Working with external data in GRASS 7

This page explains how to work with external data in GRASS 7. See also PostGIS.

Raster data

External raster data can be linked via r.external. List of supported formats can be determined by

r.external -f

To link file-based data formats, eg. GeoTiff

r.external input=ncrast/urban.tif output=urban

Vector data

See trac OGR interface and PG interface page for development issues.

Link external data

External vector data can be linked via v.external using OGR library or GRASS-PostGIS data driver (format=PostGIS). List of supported formats can be determined by

v.external -f

To link file-based data formats, eg. ESRI Shapefile using OGR library

v.external dsn=ncshape/ layer=railroads

Assuming that railroads.shp is located in directory ncshape.

To link database-based data formats, eg. PostGIS using OGR library

v.external dsn=PG:dbname=pgis_nc -l

Data source <PG:dbname=pgis_nc> (format 'PostgreSQL') contains 4 layers:
bridges
...

v.external dsn=PG:dbname=pgis_nc layer=bridges output=b

Assuming PostGIS layer named bridges is located in database pgis_nc. This layer is linked to GRASS mapset as vector map with name b. v.external also builds pseudo-topology over simple features which enables GRASS to access linked vector data on level 2. Note that data are stored as simple features, so no full topology support can be build based on this data.

PostGIS feature tables can be linked also using direct PG support (GRASS-PostGIS data driver). In this case PostGIS geometry data are accessed by GRASS directly without any abstract level (like OGR library). Accessing PostGIS directly using GRASS-PostGIS data driver (flag -p) is work-in-progress and it's highly experimental. Average time to access data is significantly reduced compared to OGR/PostgreSQL driver.

v.external -p dsn=PG:dbname=pgis_nc layer=bridges output=b

Direct access to external data

External data can be accessed using OGR library via virtual mapset OGR.

v.info map=PG:dbname=pgis_nc@OGR layer=bridges

Direct access using GRASS-PostGIS data driver is not currently possible.

Create new OGR layers using GRASS modules

v.extract showcase

OGR driver in GRASS 7 also supports write access to the external data. Showcase bellow:

v.external dsn=PG:dbname=pgis_nc layer=bridges output=b

v.out.ascii input=b where="cat < 10" --q

375171.4992779|317756.72097616|1
374247.5192779|317487.13697616|2
380230.2292779|316900.97897616|3
379191.4162779|316419.09697616|4
388958.8222779|316332.04697616|5
375875.2662779|316319.89597616|6
376393.5282779|316155.96797616|7
380647.5282779|316022.61797616|8
376739.6982779|315970.62597616|9

v.external.out dsn=PG:dbname=pgis_nc format=PostgreSQL

v.extract input=b output=b_9 where="cat < 10"

v.external dsn=PG:dbname=pgis_nc -l

Data source <PG:dbname=pgis_nc> (format 'PostgreSQL') contains 5 layers:
b_9
bridges
...

Example of direct access to external data without creating a link

v.out.ascii input=PG:dbname=pgis_nc@OGR layer=b_9

375171.4992779|317756.72097616|1
374247.5192779|317487.13697616|2
380230.2292779|316900.97897616|3
379191.4162779|316419.09697616|4
388958.8222779|316332.04697616|5
375875.2662779|316319.89597616|6
376393.5282779|316155.96797616|7
380647.5282779|316022.61797616|8
376739.6982779|315970.62597616|9

Create new empty OGR layer

Check connection settings:

v.external.out -p

dsn: PG:dbname=pgis_nc
format: PostgreSQL
options: <none>

New OGR layer can be created using v.edit, note that you need to specify feature type for newly created OGR layer (point, line or area).

v.edit map=pmap tool=create type=point

Check created OGR layer (direct access):

v.info -t map=PG:dbname=pgis_nc@OGR layer=pmap

...
points=0
...

Check created OGR layer (link):

v.external dsn=PG:dbname=pgis_nc layer=pmap
v.info -t map=pmap

Adding new point feature:

cat point.txt

P  1 1
375171.4992779 317756.72097616
1     1

v.edit -n map=pmap tool=add input=point.txt
v.info -t map=pmap

...
points=1
...

v.select speed test

Testing data:

• DBF input (attributes only)

g.mapset user1
db.connect -p

driver:dbf
database:$GISDBASE/$LOCATION_NAME/$MAPSET/dbf/

g.copy vect=bridges,b --o
g.copy vect=urbanarea,u --o

• SQLite input (attributes only)

g.mapset sqlite
db.connect -p

driver:sqlite
database:$GISDBASE/$LOCATION_NAME/$MAPSET/sqlite.db

g.copy vect=bridges,b --o
g.copy vect=urbanarea,u --o

• PostGIS input (geometry + attributes)

g.mapset pg
v.out.ogr input=bridges dsn=PG:dbname=pgis_nc format=PostgreSQL --o
v.external dsn=PG:dbname=pgis_nc layer=bridges output=b --o
v.out.ogr input=urbanarea dsn=PG:dbname=pgis_nc format=PostgreSQL --o
v.external dsn=PG:dbname=pgis_nc layer=urbanarea output=u --o

Writing output directly using OGR library:

v.external.out -p

dsn: PG:dbname=pgis_nc
format: PostgreSQL
options: <none>

• DBF input

time v.select ain=b@user1 atype=point bin=u@user1 btype=area out=b_u ope=overlap --o

real    0m6.059s
user    0m4.780s
sys     0m0.588s

• SQLite input

time v.select ain=b@sqlite atype=point bin=u@sqlite btype=area out=b_u ope=overlap --o

real    0m2.239s
user    0m1.084s
sys     0m0.524s

• PostGIS input

time v.select ain=b atype=point bin=u btype=area out=b_u ope=overlap --o

real    0m20.609s
user    0m7.920s
sys     0m1.644s

Note: Main reason of worse speed is random access used by v.select which is quite costly for OGR layers.

Note also speed of process for native output:

real    0m1.631s
user    0m0.860s
sys     0m0.768s

Digitize OGR layer using wxGUI

Example of digitizing PostGIS layer (linked via v.external) in wxGUI

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wxGUI - create and edit new OGR layer

Using wxGUI

From menu

File -> Link external formats

Link external data - Layer Manager menu

or from toolbar in Layer Manager.

Link external data - Layer Manager toolbar (1)

Link external data - Layer Manager toolbar (2)

wxGUI v.external dialog

wxGUI v.external.out dialog