[GIS] Producing raster A + B where A and B intersect, and A where they do not, using QGIS

qgisrasterraster-calculator

I have a raster A (a heightmap created from a contour file). I also have created a vector layer with some polygons which I rasterized into raster B. Raster B is completely contained within the extents of raster A.

Raster A has values at each pixel. Raster B, on the other hand, has a bunch of empty area (which I believe is represented as 0 or nodata).

I'd like to produce raster C, such that C = A + B, where A and B intersect, and C = A where A and B don't intersect.

The problem I'm running into is that when I add the two rasters together using the raster calculator, the result only has values where the two rasters intersect, but show nothing where they do not intersect. I'm guessing this is due to how QGIS treats nodata values.

How can I achieve what I'm trying to do?

Best Answer

The problem, as you observed, is that the Rasterize program sets the values outside of the polygons to NODATA (or dummy), and the raster calculator outputs a dummy wherever either raster has a dummy. In QGIS 3.6.3 I've solved this problem by specifying the output carefully in the Rasterize program to give everything outside of the polygons a value of zero.

Under the advanced parameters set the "Pre-initialize" value to zero, which will cause the raster to be all zeros wherever there is not a polygon.
Set the NODATA value to something other than zero. Zero is the default value which is really a poor choice. I just chose a large negative number that will clearly not be in the data.
Possibly not necessary, but I also found I had to set the extents and cell size to exactly match the raster I will be combining with, otherwise I got wonky results

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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

qgis – Update or Replace Raster Values from Another Raster in QGIS

OK - after some extensive googling and a fair bit of hair ripped out, I found a solution.

Instead of using a the raster calculator, I used a shape file for the marshes, and "rasterized" them. Instead of specifying a new .tiff, I wrote over the land cover raster that I wanted to update, and boom. Done. Not sure if that is the "correct" way, but it works for what I need.

Best Answer

Related Solutions

[GIS] Compare two raster extents in QGIS

Note

qgis – Update or Replace Raster Values from Another Raster in QGIS

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