Ubuntu Manpage: v.external - Creates a new pseudo-vector map as a link to an OGR-supported layer or a

Provided by: grass-doc_7.8.2-1build3_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

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)

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