Ubuntu Manpage: vectorintro - Vector data processing

name
vector data processing in grass gis

NAME

       vectorintro - Vector data processing

       Vector data processing

Vector data processing in GRASS GIS

Vector maps in general
A "vector map" is a data layer consisting of a number of sparse features in geographic space. These might
be data points (drill sites), lines (roads), polygons (park boundary), volumes (3D CAD structure), or
some combination of all these. Typically each feature in the map will be tied to a set of attribute
layers stored in a database (road names, site ID, geologic type, etc.). As a general rule these can exist
in 2D or 3D space and are independent of the GIS's computation region.

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. To import and export only attribute tables, use
db.in.ogr and db.out.ogr. The naming convention for vector maps requires that map names start with a
character, not a number (map name scheme: [A-Za-z][A-Za-z0-9_]*).

Metadata
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). Metadata such as map title, scale, organization etc. can be
updated with v.support.

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 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 object types 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 ring of boundaries;

area: the topological composition of a closed ring of boundaries and optionally a
centroid;

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 vertices in between).

Area topology also holds information about isles. These isles are located within that area, not touching
the boundaries of the outer area. Isles consist of one or more areas and are used internally to maintain
correct topology for areas.

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 the user to extract 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. A dedicated vector editing module is v.edit which supports global and local editing operations.
Additionally, new advanced topological operations are available in the wxGUI vector digitizer. 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 boundaries. 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.

Vector object categories and attribute management
GRASS vectors 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 a table
linked to a vector map.

Categories
Categories are used to categorize vector objects and link attribute(s) to each category.
Each vector object can have zero, one or several categories. Category numbers do not have
to be unique for each vector object, several vector objects can share the same category.
Category numbers are stored both within the geometry file for each vector object and within
the (optional) attribute table(s) (usually the "cat" column). It is not required that
attribute table(s) hold an entry for each category, and attribute table(s) can hold
information about categories not present in the vector geometry file. This means that e.g.
an attribute table can be populated first and then vector objects can be added to the
geometry file with category numbers. Using v.category, category numbers can be printed or
maintained.

Layers
Layers are a characteristic of the vector feature (geometries) file. As mentioned above,
categories allow the user to give either a unique id to each feature or to group similar
features by giving them all the same id. Layers allow several parallel, but different
groupings of features in a same map. The practical benefit of this system is that it allows
placement of thematically distinct but topologically related objects into a single map, or
for linking time series attribute data to a series of locations that did not change over
time.
For example, one can have roads with one layer containing the unique id of each road and
another layer with ids for specific routes that one might take, combining several roads by
assigning the same id. While this example can also be dealt with in an attribute table,
another example of the use of layers that shows their specific advantage is the possibility
to identify the same geometrical features as representing different thematic objects. For
example, in a map with a series of polygons representing fields in which at the same time
the boundaries of these fields have a meaning as linear features, e.g. as paths, one can
give a unique id to each field as area in layer 1, and a unique id in layer 2 to those
boundary lines that are paths. If the paths will always depend on the field boundaries (and
might change if these boundaries change) then keeping them in the same map can be helpful.
Trying to reproduce the same functionality through attributes is much more complicated.
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. Taking the
example of the fields and paths, only some boundaries, but not all, might have a category
value in layer 2. With option=report, v.category lists available categories in each layer.
Optionally, each layer can (but does not have to) be linked to an attribute table. The link
is made by the category values of the features in that layer which have to have
corresponding entries in the specified key column of the table. Using v.db.connect
connections between layers and attribute tables can be listed or maintained.
In most modules, the first layer (layer=1) is active by default. Using layer=-1 one can
access all layers.

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 SQLite, 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. 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 v.db.join.

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. v.generalize is module for generalization of
GRASS vector maps. 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:

Network preparation and maintenance: v.net

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

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

Iso-distances (from centers): v.net.iso

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

Traveling salesman (round trip): v.net.salesman
--b). Both directions are supported, most 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, 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 return) are imported from text files with <a
href="v.in.ascii.html">v.in.ascii (use the -b flag to not build 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).

See also
Introduction to GRASS database management

Introduction to GRASS raster map processing

Introduction to GRASS 3D raster map (voxel) processing
full index