4

After processing a DEM with a mask to get the aspect values of some polygons I would like to create a new field classifying those values as N, NE, E, SE, S, SW, W, NW.

I've written this but I can't get what I'm looking for:

CASE
WHEN  "Aspect"  < 22.5  THEN  'N'
WHEN  (22.5 >= "Aspect" AND "Aspect" < 67.5) THEN  'NE'
WHEN  (67.5 >= "Aspect" AND "Aspect" < 112.5) THEN  'E'
WHEN  (112.5 >= "Aspect" AND "Aspect" < 157.5) THEN  'SE'
WHEN  (157.5 >= "Aspect" AND "Aspect" < 202.5) THEN  'S'
WHEN  (202.5 >= "Aspect" AND "Aspect" < 247.5) THEN  'SW'
WHEN  (247.5 >= "Aspect" AND "Aspect" < 292.5) THEN  'W'
WHEN  (292.5 >= "Aspect" AND "Aspect" < 337.5) THEN  'NW'
WHEN  "Aspect"  >= 337.5  THEN  'N'
END

Is it possible to do this using CASE-WHEN, or is it better to use another kind of conditional?

I don't know how to write ranges using conditional sentences without getting an error except this way.

1 Answer 1

7

If the Aspect raster converted to polygon or you have a shapefile with "Aspect" field name in the attribute table and you need to put the text definition, then you are almost there, but you need to put "Aspect" before >= not after:

CASE
WHEN  "Aspect" < 22.5  THEN  'N'
WHEN  ("Aspect" >= 22.5 AND "Aspect" < 67.5) THEN  'NE'
WHEN  ("Aspect" >= 67.5 AND "Aspect" < 112.5) THEN  'E'
WHEN  ("Aspect" >= 112.5  AND "Aspect" < 157.5) THEN  'SE'
WHEN  ("Aspect" >= 157.5  AND "Aspect" < 202.5) THEN  'S'
WHEN  ("Aspect" >= 202.5  AND "Aspect" < 247.5) THEN  'SW'
WHEN  ("Aspect" >= 247.5  AND "Aspect" < 292.5) THEN  'W'
WHEN  ("Aspect" >= 292.5  AND "Aspect" < 337.5) THEN  'NW'
WHEN  "Aspect"  >= 337.5  THEN  'N'
END

Here is the output:

enter image description here

But if you want to add field to a raster data, then you cannot because QGIS does not support adding attribute table to raster data.

1
  • It worked! I'd swear I tried to put "Aspect" before >= and I didn't get what I was looking for. Now, I've just copy and paste what you wrote and works perfectly. Thanks so much!
    – GisAmateur
    Commented Apr 3, 2018 at 15:32

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