[Qgis-developer] Proportional Graduated Circle

[MORREALE Jean Roc]

Hi,

I would like to re-open the discussion about the way QGIS deals with
proportional symbols : for now it's is possible to assign a size scale
using a field, the symbol will take this value as its size in point.

This subject has previously been talked on this list [1] and a ticket
(#960) about it has been set as wontfix but still today, the most 
frequent question I get on meeting, on forums, from students is "how to 
make proportional symbols ?", behind it what these people are really 
asking for is if it's possible to do what they're used to on MapInfo, 
Arcgis, CrimeStat, Spotfire, etc. The point in 2008 was that the 
feature's usefulness was not obvious so here is some thoughts on it.

The downside of the actual qgis's method is :
- value = point size, if the values are hight (like for a city
population field) the biggest symbol will hide its neighborhood (causing
some visual glitches when too near)
- necessity to set by hand a correct size, by creating a new field or by
changing the size by class (throw in it someone who wnat a proportional 
circle for class range and you're set)
- the size doesn't show up into the legend

Many articles have been written about these proportional circles  such 
as James Flannery's dissertation called “The Graduated Circle: A 
Description, Analysis, and Evaluation of a Quantitative Map Symbol” with
gives a formula (used in ArcGIS [2]) :

"Taking the median of the results from several parts of the data, he
offered the following formula as his best estimate of the relationship
of apparent circular area (Yc) to the logarithm of actual area (X),
raised by an exponent and multiplied by a scaling constant" Daniel R.
Montello, Cognitive Map-Design Research in the 20th Century: Theoretical
and Empirical Approaches

While an absolute scale is more scientifically correct (Tufte, 1998), 
the compensated proportional circle still has its benefits and users.

I must admit that psychophysical's studies are quite an interesting read :)

[1]
http://osgeo-org.1803224.n2.nabble.com/proportional-symbols-td2038030.html
[2] Arcgis's implementation (with screenies) ->
http://www.geog.ucsb.edu/~jeff/gis/proportional_symbols.html#gradsym
[3] Perceptual Scaling of Map Symbols ->
http://makingmaps.net/2007/08/28/perceptual-scaling-of-map-symbols/