Using Presence/Absence method in QGIS?

Question

I seek advice for calculating presence/absence of features in QGIS. I have a polygon dataset of boundary outlines and a number of other datasets which I want to compare it against for presence/absence of features within the main boundary dataset.

Can I do this using the vector datasets as is or will I need to convert then to a grid/raster beforehand?

My example is I have OS administrative boundaries an OS open green spaces data, I would like to know whether an area of open green space is present or absent from each boundary polygon. I was thinking about using the Join attributes by location and intersect, but don't know if that is flexible enough to give a simple yes/no answer for each boundary polygon.

Answer 1

One method:

Install the refFunctions plugin.

Use field calculator to add a field with the expression

intersecting_geom_count('layer name') > 0

If you need a live-updating count, make it a virtual field by selecting "Create virtual field." Otherwise make it a static field by leaving that box un-checked.

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Another method:

Join attributes by location. Vector menu > Data management tools > Join attributes by location

• For Geometry Predicate, choose "intersects"

• For Fields to add, choose one field - any field will work as long as it has no null values

  Then use the field calculator to create a field with this expression:

  'field name' is not null

If you need a live-updating count, make it a virtual field by selecting "Create virtual field." Otherwise make it a static field by leaving that box un-checked.