Table of Contents
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
