How to scale features declining using map units in QGIS

Question

I have layer in QGIS that I want to style using SVG symbols.

I want the SVGs to increase in size when zooming in the map. I can solve this by using map units instead of millimeters.

Though, I don't want the symbols to increase 1:1.

Say, I want the symbol to be 75 map units at zoom level 1:50.000 but only 37.5 map units at zoom level 1:3.000.

I guess that using QGIS Expressions here is key, but I can't work out a proper mathematic formula that fixes my problem.

Can I use the scale_exp function in the Expression string builder in QGIS?

Can anybody help out?

Answer 1

You can try this formula applied on Data Defined Override near the size of the symbol and apply map units as in your case:

Case 
When @map_scale <=3000 then 37.5
When @map_scale > 3000 and @map_scale < 50000 then 50
when @map_scale >= 50000 then 75
end

You may need to define a symbol size between the 3000 and 50000 scales

The size of SVG symbol will be adjusted based on the map scale of the view not the layout scale.

Answer 2

This expression works only within the set input range (3,000 - 50,000), but you will get the idea.

scale_exp("value", 3000, 50000, 37.5, 75, 0.24638)