v.external(1grass) GRASS GIS User's Manual v.external(1grass)
NAME
v.external - Creates a new pseudo-vector map as a link to an OGR-sup-
ported 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 lo-
cation
-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 exter-
nal 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 differ-
ent from the topological format used by GRASS. Instead of true topol-
ogy, so called ’pseudo topology’ is created for data linked by v.exter-
nal. User should learn the difference between those two formats, be-
cause some modules working correctly with GRASS native data, can pro-
duce 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 at-
tributes 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_VEC-
TOR_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 di-
rectory (".").
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 Tech-
nical University in Prague, Czech Republic
SOURCE CODE
Available at: v.external source code (history)
Accessed: unknown
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GRASS 7.8.7 v.external(1grass)
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