OGR data provider in GRASS 7

See also ​programmer's manual page and user-oriented ​wiki page on GRASS ​Wiki.

Direct OGR read access

Capability to read OGR layers directly without linking them via v.external.

Example 1: Shapefile 'busstopsall.shp' located in directory '~/shapefiles'

v.info map=~/shapefiles/@OGR layer=busstopsall

Example 2: PostGIS table 'busstopsall' located in database 'pgis_nc'.

v.extract input=PG:dbname=pgis_nc@OGR layer=busstopsall where="STREET_1 = 'William Moore Dr.'" output=b1

List of modules which supports direct OGR read access ((*) indicates work in progress):

• v.info (r39161), v.db.connect (r39538), v.extract (r39621), v.db.select (r39556), v.buffer (r39809), v.category (r39812), v.class (r39813), v.clean (r39814), v.delaunay (r39821), v.dissolve (r39822), v.distance (r39823), v.drape (r39826), v.extrude (r39827), v.generalize (r39828), v.hull (r39829), v.kcv (r39832), v.neighbors (r39834), v.normal (r39839), v.out.ascii (r39881), v.out.dxf (r39882), v.out.ogr (r39883), v.out.pov (r39884), v.out.svg (r39885), v.overlay (r39895), v.parallel (r39896), v.pertrub (r39897), v.qcount (r39898), v.report (r39905), v.sample (r39906), v.segment (r39907), v.reclass (r39908), v.random (r39909), v.rast.stats (r39910), v.split, v.surf.idw, v.surf.rst, v.to.3d, v.to.db, v.to.points, v.to.rast, v.to.rast3, v.transform, v.type, v.univar, v.what (r40188), v.select (r48936)

Source code: read_ogr.c​

Status: Currently all modules in GRASS 7 should support direct OGR read access.

Direct OGR write access

Capability to write OGR layers directly without need to store data in the native GRASS vector format and then exporting data using v.out.ogr.

Testcase:

• GRASS location: nc_spm_08 - ​http://grass.osgeo.org/sampledata/nc_spm_latest.tar.gz
• Shapefile directory: ~/shapefiles - ​http://grass.osgeo.org/sampledata/north_carolina/nc_shape.tar.gz
• PostGIS database: pgis_nc (see how to ​export GRASS data to PostGIS using ​wxGUI Modeler)

Scenario 1 (not implemented)

Use parameters output for output OGR datasource and olayer for OGR layer. Example:

# input: GRASS data
# output: PostGIS database
v.extract input=bridges where="BRIDGE_NUM=417" output=bridges_417

# input: PostGIS layer, ESRI Shapefile
# output: PostGIS database
v.select ainput=PG:dbname=gisdb@OGR alayer=zeleznice \
 binput=shps@OGR blayer=obce operator=intersects \
 output=PG:dbname=gisdb@OGR olayer=obce_zelez

Status: not planned to be implemented

Scenario 2 (implemented)

Design v.external.out (similarly to r.external.out)

# input: GRASS data
# output: PostGIS database
v.external.out dsn=PG:dbname=pgis_nc format=PostgreSQL
v.extract input=bridges where="BRIDGE_NUM=417" output=bridges_417

# input: PostGIS layer, ESRI Shapefile
# output: PostGIS database
v.select ainput=PG:dbname=pgis_nc@OGR alayer=roadsmajor \
 binput=~/shapefiles@OGR blayer=boundary_municp operator=intersects \
 output=municp_roads

Status: implemented grass/trunk/vector/v.external.out. Only limited number of modules in GRASS 7 (those which writes data in simple-features-like form) allows to write output vector data directly using OGR library.

Tasks

• Attribute table (write) access for OGR-linked vectors (direct or linked by v.external)
  • Implemented in r47225

Changes in vector library

Source code: write_ogr.c​

• New functions
  • V1_write_line_ogr(), V2_write_line_ogr()
  • V1_rewrite_line_ogr(), V2_rewrite_line_ogr()
  • V1_delete_line_ogr(), V2_delete_line_ogr()
• Vect_build_ogr() support all build levels - r48960
• Implement V2__add_line_to_topo_ogr() - r49178

Supported GRASS modules

Current status

Module	Read access (link)	Direct read access	Direct write access
v.build	Y	N	N
v.build.all	Y	N	N
v.clean	Y	Y	N
v.edit	Y	Y	Y
v.extract	Y	Y	Y
v.out.dxf	Y	Y	N
v.select	Y	Y	Y
v.random	Y	Y	Y
v.voronoi	Y	Y	N
v.what	Y	Y	X
v.what.rast	Y	Y	Y
v.what.vect	Y	Y	Y

Optimalization

Most of GRASS modules are designed to access features randomly which is basically quick for GRASS native vector data, but very very slow when accessing features via OGR library, see example bellow

    nlines = Vect_get_num_lines(&In);
    for (line = 1; line <= nlines; line++) {
        Vect_read_line(&In, Points, Cats, line);
    }

• Test for vector map with 100.000 points

Native

real    0m0.194s
user    0m0.164s
sys     0m0.024s

OGR-link

real    0m35.616s
user    0m7.880s
sys     0m3.700s

Rewriting the code to use sequential access causes significant speed-up for OGR-links:

    while(TRUE) {
        if (Vect_read_next_line(&In, Points, Cats) == -2)
            break;
    }

OGR-link

real    0m0.831s
user    0m0.444s
sys     0m0.016s

Simple test of rendering speed

See r47875

• vector map 'bridges' (points=10938)

d.vect bridges type=point

Native:

real    0m0.400s
user    0m0.388s
sys     0m0.008s

Linked PostGIS layer:

real    0m0.828s
user    0m0.568s
sys     0m0.016s

(before r47875)

real    0m5.850s
user    0m1.860s
sys     0m0.464s

• vector map 'roadsmajor' (lines=355)

d.vect roadsmajor type=line

Native:

real    0m0.103s
user    0m0.092s
sys     0m0.008s

Linked PostGIS layer:

real    0m0.521s
user    0m0.128s
sys     0m0.028s

• vector map 'urbanarea' (areas=666)

d.vect urbanarea type=centroid,area

Native:

real    0m0.540s
user    0m0.520s
sys     0m0.016s

Linked PostGIS layer:

real    0m1.236s
user    0m0.736s
sys     0m0.068s

Related OGR tickets

• ​#4202
• ​#4217