Provided by: grass-doc_8.3.0-1_all 
NAME
v.external - Creates a new pseudo-vector map as a link to an OGR-supported layer or a
PostGIS feature table.
KEYWORDS
vector, import, external, OGR, PostGIS, level1
SYNOPSIS
v.external
v.external --help
v.external [-ojfltb] input=string [layer=name] [where=sql_query] [output=name]
[--overwrite] [--help] [--verbose] [--quiet] [--ui]
Flags:
-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
Parameters:
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)
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
SOURCE CODE
Available at: v.external source code (history)
Accessed: Tuesday Jun 27 11:14:09 2023
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