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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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       © 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual