QGIS Field Calculator – How to Use the Intersects Function

field-calculatorintersectionqgis

I would like to have an example on how to use the Intersects function in the Field Calculator of QGIS.

Basically, I want to know which feature from a polylines layer intersects a polygons layer and store the result in an attribute. (0 = not intersecting, 1 = Intersecting).

I'm aware of the geoprocessing tools "Clip" or "Intersect", but is it possible in the Field calculator ?

Best Answer

This is not possible in field calculator. The intersect function in the field calculator does not work with two layers. What you can do is calculate intersections with specific geometries, e.g.

intersects( $geometry, geomFromWKT( 'LINESTRING(765145.88 6274561.22, 776031.67 6284189.52)' ) )

For more on these spatial operators check http://docs.qgis.org/latest/en/docs/user_manual/working_with_vector/expression.html#geometry-functions

Instead, use the Spatial query plugin to first select all features that intersect the polygon layer. Then use field calculator and only update selected features to set the intersection attribute to 1.

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[GIS] Batch Update Fields in Field Calculator or Model

You should be able to write this in a python scripting using an update cursor fairly easily.

import arcpy
tablename = "c:\temp\*NAMEOFYOURTABLE*"
fieldvalues = ['fieldname1','fieldname2','fieldname3'......'fieldnameX']

cur = arcpy.UpdateCursor(tablename)
for row in cur:
    sourcevalue = row.getValue('*name of field where source value coming from*')
    for f in fieldvalues:
        row.setValue(f, sourcevalue)
    cur.updateRow(row)

[GIS] Using replace string function in QGIS Field Calculator

replace( '(B)','(B)','County')

tells QGIS to replace the '(B)' in '(B)' with 'Country'.

You want

replace("Name",'(B)','Country')

= replace the '(B)' in "Name" with 'Country'.

Note that the double quotes indicate a field name while the single quotes refer to a static string. These are not interchangeable.

Best Answer

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[GIS] Batch Update Fields in Field Calculator or Model

[GIS] Using replace string function in QGIS Field Calculator

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