DESCRIPTION

v.external.out instructs GRASS to write vector maps in external data format (e.g. ESRI Shapefile, Mapinfo, and others) using OGR library. PostGIS data can be also written by built-in GRASS-PostGIS data provider.

NOTES

Number of available output formats (v.external.out -f) depends on OGR installation. 'PostgreSQL' format is presented also when GRASS comes with PostgreSQL support (check for '--with-postgres' in g.version -b output).

To store geometry and attribute data in PostGIS database ('PostgreSQL' format) GRASS uses built-in GRASS-PostGIS data provider. PostGIS data can be written also by OGR library when GRASS_VECTOR_OGR environmental variable is defined or GRASS is not compiled with PostgreSQL support.

Creation options refer to the output format specified by format option. See the list of valid creation options at OGR formats specification page, example for ESRI Shapefile or PostgreSQL/PostGIS format (section "Layer Creation Options"). Options are comma-separated pairs (key=value), the options are case-insensitive, eg. options="SCHEMA=myschema,FID=cat".

PostgreSQL/PostGIS Creation Options

Note that built-in GRASS-PostGIS data provider (format=PostgreSQL) supports different creation options compared to PostgreSQL/PostGIS driver from OGR library:

SCHEMA=<name> - name of schema where to create feature tables. If schema doesn't exists, it's automatically created when writing PostGIS data.
FID=<column> - name of column which will be used as primary key (feature id), default: fid
GEOMETRY_NAME=<column> name of column which will be used for storing geometry data in feature table, default: geom
SPATIAL_INDEX=YES|NO - enable/disable spatial index on geometry column, default: YES
PRIMARY_KEY=YES|NO - enable/disable primary key on FID column, default: YES
SRID=<value> - spatial reference identifier, default: not defined
TOPOLOGY=YES|NO - enable/disable native PostGIS topology, default: NO

Options relevant only to topological output (TOPOLOGY=YES):

TOPOSCHEMA_NAME=<schema name> - name of PostGIS Topology schema (relevant only for TOPOLOGY=YES), default: topo_<input>
TOPOGEOM_NAME=<column> - name of column which will be used for storing topogeometry data in feature table, default: topo
TOPO_TOLERANCE=<value> - tolerance for PostGIS Topology schema, see CreateTopology function for defails, default: 0
TOPO_GEO_ONLY=YES|NO - store in PostGIS Topology schema only data relevant to Topo-Geo data model, default: NO
SIMPLE_FEATURE=YES|NO - build simple features geometry in GEOMETRY_NAME column from topogeometry data, default: NO

Note that topological output requires PostGIS version 2 or later.

EXAMPLES

ESRI Shapefile

v.external.out can be used along with v.external to process external geodata in GRASS while writing out the results directly eg. in ESRI Shapefile format:

# register Shapefile in GRASS mapset:
v.external dsn=/path/to/shapefiles layer=cities

# define output directory for GRASS calculation results:
v.external.out dsn=$HOME/gisoutput

# do something (here: spatial query), write output directly as Shapefile
v.select ainput=cities atype=point binput=forests btype=area operator=within output=fcities

Current settings can be printed using -p or -g flag.

v.external.out -p

dsn: /path/to/home/gisoutput
format: ESRI Shapefile

PostGIS (simple features)

PostGIS data can be accessed directly using GRASS-PostGIS data provider (GRASS must be compiled with PostgreSQL support).

# register PostGIS table in GRASS mapset:
v.external dsn=PG:dbname=gisdb layer=cities

# define output PostGIS database for GRASS calculation results stored as simple features:
v.external.out dsn=PG:dbname=gisdb format=PostgreSQL

# do some processing...

Note: If the environment variable GRASS_VECTOR_OGR is defined, or GRASS is compiled without PostgreSQL support then GRASS will use PostgreSQL driver from OGR library for reading and writing PostGIS data.

PostGIS Topology

# define output PostGIS database for GRASS calculation results stored as topological elements:
v.external.out dsn=PG:dbname=gisdb format=PostgreSQL options=topology=YES

# do some processing...

Note: PostGIS topological access is supported only in built-in GRASS-PostGIS data provider.

GRASS native format

To restore original settings, ie. use GRASS native format, type:

v.external.out -r

Restore settings

Current settings can be stored to file by specifying output option.

# define output PostGIS database for GRASS calculation results stored as topological elements:
v.external.out dsn=PG:dbname=gisdb format=PostgreSQL options=topology=YES output=gisdb_topo.txt

# do some processing in PostGIS Topology

Back to native format:

v.external.out -r

# do some processing in native format

Restore previous settings from "gisdb_topo.txt" file by specifying input option.

v.external.out input=gisdb_topo.txt

# do some processing in PostGIS Topology

REFERENCES

AUTHOR

Martin Landa, Czech Technical University in Prague, Czech Republic (development supported by Fondazione Edmund Mach and Comune di Trento, Italy)

Last changed: $Date$