Area scale field: make standardized symbol dimension and legend

[giohappy]

I find the "Area scale field" an important enhancement from QGis 0.9.2, but two enhancements should be

1 - make the symbol dimensions appear "standardized", and not directly proportional to the attribute values. The problem raises when we have big values, and the symbols appear to large, even if one reduces the dimension to the minimum (size 3).

2 - make appear the symbol classes in the legend, otherwise we loose the visual usefullness of having graduated symbols.

[timlinux]

Could you provide more detail of your requirements here? I couldnt understand exactly what you are looking for from your description.

I'm going to mark this for Magnus' attention since he implemented the original symbol scaling logic.

Many thanks

Tim

[giohappy]

Hi Tim. I'm referring to the symbology option for punctual features, that lets you choose a single layer whose dimension (and rotation) may be controlled by means of a selected attribute. You can choose a not better defined "dimension" factor that applies to the symbols size proportionally to the attribute value. Now, if you have huge values you can get enormous balloons even if you set the minimum "dimension" (by default the lower limit is 3). I think it could be better to stanrdardize the dimensions of the symbols on the basis of a minimum-maximum value. In this way the user could choose the best visualization it needs.

In formula it could be: (attribute_value - min_dimension)/(max_dimension-min_dimension) or something similar...

[homann]

I'm not so sure this is a good idea. The question then arise, what kind of expressions should we allow? Scale based on a generic forumla on 1 or more fields?

No, I propose that should be up to the layer to proivde the correct values.

[giohappy]

Ok, you can advocate my suggestion, but the problem remains: these days I'm working on a layer, and I need to make graduated symbols on the base of a floating attribute. The values are so high that the balls cover the entire screen. Should I create an attribute field with scaled values?! Maybe I miss something from the new Qgis versions... How would you deal with this?

Anyway, before closing a ticket it would be nice to propose a solution...

[homann]

Set the fields to values that can be used for scaling directly.

[giohappy]

Ok, I leave this ticket but your idea is, but I haven't seen any GIS needing that! I select the classes, and max-min values, and that's all...

[giohappy]

Sorry, there were problem pasting the text

*Ok, I leave this ticket, but I haven't seen any GIS needing that. I usually select the bins/classes, max-min values, and classification method (linear, quantiles, etc.), and that's all... I can't uderstand why it cannot be done for symbol dimensions. Maybe I can't expain what I mean...Anyway, don't mind.

[homann]

Have you tried using Graduated Symbol?

[giohappy]

I was talking about that... Look at the attached images:

Property dialog:  http://www.geospatial.it/allegri/property.png Result:  http://www.geospatial.it/allegri/graduated.png As you can see the Dimensione is set to minimum (3). Maybe it could be enough allowing lower dimensions (decimals too).

[homann]

I still dont understand what you really want. Could you please email the qgis-user list and we take the discussion there?

[anonymous]

Milestone Version 1.0.0 deleted

[jrm]

Biased proportional symbol are possible in most GIS software, here a few methods :

 / ArcGIS -> use a size range (e.g. lowest is at 4pt and the biggest at 22pt) or use Flannery's compensation

 / MapInfo -> use a size range (you can determine the interpolated symbol size using the square root, a constant or a log)

 / GvSIG -> use a size range

 / MapFish -> use a size range

Some good read on the subject :  / Perceptual Scaling of Map Symbols

Daniel R. Montello, Cognitive Map-Design Research in the 20th Century: Theoretical and Empirical Approaches  / PDF

James Flannery, The Graduated Circle: A Description, Analysis, and Evaluation of a Quantitative Map Symbol

Susumu Tanimura and al., Proportional Symbol Mapping in R  / PDF

Bernhard Jenny, Self-Adjusting Legends for Proportional Symbol Maps  / PDF  / Exemples

Original  / post on qgis-dev's ml

Making it possible to show the size in the legend would be a small step that would still be compliant with Tufte's rigorous ideas.