GRASS-OGR data provider

Table of Contents

• Pseudo-topology
• Frmt file format specification
• Fidx file format specification

by GRASS Development Team (https://grass.osgeo.org)

Pseudo-topology

Reduced topology: each boundary is attached to one area only, i.e. smoothing, simplification, removing small areas etc. will not work properly for adjacent areas or areas within areas.

Full topology is only available for native GRASS vectors or can only be built after all polygons are converted to areas and cleaned as done by v.in.ogr.

Frmt file format specification

Frmt is a plain text file which contains basic information about external format of linked vector map. Each line contains key, value pairs separated by comma.

OGR specific format is described by:

• FORMAT - ogr
• DSN - OGR datasource name
• LAYER - OGR layer name

Example:

FORMAT: ogr
DSN: /path/to/shapefiles
LAYER: cities

OGR layer can be linked via v.external command. When linking OGR layer pseudo-topology ('topo') is built including spatial index file ('sidx') and category index file ('cidx'). Additionally also feature index file (see Fidx file format specification) is created.

Fidx file format specification

Note: finfo is an instance of Format_info structure.

Name	Type	Number	Description
Version_Major	C	1	file version (major)
Version_Minor	C	1	file version (minor)
Back_Major	C	1	supported from GRASS version (major)
Back_Minor	C	1	supported from GRASS version (minor)
byte_order	C	1	little or big endian flag; files are written in machine native order but files in both little and big endian order may be readl; zero for little endian
length	L	1	header size
fInfo.ogr.offset_num	I	1	number of records
fInfo.ogr.offset	I	offset_num	offsets