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.
v.external -f
v.external input=/home/user/shape_data layer=test_shape output=grass_map
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
v.external input=./ layer=mapinfo_test output=grass_map
v.external input=CITXCATD.DDF output=cities
v.external input=input/2000/56015/ layer=CompleteChain,PIP output=t56015_all
v.external in="PG:dbname=tiles" layer=wrs2_descending where="pr=191026" ... Number of areas: 1 ...
See also GRASS user wiki page for more examples.
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