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?

  • 1
    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 Apr 21 '17 at 22:12

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.

  • 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 Apr 24 '17 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 Apr 24 '17 at 13:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.