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Is there any option to SAGA to calculate the Topographic Wetness Index ? I have a big area and I tried many times with SAGA, the last time I thought the results was ok (before SAGA would crash or give me a empty raster), however I just noticed that for a small part of my study area the TWI raster is empty.

For the last TWI created in SAGA I decided to calculate the slope also in SAGA, but I still have problems with the file. The area with empty cells is near the right margin of the raster.This is how it looks like (black cells are NODATA cells): enter image description here

I need to recalculate this asap but I can't find any another solution (in arcgis or qgis).


I've just saw that I have the same problem in other small areas of the image, that are +/- flat. After overlaying with the SRTM, I can see that when the SRTM value is negative that the TWI cells have NODATA values. I guess this is the reason for the error.

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there is also a grass-tool named r.topidx. Maybe you can try it out too. I had the same problem as you in the past (see my post in the related posts list), but i somehow managed to do it with SAGA –  Curlew Apr 28 '13 at 18:45
For the topographic wetness index in GRASS GIS, see grasswiki.osgeo.org/wiki/Hydrological_Sciences#Flow_calculation -> r.topidx. You can reach it from QGIS through the Sextante plugin (see also grasswiki.osgeo.org/wiki/GRASS_and_Sextante). BTW: GRASS can deal with large files in case you hit the 2Gb barrier on a 32bit system. –  markusN Apr 28 '13 at 18:54
An ArcGIS solution to exactly the same problem is presented at gis.stackexchange.com/questions/43276/…. –  whuber Apr 29 '13 at 14:46
thanks @whuber ! –  A.R Apr 30 '13 at 11:22
@Fezter Understood--for otherwise I would have closed this question as a duplicate and not just put the link in a comment. It was offered in the expectation that seeing one way to compute the TWI could be helpful regardless of the software one finally intends to use. –  whuber May 23 '13 at 12:34
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