i want to make a 3d modell out of the mount everest and some other mountains. what i need therefore is a 14km² square around the peak from the mountain. so basically i use qgis to extract the area from the dowlnoaded SRTM geoTIFF (which i downloaded here http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp). qgis only needs the left upper and the right bottom corner of the areal, which i calculate with this site http://www.gpsvisualizer.com/calculators. So far, so good... now qgis cuts a rectangle!! but if i use microDEM to convert to a greyscale BMP i get a square shaped picture again!?!

the dem information of the bmp are the following:

Size: 13.92x13.95 km-------------> square (but not 14km²)

DEM size: 171x152----------------> no square

Screen Pixels: 517 x 518----------> almost square

Screen pixel size: 26.94 m

z range: 5040 to 8806 m

does anyone know what the heck is going on? i am really baffled and really dont know how to deal with this situation...


SRTM data is in geographical coordinates, if you want exactly (ish) 14 square km around everest download extra, project to a suitable projected coordinate system and then extract. The cells are not 'square' as the data is originally in arc seconds and is only nearly square at the equator and become more rectangular the closer you get to the poles.

  • thanks for the fast answer! what do you mean by "download extra"? is extra a program? and how do i project to another coordinate system? Apr 29 '14 at 12:50
  • How did you get your SRTM data originally? You may need to get more - hence the download, or lower your expectations. I believe that you can project using GDALwarp gdal.org/gdalwarp.html. Based on the cell size your area of interest is not evenly divisible so you will not get exactly 14km x 14km unless you resample (14000 % 26.94 = 18.14) you should be able to set your output cell size during the warp to a factor of 14000 (28 is good) and achieve your result to exactly 14km. Apr 29 '14 at 21:31
  • i downloaded the geoTIFFs from this link srtm.csi.cgiar.org/SELECTION/inputCoord.asp thanks! i'll try what you wrote.. Apr 29 '14 at 21:46

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