PostGIS: Difference between revisions
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Revision as of 21:35, 30 December 2011
See also working with external data in GRASS 7.
Help pages
- Database management in GRASS GIS help page
- SQL support in GRASS GIS help page
- PostgreSQL driver in GRASS help page
- ODBC driver in GRASS help page
Link to external dataset
- input maps: v.external
- (in GRASS 6 links are read only, for GRASS 7 see working with external data in GRASS 7)
- output maps: v.external.out (GRASS 7 only)
Import into GRASS
There is a fundamental difference between the PostGIS format which is non-topological (OGC simple feature-based) and the internal GRASS format which is topological and which, thus, does not really allow for overlapping polygons. You can digitize them, but they are not really useful...
Link to GRASS
You can register a PostGIS layer into GRASS using v.external, eg. PostGIS layer 'lakes' from database 'pgis_nc'
v.external dsn=PG:dbname=pgis_nc layer=lakes
It will create in the current mapset new vector map 'lakes', the module also builds pseudo-topology for this map,
To link a PostGIS layer from non-public schema
v.external dsn=PG:<connection sting> layer=<schema>.<layer> output=<layer>
Important note: In GRASS 6 this link is read-only, in GRASS 7 it's possible to modify linked layers directly via OGR library, see working with external data in GRASS 7.
You can also link a GRASS layer to a PostgreSQL attribute table, though v.db.connect.
Export to PostGIS
To export GRASS vector map layer to PostGIS layer use v.out.ogr, eg.
v.out.ogr in=lakes@PERMANENT dsn=PG:dbname=pgis_nc format=PostgreSQL type=area
Note that exporting data can be quite time-consuming task, especially when input vector map attributes are stored in DBF format. It's recommended to store attribute data in SQLite format rather then in old-fashioned DBF format. For example when exporting vector map 'lakes' from North Carolina sample dataset.
- Attributes in DBF format
real 1m15.072s user 1m6.160s sys 0m3.824s
- Attributes in SQLite format
real 0m11.796s user 0m5.564s sys 0m4.148s
1) create schema with name of input mapset
2) copy vector maps from selected mapset to the current and stores their attributes in SQLite database
3) exports GRASS vector maps to the selected PostGIS database
See also wxGUI Modeler.
Direct access to PostGIS data
Important note: Direct read/write access is implemented only in GRASS 7.
Direct read access
GRASS 7 enables the users to access PostGIS layers directly via virtual mapset 'OGR' and modules parameters map=OGR_datasource@OGR and layer=OGR_layer, eg. to access PostGIS layer 'lakes' from database 'pgis_nc'
v.info map=PG:dbname=pgis_nc@OGR layer=lakes
The direct read access avoids need of creating a link via v.external and accessing PostGIS data directly without creating any data elements in the current mapset. The main drawback of direct read access is that the pseudo-topology is built each time when accessing the data.
Direct write access
Note: direct write access via OGR library is currently under development (GRASS 7 only).
GRASS 7 allows to write output vector map directly via OGR library. For defining output vector data format is designed v.external.out module. For example
v.external.out dsn=PG:dbname=pgis_nc format=PostgreSQL
causes that every newly created vector map will be stored as PostGIS layer in database 'pgis_nc' without any data elements created in the current mapset. Such PostGIS layer can be linked afterwards via v.external or accessed directly as described in the section above. For example
v.extract input=lakes out=reservoir where="FTYPE = 'RESERVOIR'" v.info map=PG:dbname=pgis_nc@OGR layer=reservoir
To switch back to GRASS native format enter
v.external.out -r