PostGIS

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

1. v.in.ogr (import only geometry column from postgresql/postgis)
2. v.clean

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,

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

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

See also

• Tutorial by Micha Silver