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When I tried to compare two rasters in Arcgis by using Raster Calculator, the resulting raster includes positive and negative values.

How can I interpret these values?

What does the mean of the negative values?

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assuming that both rasters reflect the same variable on different dates and value scales, for example NDVI start date (NDVI-i) - NDVI end date (NDVI-f). Assuming you performed the operation NDVI-f - NDVI-i:

  1. Positive values indicate a recovery of the vegetation cover at the end date.
  2. Negative values: a loss of coverage between the initial and final date.
  3. Value 0: this zone remained unchanged.

I understand that ArcGis has tools for detecting changes, so you might want to look into it.

Another aspect, instead of working with the change of signs, negative or positive. It is advisable to define a threshold of change, ie, what range you consider a significant change. For example, for a scale of 0 to 1, you could decide as threshold value 0.03, values below this range are not considered a change.

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  • Hey Luis, thanks for the reply. I was trying to compare two rasters of wind speed. Range of the values of A raster is 1.05 to 12.6 and range of the values of B raster is 1.2 to 9.1. When I performed the operation Raster A - Raster B, range of the values of the resulting raster is from -3 to 9. I couldn't quite understand the meaning of the minus value.
    – symcln
    Aug 28, 2020 at 19:46
  • An example, rasterA- RasterB, 9-12=-3 or 1-4=-3 interprets the resulting raster, not as wind values, but as variations. In this way, -3 indicates an increase of 3 units of B in relation to A
    – Luis Perez
    Aug 28, 2020 at 21:35

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