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I have two layers: 1 polygon layer where the depth of an area is written in the database (as DRVAL1) and and where the depth is exported as a Z-value to the geometry. 1 point layer that I use to plan Aids-to-Navigation. two test opjects.

In the screenshot to test objects are placed in the point layer. I want to write the depth from the polygonlayer (that is displayed under the pointlayer) that is true for the position of the point, to the poinlayer.

I've tried to make a virtual field in the pointlayer with:

 CASE
 (WHEN intersects($geometry,geometry(get_features('layer_id',"DRVAL1", 
 '50')))) = true THEN '50'
 END

I've repeated the WHEN->THEN for all the values in the polygonlager. but I get a reply with (can't convert to double) or (can't find DRVAL1)

what do I need to change or are there better ways to tackle this issue?

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2 Answers 2

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1

If you install the RefFunctions plugin as @Joseph suggests, there's no need to use conditional statements. Instead, use the geomintersects() function to get the 'DRVAL' value of the polygon that the point intersects, like this:

geomintersects( 'polygonlayer', 'DRVAL1')

Note: Your expression uses a function called get_features() (plural). That function doesn't exist, but there's a function get_feature() (singular). It "returns the first feature of a layer matching a given attribute value." So if your polygon layer has more than one feature with 'DRVAL1' = 50, intersects($geometry,geometry(get_feature('layer_id',"DRVAL1", '50'))) will return false if the current point intersects a different polygon than whichever one QGIS decides is the first. This limitation of the built-in geometry functions is exactly why the RefFunctions plugin exists.

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5
  • I wasn't aware of the limitation you described in get_feature, neither of the existence of RefFunctions, many thanks. Still getting a NULL reply if i use this method. Will try harder.
    – CaptainAhab
    Apr 4, 2019 at 19:09
  • A few suggestions: Make sure you substituted the correct layer and field names into the expression (check your spelling and look for leading or ending whitespace). Double-check that the "polygon layer" is actually polygons, not lines. Try geomwithin() instead of geomintersects().
    – csk
    Apr 4, 2019 at 19:15
  • I tried both but somewhere i’m going wrong. I know the spelling is right cause if it isn’t i get an error. I tried dissolving and then singleparts to multiparts. Both no succes, i just get a read-out (NULL). Is it possible both layers need to have the same CRS?
    – CaptainAhab
    Apr 7, 2019 at 20:05
  • yes. always use the same crs for geometry operations (union, intersect, etc).
    – csk
    Apr 8, 2019 at 14:00
  • problem solved, works flawless, thnx!
    – CaptainAhab
    Apr 8, 2019 at 18:24
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I find it easier to install the refFunctions plugin as this contains a set of geospatial expressions which can be called from the field calculator. You could then use something like:

CASE 
WHEN geomintersects( 'polygonlayer', 'DRVAL1') = '50'
THEN '50'
END

where polygonlayer is the name of your polygon layer.

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