Mrthfx

Using QGIS, I have two polygon layers, a vegetation layer and a treatment layer. I want to add a field to the attribute layer of the vegetation layer to indicate if any part of the polygon has been treated (overlaps with a treatment layer).

For the above example - I want an attribute table that ends up looking like

I do not want to intersect the two layers - I need to keep the full polygons in the vegetation layer, I just want to know if they've been treated. There are too many polygons to do this manually.

I have looked through the vector tools to try identify one that does this, this was not successful. I've tried to find a workflow via rasters instead of polygons, but cannot figure out what would be needed for this to work. I have searched online but not been able to find anything that answers this question, likely because I do not know what this is called so do not know what terms to search for.

The only solutions I have found are manual, but there are several hundred polygons so I would like to avoid that approach unless there is no other option.

You can do this using Aggregate function. Add a new field isTreated in the vegetation layer with an expression like below

if(aggregate(

 layer:= 'treatment',

 aggregate:='count',

 expression:=fid,

 filter:=intersects($geometry, geometry(@parent))

 ) > 0, 1, 0)

The aggregate function returns number of features from the treatment layer that are intersecting. As you are only interested whether they intersect at least 1 feature, you can add the if condition to assign 0 or 1.

See my post about aggregate functions in QGIS to learn more https://spatialthoughts.com/2019/04/12/summary-aggregation-qgis/

Tested on QGIS 2.18 and QGIS 3.4

I can suggest using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer...

Let's assume we have three features in "vegetation" and four in "treatment" accordingly, see image below.

With the following Query, it is possible to achieve the result

SELECT vegetation.*,

(CASE

    WHEN vegetation.id IN

        (SELECT vegetation.id

        FROM vegetation, treatment

        WHERE st_intersection(vegetation.geometry, treatment.geometry) IS NOT NULL)

    THEN '1'

    ELSE '0'

    END) AS Is_Treated

FROM vegetation

The output Virtual Layer will maintain initial attributes and geometries and add an additional field representing overlaps.

Additionally, you may extend your output layer as was earlier suggested by @spatialthoughts with several lines

SELECT vegetation.*,

(CASE

    WHEN vegetation.id IN

    (SELECT vegetation.id

    FROM vegetation, treatment

    WHERE st_intersection(vegetation.geometry, treatment.geometry) IS NOT NULL)

    THEN '1'

    ELSE '0'

    END) AS Is_Treated,

SUM(st_intersection(vegetation.geometry, treatment.geometry) IS NOT NULL) AS Intersections

FROM vegetation, treatment

GROUP BY vegetation.id

Now, the output Virtual Layer will look as following

References:

• Chapter 8. PostGIS Reference | 8.5. Geometry Accessors


• Counting attributes from line intersections with polygons on QGIS?


• Documentation “Join Attributes by Location” QGIS 2.18