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Vector data processing in GRASS GIS

Vector data import and export

The v.in.ogr module offers a common interface for many different vector formats. Additionally, it offers options such as on-the-fly creation of new locations or extension of the default region to match the extent of the imported vector map. For special cases, other import modules are available, e.g. v.in.ascii for input from a text file containing coordinate and attribute data, and v.in.db for input from a database containing coordinate and attribute data. With v.external external maps can be virtually linked into a mapset, only pseudo-topology is generated but the vector geometry is not imported. The v.out.* set of commands exports to various formats.


The v.info display general information such as metadata and attribute columns about a vector map including the history how it was generated. Each map generating command stores the command history into the metadata (query with v.info -h mapname).

Vector map operations

GRASS vector map processing is always performed on the full map. If this is not desired, the input map has to be clipped to the current region beforehand (v.in.region, v.overlay,v.select).

Vector model and topology

GRASS is a topological GIS. This means that adjacent geographic components in a single vector map are related. For example in a non-topological GIS if two areas shared a common border that border would be digitized two times and also stored in duplicate. In a topological GIS this border exists once and is shared between two areas. Topological represenation of vector data helps to produce and maintain vector maps with clean geometry as well as enables certain analyses that can not be conducted with non-topological or spaghetti data. In GRASS topological data are refered to as level 2 data and spaghetti data is referred to as level 1.

Sometimes topology is not necessary and the additional memory and space requirements are burdensome to a particular task. Therefore two modules allow for working level 1 (non-topological) data within GRASS. The v.in.ascii module allows users to input points without building topology. This is very useful for large files where memory restrictions may cause difficulties. The other module which works with level 1 data is v.surf.rst which enables spatial approximation and topographic analysis from a point or isoline file.

In GRASS, the following vector objects are defined:

  • point: a point;
  • line: a directed sequence of connected vertices with two endpoints called nodes;
  • boundary: the border line to describe an area;
  • centroid: a point within a closed boundary;
  • area: the topological composition of centroid and boundary;
  • face: a 3D area;
  • kernel: a 3D centroid in a volume (not yet implemented);
  • volume: a 3D corpus, the topological composition of faces and kernel (not yet implemented).
  • Note that all lines and boundaries can be polylines (with nodes in between).

    The v.type module can be used to convert between vector types if possible. The v.build module is used to generate topology. It optionally allows to extract the erroneous vector objects into a separate map. Topological errors can be corrected either manually within v.digit or, to some extent, automatically in v.clean. Adjacent polygons can be found by v.to.db (see 'sides' option).

    Attribute management

    GRASS can be linked to one or many database management systems (DBMS). The db.* set of commands provides basic SQL support for attribute management, while the v.db.* set of commands operates on the vector map. When creating vector maps from scratch, in general an attribute table must be created and the table must be populated with one row per category (using v.to.db). However, this can be performed in a single step using v.db.addtable along with the definition of table column types. Column adding can be done with v.db.addcol. To drop a table from a map, use v.db.droptable. Values in a table can be updated with v.db.update.

    Editing vector attributes

    To manually edit attributes of a table, the map has to be queried in 'edit mode' using d.what.vect. To bulk process attributes, it is recommended to use SQL (db.execute).

    Geometry operations

    The module v.in.region saves the current region boundary into a vector area. Split vector lines can be changes to polylines by v.build.polylines. Long lines can be split by v.split and v.segment. Buffer and circles can be generated with v.buffer and v.parallel. 2D vector maps can be changed to 3D using v.drape or v.extrude. If needed, the spatial position of vector points can be perturbed by v.perturb. The v.type command changes between vector types (see list above). Projected vector maps can be reprojected with v.proj. Unprojected maps can be geocoded with v.transform. Triangulation and point-to-polygon conversions can be done with v.delaunay, v.hull, and v.voronoi. The v.random command generated random points.

    Vector overlays and selections

    Geometric overlay of vector maps is done with v.patch, v.overlay and v.select, depending on the combination of vector types. Vectors can be extracted with v.extract and reclassified with v.reclass.

    Vector statistics

    Statistics can be generated by v.qcount, v.sample, v.normal, and v.univar. Distances between vector objects are calculated with v.distance.

    Vector-Raster-DB conversion

    The v.to.db transfers vector information into database tables. With v.to.points, v.to.rast and v.to.rast3 conversions are performed.

    Vector queries

    Vector maps can be queried with v.what and v.what.vect.

    Vector-Raster queries

    Raster values can be transferred to vector maps with v.what.rast and v.rast.stats.

    Vector network analysis

    GRASS provides support for vector network analysis. The following algorithms are implemented: Vector directions are defined by the digitizing direction (a-->--b). Both directions are supported, network modules provide parameters to assign attribute columns to the forward and backward direction.

    Vector networks: Linear reference system (LRS)

    LRS uses linear features and distance measured along those features to positionate objects. There are the commands v.lrs.create to create a linear reference system, v.lrs.label to create stationing on the LRS, v.lrs.segment to create points/segments on LRS, and v.lrs.where to find line id and real km+offset for given points in vector map using linear reference system.

    The LRS tutorial explains further details.

    Interpolation and approximation

    Some of the vector modules deal with spatial or volumetric approximation (also called interpolation): v.kernel, v.surf.idw, v.surf.rst, and v.vol.rst.

    Lidar data processing

    Lidar point clouds (first and last ping) are imported with v.in.ascii (-b flag to not build the topology). Outlier detection is done with v.outlier on both first and last ping data. Then, with v.lidar.edgedetection, edges are detected from last ping data. The building are generated by v.lidar.growing from detected edges. The resulting data are post-processed with v.lidar.correction. Finally, the DTM and DSM are generated with v.surf.bspline (DTM: uses the 'v.lidar.correction' output; DSM: uses last ping output from outlier detection).

    See also

    Main index - vector index - full index