Documentation missing explanation on symbol scaling methods

[whrcdontfearme22]

Requesting clarification on symbol scaling

Hello, first off thank you for developing QGIS. Its excellent software.

There are several methods available for scaling symbols, but no documentation exists on what these methods are or how they function.

Page URL: https://docs.qgis.org/3.22/en/docs/user_manual/introduction/general_tools.html#using-the-data-defined-assistant-interface

Screenshot:

Anita Grazer mentioned that Vincent Mora may have been the one to code this originally, from Oslandia.

[vpicavet]

Some references ( ⚠️ not to use in QGIS documentation, we need GNU FDL licence ) :

• Article on Flannery : https://makingmaps.net/2007/08/28/perceptual-scaling-of-map-symbols/
• Flannery academic article : https://search.library.wisc.edu/catalog/999873270802121
• ArcGIS documentation : https://pro.arcgis.com/en/pro-app/latest/help/mapping/layer-properties/proportional-symbology.htm

[vpicavet]

ping @vmora @troopa81

[vmora]

The scaling function, once created with the assistant, is converted into an expression (useful to check what I'm saying):

• Radius: the *diameter" of the symbol varies linearly between the "size from" and the "size to" when the "source" expression value varies from "value from" and "value to"
• Surface: since human percieves visualy the "intensity"/"strength" of a symbol by it's area retlative to another rather than it's relative "diameter", the surface scaling scales according to the square root of the "source" expression, therefore scaling the area of the symbol linearly betweeen... (same as above)
• Flannery: I did not read the paper, but apparently the perseption of relative intensity as the area of the symbol is not exactly what happens in our brain, and thus the nice and simple square root (power 0.5) is replaced by a power 0.57.
• Exponanetial: same principle, but the user can choose the exact power that is used for scaling.

The scale_exp function documentation is available from the expression editor.

Hope this helps.

[vpicavet]

John Krieger gave us the right to reuse part of entirety of his article on Flannery method in QGIS documentation : 
https://makingmaps.net/2007/08/28/perceptual-scaling-of-map-symbols/
It would be great to mention him as a contributor if we reuse his material.