QGIS Raster Calculator – How to Keep Original Values Instead of 1s and 0s

qgisraster-calculator

I have a crop data raster layer in QGIS with one band with values from 10 to 230.

I have written an expression in the Raster Calculator to keep only a number of values (e.g. when = 145 OR when = to 146, etc..

This works but the resulting layer has 0 values for areas that did not satisfy the expression and 1's for areas that did. As a result, I appear to have lost the original raster values which indicate crop types.

Am I doing this wrong or maybe there's another way to filter that will keep my original values?

Best Answer

If you use QGIS 3.22 or later, you have the new conditional if in raster calculator:

IF ("RASTER@1" >= 145 AND "RASTER@1" <= 146, "RASTER@1", 0)

For older versions, use:

("RASTER@1" >= 145 and "RASTER@1" <= 146) * "N45E006@1"

I would not advice using something like "RASTER@1" = 145 or "RASTER@1" = 146: probably pixel values will not match exactly integers (145.00, 146.00), but be rather like 145.7 and using this expression, it will not find anything.

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QGIS Raster Calculator – Set Values to 0 Instead of No Data

I just found the SAGA tools and the better raster calculator provided. I solved the problem using the following formula:

ifelse(eq(a,190), 1, 0)

which actively sets all cells with a value other than 190 to zero.

Edit: I just found the reason why QGIS dealt differently, too: I was wrong stating that both rasters had the same format. In the first case my input raster was in ASCII format and QGIS set zeros. Doing the same operation with a tif format, it sets no data values.

[GIS] QGIS 2.18 Raster Calculator not working properly

Although I cannot see the raster value of 3110 maybe due to the min and max values in the raster statistics are not set properly, but anyway you can achieve what you want by applying the following expression:

("dblbnd@1" > 1525  AND  "dblbnd@1" < 3110)  *  "dblbnd@1"

Please note that the values below or equal to 1525 and above and equal to 3110 will assigned 0 because they don't meet the condition above.

Here is an example to extract the values above 800 and below 2000 from the image below:

Using raster calculator, I wrote the following expression:

("Test@1"  > 800  AND "Test@1" <2000) * "Test@1"

Here is the output:

You can set the zero value to null using r.null tool in GRASS and replace 0 with -9999 asfollows:

The final output raster will have values ranges from 801 to 1999 as extracted from raster calculator:

Best Answer

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