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I want to compare 2 same tiles at different dates.

I use the raster calculator like this :

(IR_band_frb - IR_band-aug)^2 

But I get this result : squared diff between 2 images

The data range goes from -6049 to 4486 ... Why I get negativ results ?

(Same when I write "abs" instead of "^2" symbol)

I tried

sqrt((im1-im2)**2)

But I get missing values (transparent "colors" in the output raster). So negatives values are passed to the square root operation.

Maybe do you have a better way for comparing 2 rasters in QGIS, do not hesitate !

Ty !

  • 1
    It may be that the squared numbers are too large, and they exceed the storage capacity of your raster format. – csk Aug 6 at 19:24
  • But when I apply sqrt((im1-im2)^2), I get missing values resulting transparent "color" in the output raster. That indicates some negatives values are passed to the root squared operation. – Antoine Mathu Aug 7 at 7:26
2

Why would you square the difference between the images anyway?

If you just do (Raster_1 - Raster_2) as the equation then any values you get that are negative mean that Raster_2 > than Raster_1 whereas positive values show that Raster_1 > Raster_2.

Would this information not be helpful?

  • 1
    This does not really answer the question. If you have a different question, you can ask it by clicking Ask Question. You can also add a bounty to draw more attention to this question once you have enough reputation. - From Review – whyzar Aug 6 at 18:20
  • 2
    @whyzar I believe this answer addresses the part of the question where they asked for "a better way for comparing 2 rasters in QGIS." – csk Aug 6 at 19:26
  • I wantt to have a positiv difference because I dont care if image 1 > image 2 or vice versa. I just want to measure "the difference" between them and then apply a threshold to keep only the constant structures – Antoine Mathu Aug 7 at 7:21

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