Creating a quality raster map in QGIS?

I've got 2 raster maps (both with the same values: 0-100). After calculating the average I need to know how precise the map is. Is there any tool in QGis?

I try to explain my problem in another way:

I have 2 or maybe 3 percentaged raster maps. After putting together ((a+b+c)/3) I need to get a map that shows if the result map is statistically firm.

For instance: If all 3 maps have the same value in an area the result must be reliable/firm. If 2 maps have the same values and 1 map is totally different the result is a little bit unreliable. If 3 maps are totally different the result is unreliable.

I want to create a colored map (green, orange, red). I already have processed the average map!

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Definitely going to need some more information about your data and exactly what you're trying to find out. – Rayner Nov 15 '12 at 14:09
Ok. I have 2 yield maps (percentaged). I want to create an average map ((a+b/2)). After having the average I need to know if the result is reliable or not (green=reliable, orange=maybe, red=not reliable). – Pimpel Nov 15 '12 at 14:45

1 Answer

You can do this using QGIS' Raster calculator. Using 1.8.0:

• Add your two layers.
• Zoom to their extent.
• Navigate to: `Raster -> Raster Calculator`
• Enter your expression. So it will be something like: `round((LayerA@1 + Layerb@1) / 2)`
• Go to the layer properties for the new layer. Set colour map to "Pseudocolor" or play around to set actual desired colours.

Note: The "round" isn't strictly necessary. It just means your output will be integers rather than floats.

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I have the average map on the screen. Now I tried to calculate the difference between the 2 maps. With a legend (-100 to +100) I can see where the data is reliable or not. But I think its just an easy and imprecise result. – Pimpel Nov 16 '12 at 10:13
You can also use Raster masks to make comparisons between rasters. i.e. `a@1 = b@1 AND a@1 = c@1` See also: linfiniti.com/2010/11/raster-masks-in-qgisfossgis and spatialgalaxy.net/2012/01/25/using-the-qgis-raster-calculator – GIS-Jonathan Nov 16 '12 at 10:16