1v.decimate(1) GRASS GIS User's Manual v.decimate(1)
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6 v.decimate - Decimates a point cloud
7 Copies points from one vector to another while applying different deci‐
8 mations
9
11 vector, LIDAR, generalization, decimation, extract, select, points,
12 level1
13
15 v.decimate
16 v.decimate --help
17 v.decimate [-gfczxbt] input=name [layer=string] output=name
18 [zrange=min,max] [cats=range] [skip=integer] [preserve=integer]
19 [offset=integer] [limit=integer] [zdiff=float] [cell_limit=inte‐
20 ger] [--overwrite] [--help] [--verbose] [--quiet] [--ui]
21 Flags:
23 -g
24 Apply grid-based decimation
25 -f
27 Use only first point in grid cell during grid-based decimation
28 -c
30 Only one point per cat in grid cell
31 -z
33 Use z in grid decimation
34 -x
36 Store only the coordinates, throw away categories
37 Do not story any categories even if they are present in input data
38 -b
40 Do not build topology
41 Advantageous when handling a large number of points
42 -t
44 Do not create attribute table
45 --overwrite
47 Allow output files to overwrite existing files
48 --help
50 Print usage summary
51 --verbose
53 Verbose module output
54 --quiet
56 Quiet module output
57 --ui
59 Force launching GUI dialog
60 Parameters:
62 input=name [required]
63 Name of input vector map
64 Or data source for direct OGR access
65 layer=string
67 Layer number or name (’-1’ for all layers)
68 A single vector map can be connected to multiple database tables.
69 This number determines which table to use. When used with direct
70 OGR access this is the layer name.
71 Default: -1
72 output=name [required]
74 Name for output vector map
75 zrange=min,max
77 Filter range for z data (min,max)
78 cats=range
80 Category values
81 Example: 1,3,7-9,13
82 skip=integer
84 Throw away every n-th point
85 For example, 5 will import 80 percent of points. If not specified,
86 all points are copied
87 preserve=integer
89 Preserve only every n-th point
90 For example, 4 will import 25 percent of points. If not specified,
91 all points are copied
92 offset=integer
94 Skip first n points
95 Skips the given number of points at the beginning.
96 limit=integer
98 Copy only n points
99 Copies only the given number of points
100 zdiff=float
102 Minimal difference of z values
103 Minimal difference between z values in grid-based decimation
104 cell_limit=integer
106 Preserve only n points per grid cell
107 Preserves only the given number of points per grid cell in
108 grid-based decimation
109
111 v.decimate reduces number of points in the input vector map and copies
112 them over to the output vector map. Different point decimation tech‐
113 niques can be applied to reduce the number of points.
114 Two main decimation techniques are:
116 · count-based decimation (skip, preserve, offset and limit
118 options)
119 · grid-based decimation (-g flag)
121 The grid-based decimation will remove points based on:
123 · similar z coordinates (-z flag and zdiff option)
125 · same categories (-c flag)
127 · count of points (-f flag and cell_limit option)
129 The grid-based decimation is currently using a 2D grid, so the points
131 are placed and compared within this 2D grid. The comparison can happen
132 using z coordinates or categories. Note that although the grid is only
133 2D, the module works with 3D points.
134 The grid-based decimation extent and resolution depend on the current
136 computational region as set by g.region. As a consequence, the output
137 is limited only to computational region in this case.
138 TODO: Currently, any output is limited by the region.
140 The count-based decimation result highly depends on how the data are
142 ordered in the input. This applies especially to offset and limit
143 options where the resulting shape and densities can be surprising. The
144 options skip and preserve are influenced by order of points in a simi‐
145 lar way but they usually keep relative density of points (which may or
146 may not be desired). On the other hand, the grid-based decimation will
147 generally result in more even density of output points (see Figure 1).
148 Besides decimation, point count can be reduced by applying different
150 selections or filters, these are:
151 · selection by category (cats option)
153 · selection by z values (zrange option)
155
157 The grid-based decimation requires all points which will be saved in
158 output to fit into the computer’s memory (RAM). It is advantageous to
159 have the region only in the area with the points, otherwise unnecessary
160 memory is allocated. Higher (finer) resolutions and higher amount of
161 preserved points per cell require more memory. The count-based decima‐
162 tion has no limitation regarding the available memory.
163 Significant speed up can be gained using -b flag which disables build‐
165 ing of topology for the output vector map. This may limit the use of
166 the vector map by some modules, but for example, this module works
167 without topology as well.
168
170 Keep only every forth point, throw away the rest:
171 v.decimate input=points_all output=points_decimated_every_4 preserve=4
172 Keep only points within a grid cell (given by the current computational
174 region) which has unique categories (e.g. LIDAR classes):
175 v.decimate input=points_all output=points_decimated_unique_cats layer=1 -g -c
176 Figure 1: Comparison of original points, decimation result with every
178 forth point preserved, and grid-based decimation result with points
179 with unique categories in each grid cell
180 Keep only points with category 2 and keep only approximately 80% of the
182 points:
183 v.decimate input=points_all output=points_decimated_ skip=5 cats=2 layer=1
184
186 · Petras, V., Petrasova, A., Jeziorska, J., Mitasova, H. (2016).
187 Processing UAV and LiDAR point clouds in grass GIS. The Inter‐
188 national Archives of Photogrammetry, Remote Sensing and Spatial
189 Information Sciences, 41, 945 (DOI)
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192 v.extract, v.outlier, v.select, v.category, v.build, v.in.lidar,
193 g.region
194
196 Vaclav Petras, NCSU OSGeoREL
197
199 Available at: v.decimate source code (history)
200 Main index | Vector index | Topics index | Keywords index | Graphical
202 index | Full index
203 © 2003-2020 GRASS Development Team, GRASS GIS 7.8.5 Reference Manual
205GRASS 7.8.5 v.decimate(1)