DESCRIPTION
v.out.postgis exports an existing GRASS vector map layer to
PostGIS feature table. Features without category are skipped.
By default GRASS GIS topological features are converted into simple
features
(see OGC Simple
Feature Access specification for details). Flag -l allows
to export vector features as topological elements stored
in PostGIS
Topology schema. Note that topological export requires PostGIS
version 2 or later.
Additional creation options can be defined by options parameter:
- FID=<column> - name of column which will be used as
primary key (feature id), default: fid
- GEOMETRY_NAME=<column> name of column which will
be used for storing geometry data in feature table, default: geom
- SPATIAL_INDEX=YES|NO - enable/disable creating spatial
index on geometry column, default: YES
- PRIMARY_KEY=YES|NO - enable/disable adding primary key
on FID column, default: YES
- SRID=<value> - spatial reference identifier,
default: not defined
PostGIS Topology related options (relevant only for -l flag):
- TOPOSCHEMA_NAME=<schema name> - name of PostGIS
Topology schema, default: topo_<input>
- TOPOGEOM_NAME=<column> - name of column which
will be used for storing topogeometry data in feature table,
default: topo
- TOPO_TOLERANCE=<value> - tolerance for PostGIS
Topology schema,
see CreateTopology
function for defails, default: 0
- TOPO_GEO_ONLY=YES|NO - store in PostGIS Topology schema
only data relevant to Topo-Geo data model,
default: NO
Creation options are comma-separated pairs
(key=value), the options are case-insensitive. Note
that options defined
by v.external.out are
ignored by v.out.postgis.
v.out.postgis optionally also creates a new vector map in the
current mapset if output_link is defined.
NOTES
By default v.out.postgis exports vector data as simple
features, ie. boundaries and centroids (forming topological
areas) become polygons, isles become holes. Geometry of simple feature
elements is stored in PostGIS feature table in the column named
"geom". Name of the geometry column can be changed
by options=GEOMETRY_NAME=<column>. Note that for
exporting vector features as simple features can be alternatively
used PostgreSQL driver
from OGR library
through v.out.ogr module.
Also note that it's allowed to store in the feature table only
features of the same type, ie. mixing of points and lines is not
currently possible. The feature type is determined for output feature
table from the first vector feature accessing data sequentially
(type=auto). Vector features of other types are during export
skipped. User can choose preferable feature type by type
parameter. Only single type is currently allowed (see TODO section for
details).
v.out.postgis currently supports only three basic output
simple feature types: Points, Linestrings and Polygons. Also 3D
features of the same type are supported, eg. 3D points are exported
as PointZ simple feature. Faces are exported as 3D
polygons. 3D features are written to the output automatically if input
vector map is 3D. If -2 flag is given then the output is always
2D (z-coordinate is silently ignored for 3D input vector maps).
Multigeometries are not currently supported. Features with the same
category are exported as multiple singe features.
v.out.postgis also allows exporting vector features as
topological elements
in PostGIS
Topology schema. PostGIS Topology extension uses three tables to
store basic topological elements which forms topological objects like
areas or isles in GRASS terminology. Nodes (0-dimensional
topological elements) are stored in "node"
table, edges (1-dimensional elements) in "edge"
table and faces (2-dimensional elements) in "face"
table.
- GRASS nodes are stored in node table
- GRASS points are stored in node table as regular nodes
- GRASS centroids are stored in node table as regular nodes
("containing_face" refers to related area)
- GRASS lines are stored in edge table
- GRASS boundaries are stored in edge table
- GRASS areas are stored in face table
Tables node, edge and face are stored in given
topological schema. By default v.out.postgis defines its name
as topo_<input>. Alternatively, the name for topology
schema can be defined
by options=TOPOSCHEMA_NAME=<name>.
EXAMPLES
Export Simple Features
Export vector map "urbanarea" as feature table "urbanarea"
located in database "grass", schema "public". Note that this
database schema is automatically used when not defined by the user.
v.out.postgis input=urbanarea output="PG:dbname=grass"
GRASS areas are converted into polygons, isles into holes. We can
check the number or created polygons by simple SQL query bellow.
db.select driver=pg database=grass \
sql="SELECT ST_GeometryType(geom) as geom_type, count(*) from urbanarea group by geom_type"
geom_type|count
ST_Polygon|657
Note: same procedure can be done
by v.out.ogr module, eg.
v.out.ogr input=urbanarea output="PG:dbname=grass" format=PostgreSQL
In this case GRASS vector data are exported to PostGIS database using
OGR library, namely using PostgreSQL driver. Contrary to
the v.out.ogr
module, v.out.postgis is using directly PostGIS data provider
which is part of GRASS vector engine. Beside
that, v.out.postgis is optimized for PostGIS export including
topological access to the data.
Export data into specific database schema
Database schema for storing exported data can be defined
by output_layer as
<schema_name>.<table_name>. If the specified
schema doesn't exist in the database, then it's automatically created.
Export vector map "bridges" as feature table in database
schema "grassout".
v.out.postgis input=bridges output="PG:dbname=grass" output_layer=grassout.bridges
Export data with creation options
Example bellow demonstrates how to define name for geometry column and
disable building spatial index. Spatial reference system is defined
by srid identifier which corresponds in this case with EPSG
3358 (North Carolina dataset).
v.out.postgis input=roadsmajor output="PG:dbname=grass" options="GEOMETRY_NAME=wkb_geometry,SPATIAL_INDEX=NO,SRID=3358"
Link exported data
Exported data can be linked as vector map created in the current
mapset by specifying output_link parameter. In the example below
vector map "busstopsall" from PERMANENT mapset is exported
into "grass" PostGIS database. v.out.postgis after
successful export also creates in the current mapset GRASS vector map
as a link to the PostGIS feature table.
v.out.postgis input=busstopsall@PERMANENT output="PG:dbname=grass" output_link=busstopsall_pg
Created link can be checked
by v.info:
v.info busstopsall_pg
...
|----------------------------------------------------------------------------|
| Map format: PostGIS (PostgreSQL) |
| DB table: public.busstopsall |
| DB name: grass |
| Geometry column: geom |
| Feature type: point |
| Topology: pseudo (simple features) |
|----------------------------------------------------------------------------|
...
Export data without attributes
v.out.postgis allows ignoring attributes when exporting
vector features by specifying -t flag.
Command bellow exports vector features without attributes. The feature
will contain only two columns, the fid and geometry column.
v.out.postgis -t input=railroads output="PG:dbname=grass"
Export topological data
By default v.out.postgis exports data as simple
features. Flag -l allows exporting data as topological elements
instead of simple features. Export topological elements is stored in
PostGIS
Topology schema.
v.out.postgis -l input=busroutesall output="PG:dbname=grass"
For more info about PostGIS Topology implementation in GRASS see
the wiki
page.
TODO
- Multi-feature export
- Allow mixed features (points, lines)
- Support other simple feature types like GeometryCollection and others
- Implement missing options
from v.out.ogr: -a, -s, -c, -p, -n
- Add options: cats, where
REQUIREMENTS
- PostGIS 2.x or later for topological export (flag -l)
REFERENCES
SEE ALSO
v.out.ogr,
v.external,
v.external.out,
v.in.ogr
See also PostGIS
and PostGIS
Topology wiki page from GRASS User Wiki.
AUTHOR
Martin Landa, Czech Technical University in Prague, Czech Republic
(development supported by Fondazione Edmund Mach and Comune di Trento, Italy)
Last changed: $Date$