v.external - Creates a new pseudo-vector map as a link to an
OGR-supported layer or a PostGIS feature table.
vector, import, external, OGR, PostGIS, level1
v.external
v.external --help
v.external [-
ojfltb]
input=
string
[
layer=
name] [
where=
sql_query]
[
output=
name] [--
overwrite] [--
help]
[--
verbose] [--
quiet] [--
ui]
- -o
-
Override projection check (use current location’s projection)
Assume that the dataset has the same projection as the current location
- -j
-
Perform projection check only and exit
- -f
-
List supported formats and exit
- -l
-
List available layers in data source and exit
- -t
-
List available layers including feature type in data source and exit
Format: layer name,type,projection check,geometry
- -b
-
Do not build topology
Advantageous when handling a large number of points
- --overwrite
-
Allow output files to overwrite existing files
- --help
-
Print usage summary
- --verbose
-
Verbose module output
- --quiet
-
Quiet module output
- --ui
-
Force launching GUI dialog
- input=string [required]
-
Name of input OGR or PostGIS data source
Examples:
ESRI Shapefile: directory containing a shapefile
MapInfo File: directory containing a mapinfo file
PostGIS database: connection string, eg. ’PG:dbname=db
user=grass’
- layer=name
-
Name of OGR layer or PostGIS feature table to be linked
Examples:
ESRI Shapefile: shapefile name
MapInfo File: mapinfo file name
PostGIS database: table name
- where=sql_query
-
WHERE conditions of SQL statement without ’where’ keyword
Example: income < 1000 and population >= 10000
- output=name
-
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.
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.
To list supported OGR formats, type
v.external -f
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
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
GDAL Library
PostGIS
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
Last changed: $Date: 2017-11-18 09:57:44 +0100 (Sat, 18 Nov 2017) $
Available at: v.external source code (history)
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