Re: [GRASS-user] grass topography and postgis
Topological datas are intrinsically topological: it's not about rules (that you can control with geometrical operators) but the geometrical structures. In topological geodatas, for example, two adjacent polygons share a common, single, boundary, while in Simple Feature model two polygons have their own boundaries. You could have a look at: http://grass.itc.it/grass63/manuals/html63_user/vectorintro.html This means that GRASS needs to use its own data model, because its algorithms code are based on it, and it wouldn't work on Simple Features (Postgis data model, Shapefiles, etc.). Hope this helps, Giovanni tt B [EMAIL PROTECTED]: 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 [EMAIL PROTECTED] wrote: Matt, On Tue, Dec 2, 2008 at 4:31 AM, Matt B [EMAIL PROTECTED] 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 ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] grass topography and postgis
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 [EMAIL PROTECTED] wrote: Matt, On Tue, Dec 2, 2008 at 4:31 AM, Matt B [EMAIL PROTECTED] 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 ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] grass topography and postgis
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 [EMAIL PROTECTED] wrote: Matt, On Tue, Dec 2, 2008 at 4:31 AM, Matt B [EMAIL PROTECTED] 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 ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] grass topography and postgis
On Tue, Dec 2, 2008 at 11:12 AM, Matt B [EMAIL PROTECTED] 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. Inofficial definition, without reflecting too much about it: - Geometry is the geometry (primitives like points, lines, boundaries, centroids etc). - Topology is the relationship between these primitives. - Attributes are values/descriptions logically attached to the geometry (in GRASS, the cat column is used for that, cat is also stored in the geometry). In topological GIS (eg GRASS) common boundaries are shared while in non-topological GIS (eg ESRI, PostGIS,ok that's no real GIS) they are not, obviously leading to more work to keep data consistent (i.e., avoid slivers and gaps). Markus ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] grass topography and postgis
Matt, On Tue, Dec 2, 2008 at 4:31 AM, Matt B [EMAIL PROTECTED] 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 ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] grass topography and postgis
I think there is some confusion here: The main Postgis functionality is in line with the OGC SFS model, which is inherently NOT topological. This means that under Postgis as it currently stands, GRASS topological geometries cannot be stored. It does not mean that using Postgis, topological structures cannot be stored, if suitable structures are built to support them, either with the PostGIS umbrella or as a third party toolset.. It is possible to store topological geometries in a relational database, ESRI geodatabases Oracle are a couple of examples where this can be done. See http://postgis.refractions.net/support/wiki/index.php?PostgisTopology for some work with PostGIS in this direction. If GRASS users are interested in having native PostGIS support for GRASS topological vectors, then it might be worth keeping in touch with this project, ans see if it can be kept in line with the GRASS topological data model to facilitate a future.GRASS data management role. Cheers, Brent Wood Brent Wood DBA/GIS consultant NIWA, Wellington New Zealand G. Allegri [EMAIL PROTECTED] 12/02/08 11:27 PM Topological datas are intrinsically topological: it's not about rules (that you can control with geometrical operators) but the geometrical structures. In topological geodatas, for example, two adjacent polygons share a common, single, boundary, while in Simple Feature model two polygons have their own boundaries. You could have a look at: http://grass.itc.it/grass63/manuals/html63_user/vectorintro.html This means that GRASS needs to use its own data model, because its algorithms code are based on it, and it wouldn't work on Simple Features (Postgis data model, Shapefiles, etc.). Hope this helps, Giovanni tt B [EMAIL PROTECTED]: 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 [EMAIL PROTECTED] wrote: Matt, On Tue, Dec 2, 2008 at 4:31 AM, Matt B [EMAIL PROTECTED] 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 ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user NIWA is the trading name of the National Institute of Water Atmospheric Research Ltd. ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] grass topography and postgis
Matt B 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. Is there a way to have grass store the topology data in postgis so I can easily work with the data in different applications? Thanks Matt Not that I know of (Not a definitive answer) but maybe you should look at using QGIS as your front-end to Postgis since this is what it was designed for originally. Alex ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
