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

[Otto Dassau]

Hi, 

thanks a lot for your help with the topological editing. I added some additional
info to the manual about snapping and topological editing (here is a commit
extract). I guess we can extend it when we review and proofread.

----
There are two possibilities to define the snapping tolerance. 
 
1. A general, project wide snapping tolerance can be defined choosing
\mainmenuopt{Settings} -> \dropmenuopttwo{mActionOptions}{Options}. In the
\tab{Digitizing} tab you can select between to vertex, to segment or to vertex
and segment as default snap mode. You can also define a default snapping
tolerance and a search radius for vertex edits. Remember the tolerance is in
layer units. In our digitizing project, the units are in feet. Your results
may vary, but something on the order of ?? to ?? should be fine.

2. A layer based snapping tolerance can be defined choosing
\mainmenuopt{Settings} -> \dropmenuopttwo{mActionOptions}{Project
Properties\dots}. In the \tab{General} tab, section Digitize you can click on
\button{Snapping options\dots} to enable and adjust snapping mode and
tolerance on a layer basis (see Figure~\ref{fig:snappingoptions}).

\subsubsection{Topological editing}

Besides layer based snapping options the \tab{General} tab in menu
\mainmenuopt{Settings} -> \dropmenuopttwo{mActionOptions}{Project
Properties\dots} also provides some topological functionalities. In section
Digitize you can \checkbox{Enable topological editing} and/or activate 
\checkbox{Avoid intersections of new polygons}.

\minisec{Enable topological editing}

The option \checkbox{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 will take 
care about updating the other boundary.

\minisec{Avoid intersections of new polygons}

The second topological option called \checkbox{Avoid intersections of new
polygons} avoids overlaps in polygon mosaics. 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.

regards,
 Otto

On Thu, 23 Oct 2008 09:09:32 +0200 (CEST)
"Andreas Neumann" <a.neumann at carto.net> wrote:

> > A lot of us never understood, still don't, how ESRI could have ever
> 
> * probably some people don't care about data quality ...
> * it is quicker to setup and maintain without topology
> * the topology model in old ArcInfo also could get corrupted and often
> required manual building/cleaning of the topology.
> 
> But I agree - there are also many disadvantages if correct topology is not
> enforced.
> 
> > dropped topology in the first place. It's caused no end of headaches,
> > and still is. The new approach has a lot more potential, encompassing
> > a wider variety of rules and greater number of layers, but is harder
> > to utilize because you need to set it up ahead of time and it is
> > optional. With coverages you had no choice, topology is baked into the
> > file format.
> 
> the new topology rules in ArcGIS go beyond to what was possible in old
> ArcInfo - but in my experience it is sometimes unstable. I often got
> crashes when I used the topology rules in earlier builds (tried about 2
> years ago when I still had access to ESRI software)
> 
> > I'm pleasantly surprised and encouraged to see topology has been
> > thought about and incorporated into qgis 1.0. :)
> 
> yes - but to be fair - it is not nearly as complete and powerful as the
> topology rules in ArcGIS. What Marco implemented was more an editing
> help/support than a topology rule that enforces correct topology. But is
> great to have this support at all!
> 
> Andreas
>