Get Geometry Expression String Builder to select geometry to local map in atlas

Question

I have a layer of deforestation, and I need to create many layouts of each one polygon. I'm using Atlas to do it. See the model below:

Yellow polygon in raster is deforestation map layer and it is driver of atlas. Bottom wright we can see map with overview and when it change the box of map will be changed automatically change depend on where overview be.

The map in right and bottom place of paper, I would like that when change the atlas map, the feature (município) of map keep centralized where the overview is into. See the red overview into polygon and this is the geometry that I need to get in order adjust limits of map.

So, I create one expression to get geometry of feature from name of municipio. The layer that drive atlas (desforastation layer map) has column with name of municipio and as with it that I try to get municipio geometry from municipio map layer.

So, I did the expression in extents box data defined override:

See the expression that work to get feature:

I make the expression from deforestation layer, which drive atlas. So, I try to get feature from municipio map layer with column attribute of deforestation map layer (that has the column name of municipio in order to join with municipio map layer). In expression, 'nome' came from municipio map layer and municipio (without '') came from atlas driver layer (deforestation map layer) which has column name of municipio.

But when I continue to get geometry, it doesn't work. It return NULL value as a result. Because of it, it is impossible to calc x_min, and another ones values of map box.

The project to download: link to download

Answer 1

Answer

Add the function x_min() to your expression, than it should work:

x_min (geometry (get_feature ( ... )))

Explanation

Your problem is due to the context where you called the Expression string builder. As we can see on your screenshot, there is an Expected format: double. This means: here, you can't create a geometry as an output, as the output must be a number of format double.

You probably called the expression string builder in a place like data driven override - e.g. for line width or something similar where only a number makes sense.

Answer 2

Try updating your QGIS version. I think this is actually a bug with slightly older versions of the current LTR, I had the same issue in 3.22.11 where the feature would show up in the preview but no geometry (and not even if you wrapped it with x_min() or x()) - regardless of whether you're using a data defined override or field calculator.

However this was not an issue in 3.22.13