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I need to do calculations on raster layers which require the sum of all cells of the raster. I've used the zonal statistics tool for that but it gets a little annoying to do those extra steps (run the plugin, open the Attribute table, copy the sum...).

Is there a way I can calculate and use the values of raster statistics directly inside a raster calculator?

Even better would be to calculate those values from a raster created in the same step:

A*B*C / Sum (A*B*C)

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A calculation like

A*B*C / Sum (A*B*C)

will perform the A*B*C operation twice. To avoid that duplicate effort instead do it in two steps

X = A*B*C
X / Sum(X)

Storing A*B*C has an immediate payoff. To implement the Sum operation, use a zonal summary operator with the entire raster as the zone. That requires placing a constant, non-null value at every non-null cell of the raster. A simple way to accomplish this is to equate the raster with itself, thus:

ZonalSum(X, X==X)

(The syntax for ZonalSum will depend on the platform and the version of the software.)

The full workflow therefore is

X = A*B*C
X / ZonalSum(X, X==X)
  • This sounds interesting, can you help me out with the "ZonalSum" tool? How can I use it/ where can I find more information? Would that only be possible writing an own script? I use QGIS 2.4 on Windows 8... – Gumbo Jul 31 '14 at 8:05
  • A search of our site turns up a lot of information about that. – whuber Jul 31 '14 at 12:27
  • Hey, my comment was a little quick, now I already researched quite a bit but i'm not familiar enough with python and programming in general that I could understand most of the posts. Thx anyway – Gumbo Jul 31 '14 at 13:04
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Why not just open the Attribute Table (VAT) and multiply all the values times all the counts ?

  • This is a good solution when the raster actually has a VAT ( which occurs only when it has integral values) and when a value is not needed "directly inside a raster calculator," as requested in this thread. – whuber Jul 30 '14 at 17:24

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