Working with external data in GRASS 7

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Attention!

This page is under construction

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
...

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

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)
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