I am perplexed by the results generated by the SAGA 2.3.1 Extract Values to Points tool in QGIS 2.18.

I am working with Vegetation Index raster layers, which means the values are between -1 and 1. According the min-max of the raster values (prior to the SAGA calculation), all the minimum values are above 0. The Extract Values to Points tool generates negative values up to -3. I have tried saving the No Data values to 0, but that doesn't change the output.

Has anyone else run into this problem? I understand that there was a known bug in the algorithm; however, I don't believe this is my issue. The tool is generally working, except for the occasional new negative numbers (I say new because they were not present in the original input raster). Additionally, I know people also recommend using the Point Sampling plugin in QGIS; however, I am working with huge datasets and (according to this forum), the plugin cannot handle that type of volume.

This issue has me questioning my results.

  • 2
    Are you sure the actual range is [-1,1]? If you double-click the raster, then go to the Information tab, are the max/min listed there (under Bands, maybe) in line with what you expect? You first need to diagnose whether this is a SAGA issue or a data/understanding-of-data issue. – Jon May 10 '19 at 14:21
  • Thanks @Jon - you raise a good point. I was looking at the Min-Max listed under the Style tab, but when I look at the Metadata, the minimum is less than -1. I calculated this vegetation index from the Google Earth Engine, so that shouldn't be possible. Now I have more questions I will have to answer. Thanks for your help! – Sarah May 10 '19 at 14:58
  • Make sure you're using the correct bands for computing NDVI (assuming you're computing NDVI)--they're different for Landsat 5/7 and 8 (assuming you're using Landsat). You can use max() and min() and print() functions in GEE to make sure everything is as you expect. – Jon May 10 '19 at 16:18

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