Varying layout map scale based on layer visibility in QGIS?

Question

I am looking for a method to vary the map scale of a QGIS layout according to the visibility of a layer.

The output should show the minimum scale at which the layer can be seen in the layout from a range of predefined scales (e.g. 1:1000, 1:2500 etc.).

I would like the method to work with the QGIS Atlas so that the output can be exported efficiently.

As an example, the images below show the layout at 1:1000 (site boundary only), 1:2500 (site boundary and surface water) and 1:5000 (site boundary and surface water). I would like the layout scale to automatically adjust depending on the minimum scale at which surface water can be seen from a range of predefined scales. So in this case, produce a layout at 1:2500 and not 1:1000 or 1:5000.

Answer 1

Calculate the distance to the closest surface water then use this distance to determine the appropriate scale.

• To calculate the distance to the closest water add a field (it doesn't seem to work with virtual field) in your site boundary layer and use :

  geomnearest('SURFACE_WATER_LAYER_NAME','$distance')

Then use this field to set the scale in your layout with an expression like this if you want to use a few scale:

CASE
WHEN "DISTANCE" < 500 THEN '1000'
WHEN "DISTANCE" > 5000 THEN '10000'
ELSE '5000'
END

where "DISTANCE" is your calculated field (adjust the numerical value to your need)

Or if you want something that give the biggest possible scale set the scale with :

"DISTANCE" / x

where "DISTANCE" is your calculated field and x is half of the shortest mesure of your map element en meter (so if you map element is 10 by 20 centimeter then x=0.05)