[GRASS5] GRASS -> SF

[Morten Hulden]

On Thu, 19 Feb 2004, Radim Blazek wrote:

> On Wednesday 18 February 2004 18:52, Frank Warmerdam wrote:
> > Radim Blazek wrote:
> > > In past 4 years, you are probably the second person interested in that,
> > > so I don't spend much time documenting data model ...
> >
> > I sympathize, but I still think it will be valuable if you want people
> > to be able to take advantage of the new vector capabilities effectively
> > (and without undue hand holding).

Me too. I store many vectors in Postgis nowadays, and it would be 
really cool to be able to use v.digit and other Grass tools on them, as 
well as directly accessing Grass vectors from Mapserver using Postgis as 
the main storage, i.e. getting rid of all these exports/imports. 

...

> Would it be acceptable to duplicate features in more layers (if necessary)?
> ?_point
> ?_line
> ?_all     // contains features from both ?_point and ?_line
> 
> > > GV_BOUNDARY contains geometry and it is used to build areas.
> > > GV_LINE cannot form an area.
> > > I agree, that whole polygons must be available, but boundaries and
> > > centroid in raw form should be available as well (import to another
> > > topological GIS, display errors in data (boundary is not closed)).
> >
> > I think I still don't get how the boundaries work.  In a well formed
> > polygon layer, is the one closed boundary per polygon feature?  
> 
> No, polygon may be formed by many boundaries (more primitives but connected).
> One boundary is shared by adjacent areas.
> 
> +--1--+--5--+
> |     |     |
> 2  A  4  B  6
> |     |     |
> +--3--+--7--+
> 
> 1,2,3,4,5,6,7 = 7 boundaries (primitives)
> A,B = 2 areas
> 
> > If there are centroids how are they associated with their polygon?
> 
> Centroid are assigned to area it is within/inside (geometrically).

What about centroid rules? When I imported (+ rebuilt topology) polygons
from e00 to Postgis I had to drop part of the 'polygons' (actually,
I imported them as lines just like the open ended arcs) because inner
rings touched the outer ring at more than one single point. 

Following is an illegal inner ring. In Mapserver it is not possible to
fill A with a color without filling B too.

+---------+
|    A    |
+-----+   |
|     |   |
|  B  |   |
+-----+   |
|         |
+---------+

So, would the new Grass vector format allow this topology, and could flood 
fills still be handled correctly?

rgds
Morten Hulden