vectorintro (1) - Linux Man Pages
Vector data processing in GRASS GIS
Vector data import and exportThe 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.
MetadataThe 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). Metadata such as map title, scale, organization etc. can be updated with v.support.
Vector map operationsGRASS 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 topologyGRASS 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.
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).
Many operations including extraction, queries, overlay, and export will
only act on features which have been assigned a category number. Typically
a centroid will hold the attribute data for the area between it and its
boundary. Boundaries are not typically given a category ID as it would be
ambiguous as to which area either side of it the attribute data would belong
to. An exception might be when the boundary between two crop-fields is the
center-line of a road, and the category information is an index to the road
name. For everyday use boundaries and centroids can be treated as internal
data types and the user can work directly and more simply with the "area"
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 and dropping
can be done with v.db.addcol and
v.db.dropcol. A table column can be renamed with
v.db.renamecol. To drop a table from a map, use
v.db.droptable. Values in a table can be updated
with v.db.update. Tables can be joined with with
GRASS layers do not contain any geographic objects, but they consist of links to attribute tables in which vector objects can have zero, one or more categories. If a vector object has zero categories in a layer, then it does not appear in that layer. In this fashion 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 into a single map (e.g. forests and lakes). These virtual layers are also 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.
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
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
Buffer and circles can be generated with v.buffer
v.generalize is module for generalization of GRASS vector maps.
2D vector maps can be changed to 3D using
If needed, the spatial position of vector points can be perturbed by
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
The v.random command generated random points.
Vector overlays and selections
Geometric overlay of vector maps is done with v.patch,
depending on the combination of vector types.
Vectors can be extracted with v.extract
and reclassified with v.reclass.
Statistics can be generated by v.qcount,
Distances between vector objects are calculated with v.distance.
The v.to.db transfers vector information
into database tables.
conversions are performed.
Vector maps can be queried with v.what and
Raster values can be transferred to vector maps with
Vector network analysis
GRASS provides support for vector network analysis. The following algorithms
Both directions are supported, network modules provide parameters
to assign attribute columns to the forward and backward direction.
Vector networks: Linear referencing 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,
v.lrs.where to find line id and real km+offset
for given points in vector map using linear reference system.
Interpolation and approximation
Some of the vector modules deal with spatial or volumetric
approximation (also called interpolation):
Lidar data processing
Lidar point clouds (first and last return) 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 return data.
Then, with v.lidar.edgedetection,
edges are detected from last return 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 return output
from outlier detection).
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).
Many operations including extraction, queries, overlay, and export will only act on features which have been assigned a category number. Typically a centroid will hold the attribute data for the area between it and its boundary. Boundaries are not typically given a category ID as it would be ambiguous as to which area either side of it the attribute data would belong to. An exception might be when the boundary between two crop-fields is the center-line of a road, and the category information is an index to the road name. For everyday use boundaries and centroids can be treated as internal data types and the user can work directly and more simply with the "area" meta-feature type.