[Qgis-user] Areaunits and update fiels

[Alister Hood]

Hi,

> Date: Sun, 10 Jun 2012 11:44:40 +0200
> From: "Lene Fischer" <lfi at life.ku.dk>
> To: <Qgis-user at lists.osgeo.org>
> Subject: [Qgis-user] Areaunits and update fiels
> Message-ID: <4FD488A80200002E0003D964 at gwmta2.kvl.dk>
> Content-Type: text/plain; charset="utf-8"
> 
> Hi
> 
> I?ve been searching for at setting to set areaunits. We mostly use
> hectare, so the standard alwas has to be divided with 10000.
> 
> Next question: In the fieldcalculator we can calculate an area, but the
> field isn?t updatet after editing. And suggestions?
> 
> Kind Regards
> 
> 
> Lene Fischer
> Ass. Professor -  GIS & GPS

I don't think this is possible at the moment.

There has been some work on adding Python functionality that is similar to the way VBA is used in Microsoft office - e.g. allowing the use of scripts which will run automatically when a project is opened or saved.  See http://osgeo-org.1560.n6.nabble.com/Python-support-in-project-file-td4976475.html
I guess the ability to run a script automatically when a feature in a particular layer is edited would be a nice enhancement to go along with that, and could be used to achieve what you want.

But I don't think it is good practice to store derived data like this in an attribute field.  And if you are using it for labelling, it will be unnecessary with QGIS 1.8, which has on-the-fly expression based labelling.  Do you really need it in the attribute table?  There was a discussion about implementing "virtual columns" using the same engine as the expression based labelling.  See http://lists.osgeo.org/pipermail/qgis-developer/2011-July/015383.html
You might want to look at helping implement one of these features.

Regards,
Alister