Provided by: grass-doc_7.8.2-1build3_all 
NAME
v.generalize - Performs vector based generalization.
KEYWORDS
vector, generalization, simplification, smoothing, displacement, network generalization,
topology, geometry
SYNOPSIS
v.generalize
v.generalize --help
v.generalize [-lt] input=name [layer=string] [type=string[,string,...]] output=name
[error=name] method=string threshold=float [look_ahead=integer] [reduction=float]
[slide=float] [angle_thresh=float] [degree_thresh=integer] [closeness_thresh=float]
[betweeness_thresh=float] [alpha=float] [beta=float] [iterations=integer]
[cats=range] [where=sql_query] [--overwrite] [--help] [--verbose] [--quiet] [--ui]
Flags:
-l
Disable loop support
Do not modify end points of lines forming a closed loop
-t
Do not copy attributes
--overwrite
Allow output files to overwrite existing files
--help
Print usage summary
--verbose
Verbose module output
--quiet
Quiet module output
--ui
Force launching GUI dialog
Parameters:
input=name [required]
Name of input vector map
Or data source for direct OGR access
layer=string
Layer number or name (’-1’ for all layers)
A single vector map can be connected to multiple database tables. This number
determines which table to use. When used with direct OGR access this is the layer
name.
Default: -1
type=string[,string,...]
Input feature type
Options: line, boundary, area
Default: line,boundary,area
output=name [required]
Name for output vector map
error=name
Error map with failed generalizations
Lines and boundaries causing errors (collapsed to a point or topology errors)
method=string [required]
Generalization algorithm
Options: douglas, douglas_reduction, lang, reduction, reumann, boyle,
sliding_averaging, distance_weighting, chaiken, hermite, snakes, network, displacement
douglas: Douglas-Peucker Algorithm
douglas_reduction: Douglas-Peucker Algorithm with reduction parameter
lang: Lang Simplification Algorithm
reduction: Vertex Reduction Algorithm eliminates points close to each other
reumann: Reumann-Witkam Algorithm
boyle: Boyle’s Forward-Looking Algorithm
sliding_averaging: McMaster’s Sliding Averaging Algorithm
distance_weighting: McMaster’s Distance-Weighting Algorithm
chaiken: Chaiken’s Algorithm
hermite: Interpolation by Cubic Hermite Splines
snakes: Snakes method for line smoothing
network: Network generalization
displacement: Displacement of lines close to each other
threshold=float [required]
Maximal tolerance value
Options: 0-1000000000
look_ahead=integer
Look-ahead parameter
Default: 7
reduction=float
Percentage of the points in the output of ’douglas_reduction’ algorithm
Options: 0-100
Default: 50
slide=float
Slide of computed point toward the original point
Options: 0-1
Default: 0.5
angle_thresh=float
Minimum angle between two consecutive segments in Hermite method
Options: 0-180
Default: 3
degree_thresh=integer
Degree threshold in network generalization
Default: 0
closeness_thresh=float
Closeness threshold in network generalization
Options: 0-1
Default: 0
betweeness_thresh=float
Betweeness threshold in network generalization
Default: 0
alpha=float
Snakes alpha parameter
Default: 1.0
beta=float
Snakes beta parameter
Default: 1.0
iterations=integer
Number of iterations
Default: 1
cats=range
Category values
Example: 1,3,7-9,13
where=sql_query
WHERE conditions of SQL statement without ’where’ keyword
Example: income < 1000 and population >= 10000
DESCRIPTION
v.generalize is a module for the generalization of GRASS vector maps. This module consists
of algorithms for line simplification, line smoothing, network generalization and
displacement (new methods may be added later).
The cats and where options are used only if a layer > 0 is specified, otherwise, those
options are ignored. Be aware that the default is layer=-1, meaning that all layers are
processed, ignoring the cats and where options.
If type=area is selected, boundaries of selected areas will be generalized, and the
options cats, where, and layer will be used to select areas.
NOTES
(Line) simplification is a process of reducing the complexity of vector features. The
module transforms a line into another line consisting of fewer vertices, that still
approximate the original line. Most of the algorithms described below select a subset of
points on the original line.
(Line) smoothing is a "reverse" process which takes as input a line and produces a
smoother approximate of the original. In some cases, this is achieved by inserting new
vertices into the original line, and can total up to 4000% of the number of vertices in
the original. In such an instance, it is always a good idea to simplify the line after
smoothing.
Smoothing and simplification algorithms implemented in this module work line by line, i.e.
simplification/smoothing of one line does not affect the other lines; they are treated
separately. For isolated loops formed by a single line/boundary, he first and the last
point of each line/boundary can be translated and/or deleted, unless the -l flag is used
to disable loop support.
Lines and boundaries are not translated if they would collapse to a single point.
Boundaries are not translated if they would intersect with themselves or other boundaries.
Such erroneous features are written to an optional error vector map. Overlaying the error
map over the generalized map indicates the kind of error. Lines/boundaries collapsing to
a point are written out as points, boundaries violating topology are written out as
boundaries. The error map can be overlaid over the generalized map to understand why some
features were not generalized.
SIMPLIFICATION
Simplification can fail for many boundaries if the simplification parameters would result
in a large reduction of vertices. If many lines/boundaries could not be simplified, try
different parameters that would cause a lower degree of simplification.
v.generalize contains following line simplification algorithms:
• Douglas-Peucker Algorithm
• Douglas-Peucker Reduction Algorithm
• Lang Algorithm
• Vertex Reduction
• Reumann-Witkam Algorithm
• Remove Small Lines/Areas
Different algorithms require different parameters, but all the algorithms have one
parameter in common: the threshold parameter, given in map units (for latitude-longitude
locations: in decimal degree). In general, the degree of simplification increases with the
increasing value of threshold.
ALGORITHM DESCRIPTIONS
• Douglas-Peucker - "Quicksort" of line simplification, the most widely used
algorithm. Input parameters: input, threshold. For more information, see:
http://geomalgorithms.com/a16-_decimate-1.html.
• Douglas-Peucker Reduction Algorithm is essentially the same algorithm as the
algorithm above, the difference being that it takes an additional reduction
parameter which denotes the percentage of the number of points on the new line
with respect to the number of points on the original line. Input parameters:
input, threshold, reduction.
• Lang - Another standard algorithm. Input parameters: input, threshold, look_ahead.
For an excellent description, see:
http://www.sli.unimelb.edu.au/gisweb/LGmodule/LGLangVisualisation.htm.
• Vertex Reduction - Simplest among the algorithms. Input parameters: input,
threshold. Given a line, this algorithm removes the points of this line which are
closer to each other than threshold. More precisely, if p1 and p2 are two
consecutive points, and the distance between p2 and p1 is less than threshold, it
removes p2 and repeats the same process on the remaining points.
• Reumann-Witkam - Input parameters: input, threshold. This algorithm quite
reasonably preserves the global characteristics of the lines. For more
information, see for example:
http://psimpl.sourceforge.net/reumann-witkam.html.
Douglas-Peucker and Douglas-Peucker Reduction Algorithm use the same method to simplify
the lines. Note that
v.generalize input=boundary_county output=boundary_county_dp20 method=douglas threshold=20
is equivalent to
v.generalize input=boundary_county output=boundary_county_dp_red20_100 \
method=douglas_reduction threshold=20 reduction=100
However, in this case, the first method is faster. Also observe that douglas_reduction
never outputs more vertices than douglas, and that, in general, douglas is more efficient
than douglas_reduction. More importantly, the effect of
v.generalize input=boundary_county output=boundary_county_dp_red0_30 \
method=douglas_reduction threshold=0 reduction=30
is that ’out’ contains approximately only 30% of points of ’in’.
SMOOTHING
The following smoothing algorithms are implemented in v.generalize:
• Boyle’s Forward-Looking Algorithm - The position of each point depends on the
position of the previous points and the point look_ahead ahead. look_ahead
consecutive points. Input parameters: input, look_ahead.
• McMaster’s Sliding Averaging Algorithm - Input Parameters: input, slide,
look_ahead. The new position of each point is the average of the look_ahead
points around. Parameter slide is used for linear interpolation between old and
new position (see below).
• McMaster’s Distance-Weighting Algorithm - Takes the weighted average of look_ahead
consecutive points where the weight is the reciprocal of the distance from the
point to the currently smoothed point. The parameter slide is used for linear
interpolation between the original position of the point and newly computed
position where value 0 means the original position. Input parameters: input,
slide, look_ahead.
• Chaiken’s Algorithm - "Inscribes" a line touching the original line such that the
points on this new line are at least threshold apart. Input parameters: input,
threshold. This algorithm approximates the given line very well.
• Hermite Interpolation - This algorithm takes the points of the given line as the
control points of hermite cubic spline and approximates this spline by the points
approximately threshold apart. This method has excellent results for small values
of threshold, but in this case it produces a huge number of new points and some
simplification is usually needed. Input parameters: input, threshold,
angle_thresh. Angle_thresh is used for reducing the number of the points. It
denotes the minimal angle (in degrees) between two consecutive segments of a line.
• Snakes is the method of minimisation of the "energy" of a line. This method
preserves the general characteristics of the lines but smooths the "sharp corners"
of a line. Input parameters input, alpha, beta. This algorithm works very well
for small values of alpha and beta (between 0 and 5). These parameters affect the
"sharpness" and the curvature of the computed line.
One of the key advantages of Hermite Interpolation is the fact that the computed line
always passes through the points of the original line, whereas the lines produced by the
remaining algorithms never pass through these points. In some sense, this algorithm
outputs a line which "circumscribes" the input line.
On the other hand, Chaiken’s Algorithm outputs a line which "inscribes" a given line. The
output line always touches/intersects the centre of the input line segment between two
consecutive points. For more iterations, the property above does not hold, but the
computed lines are very similar to the Bezier Splines. The disadvantage of the two
algorithms given above is that they increase the number of points. However, Hermite
Interpolation can be used as another simplification algorithm. To achieve this, it is
necessary to set angle_thresh to higher values (15 or so).
One restriction on both McMasters’ Algorithms is that look_ahead parameter must be odd.
Also note that these algorithms have no effect if look_ahead = 1.
Note that Boyle’s, McMasters’ and Snakes algorithm are sometimes used in the signal
processing to smooth the signals. More importantly, these algorithms never change the
number of points on the lines; they only translate the points, and do not insert any new
points.
Snakes Algorithm is (asymptotically) the slowest among the algorithms presented above.
Also, it requires quite a lot of memory. This means that it is not very efficient for
maps with the lines consisting of many segments.
DISPLACEMENT
The displacement is used when the lines overlap and/or are close to each other at the
current level of detail. In general, displacement methods move the conflicting features
apart so that they do not interact and can be distinguished.
This module implements an algorithm for displacement of linear features based on the
Snakes approach. This method generally yields very good results; however, it requires a
lot of memory and is not very efficient.
Displacement is selected by method=displacement. It uses the following parameters:
• threshold - specifies critical distance. Two features interact if they are closer
than threshold apart.
• alpha, beta - These parameters define the rigidity of lines. For larger values of
alpha, beta (>=1), the algorithm does a better job at retaining the original shape
of the lines, possibly at the expense of displacement distance. If the values of
alpha, beta are too small (<=0.001), then the lines are moved sufficiently, but
the geometry and topology of lines can be destroyed. Most likely the best way to
find the good values of alpha, beta is by trial and error.
• iterations - denotes the number of iterations the interactions between the lines
are resolved. Good starting points for values of iterations are between 10 and
100.
The lines affected by the algorithm can be specified by the layer, cats and where
parameters.
NETWORK GENERALIZATION
Used for selecting "the most important" part of the network. This is based on the graph
algorithms. Network generalization is applied if method=network. The algorithm calculates
three centrality measures for each line in the network and only the lines with the values
greater than thresholds are selected. The behaviour of algorithm can be altered by the
following parameters:
• degree_thresh - algorithm selects only the lines which share a point with at least
degree_thresh different lines.
• closeness_thresh - is always in the range (0, 1]. Only the lines with the
closeness centrality value at least closeness_thresh apart are selected. The lines
in the centre of a network have greater values of this measure than the lines near
the border of a network. This means that this parameter can be used for selecting
the centre(s) of a network. Note that if closeness_thresh=0 then everything is
selected.
• betweeness_thresh - Again, only the lines with a betweeness centrality measure at
least betweeness_thresh are selected. This value is always positive and is larger
for large networks. It denotes to what extent a line is in between the other lines
in the network. This value is large for the lines which lie between other lines
and lie on the paths between two parts of a network. In the terminology of road
networks, these are highways, bypasses, main roads/streets, etc.
All three parameters above can be presented at the same time. In that case, the algorithm
selects only the lines which meet each criterion.
Also, the outputed network may not be connected if the value of betweeness_thresh is too
large.
EXAMPLES
SIMPLIFICATION EXAMPLE
Simplification of county boundaries with DP method (North Carolina sample dataset),
threshold given in mapset units (here: meters):
v.generalize input=boundary_county output=boundary_county_dp20 \
method=douglas threshold=20 error=boundary_county_dp20_leftover
Figure: Vector simplification example (spatial subset: original map shown in black,
simplified map with 26% remaining vertices shown in red)
SMOOTHING EXAMPLE
Smoothing of road network with Chaiken method (North Carolina sample dataset), threshold
given in mapset units (here: meters):
v.generalize input=roads output=roads_chaiken method=chaiken \
threshold=1 error=roads_chaiken_leftover
Figure: Vector smoothing example (spatial subset: original map shown in black, smoothed
map with 500% increased number of vertices shown in red)
SEE ALSO
v.clean, v.dissolve
v.generalize Tutorial (GRASS-Wiki)
AUTHORS
Daniel Bundala, Google Summer of Code 2007, Student
Wolf Bergenheim, Mentor
Partial rewrite: Markus Metz
SOURCE CODE
Available at: v.generalize source code (history)
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