PostGIS data provider in GRASS 7

PostGIS data can be accessed in GRASS 7 using GRASS-OGR data provider or preferably by native GRASS-PostGIS data provider. Note that GRASS-OGR data provider supports only simple feature access.

Development of PostGIS data provider has been supported by ​Fondazione Edmund Mach and ​Comune di Trento (Italy).

This data provider allows GRASS to read (see read_pg.c​) and write (see write_pg.c​) PostGIS data directly without any external library (in opposite to GRASS-OGR data provider). GRASS-PostGIS data provider is implemented using ​libpq library.

Note that GRASS-PostGIS data provider is compiled only when GRASS is configured with --with-postgres switch.

Overview of subroutines to read PostGIS data:

Level	Sequantial access	Random access
1 (without topology)	​V1_read_next_line_pg()	​V1_read_line_pg()
2 (with pseudo-topology/topology)	​V2_read_next_line_pg()	​V2_read_line_pg()

Overview of subroutines to write PostGIS data:

Level	Write new feature	Delete existing feature	Rewrite existing feature
1 (without topology)	​V1_write_line_pg()	​V1_rewrite_line_pg()	​V1_delete_line_pg()
2 (with pseudo-topology - simple feature access)	​V2_write_line_sfa()	​V2_rewrite_line_sfa()	​V2_delete_line_sfa()
2 (with topology - PostGIS Topology)	​V2_write_line_pg()	​V2_delete_line_pg()	​V2_rewrite_line_pg()

See ​programmer's manual page for complete overview.

By default data are read/write as simple features. If PostGIS Topology schema is defined then the data provider reads/writes topological elements from provided schema.

Note that direct access similarly to GRASS-OGR data provider is not currently implemented. In other words to access PostGIS data (using GRASS-PostGIS data provider) is required to create a link using v.external command.

Known issues

• v.build opt=cdump produces empty output

Simple features access

GRASS-PostGIS provider allows to read and write PostGIS data stored as simple features on level 1 (without topology) and also on level 2 (in this case a pseudo-topology is built over simple features).

Note: For forcing GRASS to use GRASS-OGR data provider instead of GRASS-PostGIS data provider when accessing/creating PostGIS specify environmental variable GRASS_VECTOR_OGR.

Export GRASS vector data to PostGIS as simple features:

v.out.postgis input=urbanarea dsn=PG:dbname=grass

Read example:

# create a link to PostGIS table
v.external dsn=PG:dbname=grass layer=urbanarea
# access data as normal vector map in GRASS
v.info map=urbanarea
...
 |----------------------------------------------------------------------------|
 | Map format:      PostGIS (PostgreSQL)                                      |
 | DB table:        public.urbanarea                                          |
 | DB name:         grass                                                     |
 | Geometry column: geom                                                      |
 | Feature type:    polygon                                                   |
 | Topology:        pseudo (simple features)                                  |
 |----------------------------------------------------------------------------|
...

Write example:

# define output vector format
v.external.out dsn=PG:dbname=grass format=PostgreSQL
# create random points map
v.random out=test_points n=1000
# access created external data
v.info test_points
...
 |----------------------------------------------------------------------------|
 | Map format:      PostGIS (PostgreSQL)                                      |
 | DB table:        public.test_points                                        |
 | DB name:         grass                                                     |
 | Geometry column: geom                                                      |
 | Feature type:    point                                                     |
 | Topology:        pseudo (simple features)                                  |
 |----------------------------------------------------------------------------|
...

For more examples see ​wiki page from GRASS Wiki.

Topological access

Export GRASS vector data to PostGIS as topological elements:

v.out.postgis -l input=urbanarea dsn=PG:dbname=grass

Read example:

# create a link to PostGIS table
v.external dsn=PG:dbname=grass layer=urbanarea
# access data as normal vector map in GRASS
v.info map=urbanarea
...
 |----------------------------------------------------------------------------|
 | Map format:      PostGIS (PostgreSQL)                                      |
 | DB table:        public.urbanarea                                          |
 | DB name:         grass                                                     |
 | Geometry column: geom                                                      |
 | Feature type:    polygon                                                   |
 | Topology:        PostGIS (schema: topo_urbanarea)                          |
 | Topology column: topo                                                      |
 |----------------------------------------------------------------------------|
...

Write example:

# define output vector format
v.external.out dsn=PG:dbname=grass format=PostgreSQL options=topology=YES
# create random points map
v.random out=test_points n=1000
# access created external data
v.info test_points
...
 |----------------------------------------------------------------------------|
 | Map format:      PostGIS (PostgreSQL)                                      |
 | DB table:        public.test_points                                        |
 | DB name:         grass                                                     |
 | Geometry column: geom                                                      |
 | Feature type:    point                                                     |
 | Topology:        PostGIS (schema: topo_test_points)                        |
 | Topology column: topo                                                      |
 |----------------------------------------------------------------------------|
...

For more information see PostGIS Topology editing page and ​wiki page from GRASS Wiki.