Working with external data in GRASS 7

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

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 wiki page for development issues.

Link external data

External vector data can be linked via v.external. List of supported formats can be determined by

v.external -f

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

 v.external dsn=ncshape/ layer=railroads

Assuming that railroads.shp is located in directory ncshape.

To link database-based data formats, eg. PostGIS

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 that database pgis_nc has PostGIS layer named bridges. This layer is linked to GRASS mapset as vector map 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

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
...

Digitize OGR layer using wxGUI

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

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)