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I am trying to run elevation derivative tools on a provincial DEM provided by the Ontario Government website (Ministry of Natural Resources and Forestry) and I am getting strange values when bringing the data into SAGA GIS.

The values show up like normal in ArcMap but I get an unusually extremely low minimum value (that should be 0) as displayed in the image below:

enter image description here

As displayed, I am currently trying to create a Topographic Position Index (TPI) and the output values are also unusually large. Only when I adjust the min/max values can I get a proper visualization in the display windows, but the metadata in the "Description" tab does not change. On top of that, when I save the SAGA file and bring it into ArcMap I get more strange values. Only when I rescale using the "Rescale by function" (linear transformation function) tool in ArcMap can I get a normal visualization in ArcMap, similar to when I changed the raster's min/max values in SAGA and even then I am unsure if I input the parameters correctly into the ArcMap tool.

Would anyone also know what a typical standard TPI Index value range should be? And would anyone know the proper parameter inputs for a linear transformation function via the "Rescale by function" tool in ArcMap?

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    The Min. -3400000...... value in your first image looks suspiciously close to the listed noData value. Are noData values getting mixed into your statistics causing issues?
    – Rex
    Commented Apr 21, 2017 at 22:12

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It seems SAGA has an issue with its Import Raster function to read GeoTiff tags. The strange NoData value (-3.40282e+038) is probably (-32767) in the original dataset.

As already pointed out by @Rex, this strange value was used by TPI tool as a part of the surface and seriously damaged the result.

Please modify NoData value of the imported grid to -99999, which TPI tool can correctly recognize. You can use Reclassify Grid Values tool under the menu Geoprocessing | Grid | Values |.

Reclassify Grid Values tool offers various options, but you only need to work on Special casesby

  • Tick on the no data values and put -99999 in the new value
  • NoData Output Grid Assign... user defined NoData value and give -99999.

This will return a reclassified grid with _reclassifed suffix. Re-run TPI on the new grid.

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  • What do you mean re-run the TPI on the new grid? I can't run the TPI on an already-processed TPI raster since there's no raw elevation data. And I'm pretty sure -99999 is already set as NoData without running that tool so it didn't do anything.
    – ATomz
    Commented Apr 24, 2017 at 13:38
  • @ATomz "Re-run" does not mean to run TPI twice, but my apologies if the word is misleading. It should read "Run TPI on the new grid" then.
    – Kazuhito
    Commented Apr 24, 2017 at 13:47

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