[Qgis-developer] Topology editing in QGis

[Ouyang Leyan]

> Date: Mon, 4 Nov 2013 12:33:04 +0100
> From: strk at keybit.net
> To: radim.blazek at gmail.com
> CC: Qgis-developer at lists.osgeo.org; ouyang.leyan at hotmail.com
> Subject: Re: [Qgis-developer] Topology editing in QGis
> 
> On Sun, Nov 03, 2013 at 11:48:31AM +0100, Radim Blazek wrote:
> 
> > Topology models are usually using multiple geometry types (e.g. nodes,
> > edges). Multi geometry type layers may also be useful for non                                                                                    > topological layers. How difficult would it be to support multiple
> > geometry types in a single layer?
>                                                                                                                                                    The path I took for postgis topology was instead to keep layers
> separated. They are still correlated one-other (changes in one layer
> may result in changes propagating to other layers), but they are
> still seen as separate layers in the layer management, so that you
> can set their visibility independently and/or style them differently.
> 
> I think it may be useful to have support for mixed geometry types
> within a single layer but don't feel any urgent need for them to
> edit topologies.
> 
> How do you like the separated layers approach ? Do you think it could
> work for GRASS topologies ? I guess one limitation of it is that when
> you enter "edit mode" you do so in a single layer, thus implicitly
> (rather than explicitly) also editing correlated but
> not-marked-as-being-edited layers. But this would already be the case
> by opening the same datasource twice...
>
For the topology editing, I was thinking of considering the topology as a
 special group (or the vector layers as sublayers of a topology layer):
* When adding a topology, all the vector layers (node, edge, face, etc.) would be added at once
* Each vector layer would appear independently in the layer tree, but be a part of the topology group
*
 It would be impossible to turn editing on a single layer, editing state
 would be controlled at the topology group level, so if one layer is 
edited, all layers are. This would be explicitly shown.

This allows to not touch anything for the
 visualization part, all the symbology can be applied on a single vector
 layer as usual. Only the editing features would have to be modified to 
deal with the topological modifications of the other layers and keep 
everything consistent. Of course, the definition of consistent will 
depend on the kind of topology considered.

I never used GRASS 
topology, I still have to learn about it, but I think this 
representation would be flexible enough to adapt to different 
situations. It would also be possible to build "ad-hoc" topologies with 
simple normal vector layers (shapefiles, etc.) and have editing 
operations follow basic topology rules including several layers, which 
is something I would like to see (I work with networks and casual users 
who can open shapefiles, but not install PostGreSQL). This kind of 
topology would be like an extension of the existing pseudo-topology 
editing, just extended to several layers.

Pierre
 		 	   		  
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