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I'm struggling with different DEM data, because I don't know how to convert the height information in meters.

When I load ASTER and SRTM tifs in QGIS, it shows me the spectrum of the grayband (0 to max). All the time I thought I could just take this value and see it as the height of each Point in m. Later I compared the graybandvalues with the actual height of the objects and realised that it's terribly off (German Zugspitze 500m, Test Area Swellendam 600m, a mountain in Malaysia more than 2000m). I tried it with different SRTM and ASTER DEMs, but no one was correct.

I would be very happy, if someone had an idea, how to solve this problem.

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  • What units are indicated in the metadata? – Erica Jul 28 '14 at 15:19
  • Driver GDAL provider GTiff GeoTIFF Dataset Description D:/GIS/Maps Malaysia/SRTM/srtm_60_11/srtm_60_11.tif AREA_OR_POINT=Area Band 1 STATISTICS_MAXIMUM=3589 STATISTICS_MEAN=257.11711059807 STATISTICS_MINIMUM=-37 STATISTICS_STDDEV=347.34420343174 Dimensions X: 6001 Y: 6001 Bands: 1 Origin 115,10.0004 Pixel Size 0.000833333,-0.000833333 No Data Value -32768 Data Type Int16 - Sixteen bit signed integer Pyramid overviews Layer Spatial Reference System +proj=longlat +datum=WGS84 +no_defs – Rory Jul 28 '14 at 15:35
  • Layer Extent (layer original source projection) 114.9995839386829175,4.9995834301807909 : 120.0004172720162501,10.0004167635141243 Band Band 1 Band No 1 No Stats No stats collected yet These are all metadata. I am new with DEMs, so I don't really know what they mean. – Rory Jul 28 '14 at 15:36
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After I checked my data source and used different GIS Tools (under them 3DEM a really nice DEM Viever, that gave me the right heights), I came to the conclusion that the grayband values, that are shown by QGIS, when you drag your DEMs in it, are simply wrong and aren't the height values.

Both SRTM and ASTER DEMs store (in general) their data in meter. If you want to get the real height Information in QGIS, you have to use the statistic tools and process the data, the way you want it. Then you will realise, that SRTM data are quite nice to work with.

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