[GIS] Calculating the difference between two grids in percent with QGIS

gdalmap-algebraqgisrasterraster-calculator

I'm using the QGIS GDAL Raster Calculator to get the difference in percent between two grids.

Grid A contains an integer, and grid B contains an integer which may be the same as or larger than the corresponding cell in grid A. They are geographically identical (in terms of cell size, extent etc).

So in the raster calculator I use this formula:

((B-A)/A)*100

…expecting a percent value in the created raster showing the difference in percent between raster A and B.

But the created raster is almost completely zeroes, except for a few places containing 100% values (they do not correspond with my manual control samples). I've chosen Int32 as the datatype for the created raster (I don't need more precision than whole percent).

Is the formula incorrect for what I'm trying to do, or have I missed something else?

Best Answer

Your results give the impression that when you are using integers in division, the result is also an integer (0 or 1).

Can you change the datatype of the input grid A or B to float?

If changing datatypes is a problem, another option could be to change the equation like this:

(((B-A)*100)/A)

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QGIS Raster Calculator – Performing Addition in QGIS Raster Calculator

The problem is that your individual rasters are not aligned. So, when they are added the result is a weighted average for the values in the overlaping area. To solve this you have to use a merged vector layer whose extension must be used for producing individual raster. I used this procedure for testing it:

1) I created two vector layers (both with the same value field = 10) where they are overlaped in some area:

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2) I merged them and I wrote down its extension:

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3) Using the extension to rasterize (by value field = 10) the first vector layer:

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4) First vector layer rasterized:

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5) Second vector layer rasterized:

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6) sum_raster obtained by using raster calculator. You can observe that the sum is 20 in overlaped areas (Value Tool plugin). The values: 0 (black area), 10 (gray area), 20 (white area).

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[GIS] Compare two raster extents in QGIS

You can use the QGIS raster calculator for this, using boolean masks. An example:

(("grid1@1">0) AND ("grid2@1">0)) * 1
+ (("grid1@1"=0) AND ("grid2@1">0)) * 2
+ (("grid1@1">0) AND ("grid2@1"=0)) * 3
+ (("grid1@1"=0) AND ("grid2@1"=0)) * 4

where you can change 1, 2, 3, and 4 to be whatever values you like.

The way that this works is that the first part of each term is a boolean mask. ("grid1@1">0) AND ("grid2@1">0)) is True when your first condition is met, and it can be multiplied by your categorical value D (I used 1). The next three terms match your next three conditions.

Note

Steven Kay notes in the comments that the same expression can be performed without using AND by replacing it with a *. In this case, the first term looks like

(("grid1@1">0) * ("grid2@1">0)) * 1

Best Answer

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[GIS] Compare two raster extents in QGIS

Note

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