[work] Re: [Qgis-user] No reference or details about the "topological editiing" option in the documentation

[Andreas Neumann]

Tara Athan wrote:
> I don't see anything in chapter 4 about topological editing. I am 
> unfamiliar with this option- when and how is it invoked?

Hi Tara and others,

In ESRI, QGIS, Postgis and many other GIS relying on the "relatively 
primitive" spaghetti model there are no shared boundaries for adjacent 
polygons like it used to be in good old ArcInfo or in today's GRASS GIS.

This means if you want to edit a "shared" (actually not shared) boundary 
you'd have to move the same vertices in both boundaries. The 
"topological editing" on QGIS means  The option "Enable topological 
editing" is for editing and maintaining common boundaries in polygon 
mosaics. QGIS "detects" a shared boundary in a polygon mosaic and you 
only have to move the vertex once and QGIS would take care about 
updating the other boundary.

There is also another option that avoids overlaps in polygon mosaics, 
called "Avoid intersection of new polygons". It is for quicker 
digitizing of adjacent polygons. If you already have one polygon, it is 
possible with this option to digitise the second one such that both 
intersect and qgis then cuts the second polygon to the common boundary. 
The advantage is that users don't have to digitize all vertices of the 
common boundary (quoted from a previous thread on this topic).

Hope this clarifies the two settings related to "topological editing".

Andreas