[QGIS-Developer] QGIS Select by Location, but no within a distance of

[Nyall Dawson]

On Wed, 3 Mar 2021 at 04:59, C Hamilton <adenaculture at gmail.com> wrote:
>
> I just had a user tell me he had to go back to ArcGIS so that he could use the Select by Location function and specify a distance from the feature. He could not believe that it was not available in QGIS. See
>
> https://desktop.arcgis.com/en/arcmap/10.3/map/working-with-layers/using-select-by-location.htm
>
> What he is looking for is the "Are within a distance of" operator. Am I missing it in QGIS if not this is a really important operator to include. I know you can always prebuffer the layer, but for an analyst who does this regularly ours will go back to using ArcGIS.
>
> Would this be fairly easy to implement?

Reasonably so - the bulk of the required changes would all sit within
QgsLocationBasedAlgorithm , specifically in
QgsLocationBasedAlgorithm::processByIteratingOverTargetSource and
QgsLocationBasedAlgorithm::processByIteratingOverIntersectSource
(there's two code paths there, and the one called will depend on
whether its more efficient to iterate over the first or second layer
-- this dual-logic approach is why these algorithms and join by
location are much faster then the intersection algorithm as previously
discussed!). In both of these you'd basically expand out the size of
the bounding box used to request matching features via
QgsRectangle::grow (so the larger bounding box is handed off to the
underlying data provider spatial index for filtering), and then change
the "isMatch" test to use engine->distance( .... ) <= buffer distance.
(Might also be worth first testing if the geometries intersect before
calculating the distance as it may be more efficient to do this, but
would need to be tested to verify.). On GEOS >= 3.9 based builds the
distance(... ) check is optimised and uses indexes, so it should be
very fast to perform this check! (As opposed to the inefficient
"buffer the geometries" approach). Basically we'd want to follow
PostGIS ST_DWithin approach, not a ST_Intersects(ST_Buffer(...))
approach.

If I was quoting for a customer I'd estimate 5 hours total for the
change, but of course YMMV.

Nyall


>
> Calvin
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