vector, import, external, OGR, PostGIS, level1
v.external [-ojfltb] input=string [layer=name] [where=sql_query] [output=name] [--overwrite] [--help] [--verbose] [--quiet] [--ui]
Override projection check (use current location’s projection)
Assume that the dataset has the same projection as the current location
Perform projection check only and exit
List supported formats and exit
List available layers in data source and exit
List available layers including feature type in data source and exit
Format: layer name,type,projection check,geometry
Do not build topology
Advantageous when handling a large number of points
Allow output files to overwrite existing files
Print usage summary
Verbose module output
Quiet module output
Force launching GUI dialog
- input=stringÂ [required]
Name of input OGR or PostGIS data source
Â Â Â Â Â Â Â Â ESRI Shapefile: directory containing a shapefile
Â Â Â Â Â Â Â Â MapInfo File: directory containing a mapinfo file
Â Â Â Â Â Â Â Â PostGIS database: connection string, eg. ’PG:dbname=db user=grass’
Name of OGR layer or PostGIS feature table to be linked
Â Â Â Â Â Â Â Â ESRI Shapefile: shapefile name
Â Â Â Â Â Â Â Â MapInfo File: mapinfo file name
Â Â Â Â Â Â Â Â PostGIS database: table name
WHERE conditions of SQL statement without ’where’ keyword
Example: income < 1000 and population >= 10000
Name for output GRASS vector map (default: input layer)
v.external creates new vector map as a link to external OGR layer or PostGIS feature table. OGR (Simple Features Library) is part of the GDAL library, so you need to install GDAL to use v.external for external OGR layers. Note that a PostGIS feature table can be linked also using built-in GRASS-PostGIS data driver (requires GRASS to be built with PostgreSQL support).
The simple feature data model used by OGR (or PostGIS) is very different from the topological format used by GRASS. Instead of true topology, so called ’pseudo topology’ is created for data linked by v.external. User should learn the difference between those two formats, because some modules working correctly with GRASS native data, can produce wrong results with input vector maps created by v.external.
Due to these data model differences v.external does not work with all data formats. In general, for all formats that do not have a key column (e.g. SHAPE file), attributes are not accessible, and attributes would get lost when modifying the geometries. Therefore it is generally not safe to link vector data with v.external. In many cases it does not make sense to use v.external linked data with simple features, instead vector data should be imported with v.import or v.in.ogr to get true topology support. Importantly, point cloud data which do not have topology, can be linked with v.external as long as there are no attributes attached to these point cloud data, or if the format of the point cloud data has a key column that allows linking vector geometries to attributes.
See v.db.connect for an example of maintaining attributes in external DBMS in also writable mode.
Supported OGR vector formats
To list supported OGR formats, type
For details see GDAL web site.
Assuming that ’test_shape.shp’ is located in directory ’/home/user/shape_data’.
v.external input=/home/user/shape_data layer=test_shape output=grass_map
By default, PostGIS links are created by built-in PostGIS support, ie. using GRASS-PostGIS data driver. If the environment variable GRASS_VECTOR_OGR exists, or GRASS is compiled without PostgreSQL support then GRASS will use OGR-PostgreSQL driver for creating a link.
List of layers for given data source can be printed by -l flag.
v.external input="PG:host=localhost user=postgres dbname=postgis" -l ... polymap ...
v.external input="PG:host=localhost user=postgres dbname=postgis" layer=polymap
Note: Authentication details (user password) can be preferably defined by db.login.
Assuming that ’mapinfo_test’ MapInfo file is located in the current directory (".").
v.external input=./ layer=mapinfo_test output=grass_map
Note: you have to select the CATD file
v.external input=CITXCATD.DDF output=cities
v.external input=input/2000/56015/ layer=CompleteChain,PIP output=t56015_all
Linking subset of features
By where option only subset of features can be linked. In the example below only one feature (in this case a vector tile) will be linked as a new GRASS vector map.
v.external in="PG:dbname=tiles" layer=wrs2_descending where="pr=191026" ... Number of areas: 1 ...
OGR vector library C API documentation
v.external.out, v.clean, v.db.connect, v.import, v.in.db, v.in.ogr, v.out.ogr
See also GRASS user wiki page for more examples.
Radim Blazek, ITC-Irst, Trento, Italy
PostGIS support by Martin Landa, GeoForAll (OSGeoREL) Lab, Czech Technical University in Prague, Czech Republic
Available at: v.external source code (history)
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