[GRASS-user] grass topography and postgis

[Alex Mandel]

The simple answer is that Postgis and Shapefiles etc store data per
record, all the information for a polygon is stored in a geometry which
defines it's points and if several points then a line or a polygon.
This system allows for overlap and duplication as each object is
independent.

GRASS, Arc Coverages etc store points, and lines between them. Each line
 has an object to either side of it. Even if that object is empty space,
that just means there's not related attribute table. In this type of
storage it's impossible to have overlap or duplicates all points and
lines are stored only once. The spatial information is stored separately
from the attribute data and every part of it can have a relationship to
many different tables.

I realize thats not a very clear answer but it's a fundamental difference.

You are correct that you want to snap features while digitizing in order
to create shared boundaries. In a shapefile the shared boundary would
exist twice in GRASS that line would only exist once.

So I see a few options:
1. digitize in GRASS and export to Postgis afterward converting to
non-topological format with ogr. v.out.ogr should make it easy to write
out your data.

2. digitize in QGIS or some other editor that edits directly on Postgis

Alex

Matt B wrote:
> Hi Markus,
> I possibly don't have a full understanding of what topology is. I thought it
> was more or less the geometry, with a few rules relating different objects
> to each other. Attributes are things like place names or a value at a
> location, within a boundary etc. As postgis allows the storage of geometries
> then isn't this the topology? I've not had much luck with grass getting it
> to recognise things like shared boundaries (so I only have to digitise them
> once) apart from using the snap to vertice feature (though as postgis seems
> the most useful at the moment I'm concentrating on that).
> 
> Thanks
> Matt
> 
> 
> 
> On Tue, Dec 2, 2008 at 7:45 PM, Markus Neteler <neteler at osgeo.org> wrote:
> 
>> Matt,
>>
>> On Tue, Dec 2, 2008 at 4:31 AM, Matt B <mattslists at gmail.com> wrote:
>>> Hi Group,
>>> I'd like to set up grass to edit / work with data in a postgis database.
>> So
>>> far I've figured out (and been informed, thanks Richard) that grass
>> stores
>>> the topologoly data in it's own database and the external database is
>> just
>>> used for attributes.
>> GRASS stores per default all data (geometry + attributes) in its own format
>> (i.e., attributes in a DBF file in GRASS 6 and in SQLite in GRASS 7).
>>
>> Optionally attributes can be stored in PostgreSQL (not PostGIS), Mysql,
>> SQLite, and ODBC.
>>
>>> Is there a way to have grass store the topology data in
>>> postgis so I can easily work with the data in different applications?
>> Yes, using v.external. But note that this is a read-only connection.
>> We hope that in future this will be read-write.
>>
>> The issue behind this is that GRASS is a topological GIS while
>> PostGIS is Simple Features (no topology per default). So there
>> is a "conversion" needed between these two worlds.
>>
>> Markus
>>
> 
>