## DESCRIPTION

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).

## NOTES

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.

Limitations:

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

    v.external -f

For details see GDAL web site.

## EXAMPLES

### ESRI Shapefile

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

### PostGIS layers

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.

### MapInfo files

Assuming that 'mapinfo_test' MapInfo file is located in the current directory (".").

    v.external input=./ layer=mapinfo_test output=grass_map

### SDTS files

Note: you have to select the CATD file

    v.external input=CITXCATD.DDF output=cities

### TIGER files

    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
    ...

## REFERENCES

OGR vector library C API documentation

## SEE ALSO

v.external.out, v.clean, v.db.connect, v.import, v.in.db, v.in.ogr, v.out.ogr

GDAL Library
PostGIS

See also GRASS user wiki page for more examples.

## AUTHORS

Radim Blazek, ITC-Irst, Trento, Italy
PostGIS support by Martin Landa, GeoForAll (OSGeoREL) Lab, Czech Technical University in Prague, Czech Republic

Last changed: $Date$