[Qgis-developer] Expression Based.. Styling

Nathan Woodrow madmanwoo at gmail.com
Mon Jul 9 01:38:13 PDT 2012


Hey Régis,

I think that would a good feature to add. It would add a lot of power
to the rendering engines, including the rule based render. +1 from me
for adding support for this if we can.

- Nathan

On Mon, Jul 9, 2012 at 6:18 PM, haubourg
<regis.haubourg at eau-adour-garonne.fr> wrote:
> Hi Andreas,
> maybe I missed something, please tell me.
>
> This is how I see the matter:
> Rule based styling allows to create rules based on *existing* fields with
> SQL queries, but, *Refine assistants only have access to existing fields*.
> Once refined rules on single or multiple fields are created, you then have
> the possibility to use expression evaluator to modify each rule, reorder,
> one after each other. This is awesome for many use cases, but not for thoses
> ones :
>
> * Example 1:classify and normalize numeric data*  You want to apply a sqrt
> function on a numeric field and classify it with graduate value. Having the
> expression builder accessible instead of fields list allows it directly.
> Mapinfo allows you to classify a expression (sqrt(myfield)/max(myfield) *
> 100 for example).  *Doing it in QGIS requires to create a new field. So the
> user need to have rights on it or duplicate it.* I don't like that users
> duplicate data, since I am data administrator. Modifying data for
> representation purposes is bad too.
>
>
> *Example 2: Calculate advanced rotation and size fields : *I have waste
> water plants to map. I want them red symbols for dysfunctionning plants and
> green for the others. I want a point size varying based on size (~number of
> inhabitants). If I want a correct representation, I want point size to vary
> between 40 mm on screen and 0.1. With an expression instead of existing
> fields, this can be easy. *I do know how to do that in postgis or
> spatiallite view by myself. Common users don't *
> This would be even easier with a dedicated gui to help user choose an d
> preview Sqrt, exp , or Ln dynamic.
>
> *Example 3: generate unique values for legend labels*I need to categorize
> data on a field, but with a display order (40 classes). I have a code for
> classe number, and a label field.  In QGIS, I have two choices: Classify on
> code and manually change labels (long and painful), or classify on label
> field with ruled based labeling, and then order classes with priority order.
> Long and painful too. With an expression, I could evaluate "code" || ' - '
> || "label" , and have it sorted and readable directly. Not perfect, but
> fast. *This is what we need when having a meeting with authorities and only
> 3 hours to prepare a draft map. *
>
> Did I miss something? I think QGIS still need some work to achieve all
> cartographic representation rules (
> http://www.scribd.com/doc/33408233/SUG243-Cartography-Proportional-Symbol
> http://www.scribd.com/doc/33408233/SUG243-Cartography-Proportional-Symbol )
>
> Are you interested if my organisation supports such a functionnality?
>
>
> --
> View this message in context: http://osgeo-org.1560.n6.nabble.com/Expression-Based-Styling-tp4986594p4986858.html
> Sent from the Quantum GIS - Developer mailing list archive at Nabble.com.
> _______________________________________________
> Qgis-developer mailing list
> Qgis-developer at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/qgis-developer


More information about the Qgis-developer mailing list