations is time-consuming, the event handling during the calculation by interrupting the processing loop can be are allowed bei calling the function seteventloop(). ASCCI files of the CGIAR versions 2 through 4 are supported. This fle formats are possible: Example of a file name in CGIAR V2 syntax: Z_38_2.ASC. Example of a file name in CGIAR V3/V4 syntax: SRTM_38_02.ASC. The file names are not upper or lower case sensitive. 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 3 x 3 seconds raster field containing the coordinate. The computation is exact, but relative slow. FALSE: The ground elevation corresponds to the value of the 3 x 3 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. |