vectorintro(1)

Vector data processing in GRASS GIS

  Vector 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.

  Information
The v.info display general information such as metadata and attribute columns about a vector map including the history how it was generated.

  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.select).

  Vector model and topology
GRASS is a topological GIS. This means that adjacent geographic components in a single vector file 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 object types are available:

         point: a point

         line: a sequence of vertices connected by line(s) with two endpoints called nodes

         boundary: the border line of an area

         centroid: the label point of an area

         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 (not yet implemented)
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.

        Categories: The category number is the vector ID. It is used to link attribute(s) to each vector object. A vector object can have zero, one, two, or more categories. The category number is stored within the geometry file for each vector object. Using v.category the category numbers can be printed or maintained.

        Layers: It is possible to link the geographic objects in one vector file to one or more tables. Each link to a distinct attribute table is called a layer. GRASS layers do not contain any geographic objects, but consist of links to attribute tables in which vector objects can have zero or more categories. If a vector object has zero categories in a layer, then it does not appear in that layer. In this fasion some vector objects may appear in some layers but not in others. The practical benefit of this system is that it allows placement of thematically distinct but topologically related objects in a single file (e.g. forests and lakes). These virtual layers also are useful for linking time series attribute data to a series of locations that did not change over time. By default the first layer is active, i.e. the first table corresponds to the first layer. Further tables are linked to subsequent layers. Using v.db.connect layers can be listed or maintained.

        SQL support: The DBF driver provides only very limited SQL support (as DBF is not an SQL DB) while the other DBMS backends (such as PostgreSQL, MySQL etc) provide full SQL support since the SQL commands are sent directly to the DBMI. SQL commands can be directly executed with db.execute, db.select and the other db.* modules.
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.

  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.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 <a href="v.delaunay.html">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.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.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.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 maintenance: v.net

         Shortest path: d.path and v.net.path

         Traveling salesman (round trip): v.net.salesman

         Allocation of sources (create subnetworks, e.g. police station zones): v.net.alloc

         Minimum Steiner trees (star-like connections, e.g. broadband cable connections): v.net.steiner

         Iso-distances (from centers): v.net.iso
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 <a href="v.in.ascii.html">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 putput from outlier detection).

  See also

        Introduction to GRASS database management

        Introduction to GRASS raster map processing

        Introduction to GRASS 3D raster map (voxel) processing
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