[GRASSLIST:725] Re: tiger2k questions (listproc is driving me
warmerdam at pobox.com
Tue Jul 15 09:15:21 EDT 2003
Radim Blazek wrote:
> On Friday 11 July 2003 16:53, Frank Warmerdam wrote:
>>OGR's data model for Tiger is closely tied to the underlying tiger data
>>model. One downside of that is that the polygon layer consists of features
>>with the polygons attributes but no geometries.
> It is not downside but very important advantage for GRASS :)
> It is not very practical to use 8 commands instead of 1. As it is
> now, it cannot be too much simplified and the solution within GRASS
> would be probably v.in.tiger based on OGR, but that is what
> we wanted to avoid and to have only one import module, see below.
>>Ideally the OGR TIGER/Line driver should do this for you (at least
>>optionally) but I have not implemented that.
> I thought that OGR does this for all supported layers. What it does
> for ArcInfo coverage? But as I mentioned above, for GRASS, SFS polygon
> is the problem not raw data.
> The best for GRASS would be if OGR could optionaly offer data in both
> SFS and topological form, if available (Tiger, Coverage).
> Otherwise OGR builds polygons from topology, v.in.ogr imports polygons
> and v.clean must be used to split polygons and remove duplicates.
I would agree that it is desirable for OGR to offer access to topological
vector datasets in some means where v.in.ogr could take care of the eight
steps you delimited automatically. To do this, as I see it, I would need
to modify OGR to keep track of the relationship between polygon records, and
their corresponding arcs in a standard means. In other systems I have kept
metadata on polygon layers indicating the corresponding arc layer, and marking
which arc attributes were the right/left polygon indicator. This would
in theory be enough information for v.in.ogr to build a topological dataset
on import, right?
I notice you utilized the PIP (point-in-polygon) layer. Is this important?
I set the clouds in motion - turn up | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush | Geospatial Programmer for Rent
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