v.qcount  Indices for quadrat counts of vector point lists.
vector, statistics, point pattern
v.qcount
v.qcount help
v.qcount [
g]
input=
name
[
layer=
string] [
output=
name]
nquadrats=
integer radius=
float
[
overwrite] [
help] [
verbose] [
quiet]
[
ui]
 g

Print results in shell script style
 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
 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
 output=name

Name for output quadrat centers map (number of points is written as
category)
 nquadrats=integer [required]

Number of quadrats
 radius=float [required]

Quadrat radius
v.qcount computes six different quadrat count statistics that provide a
measure of how much an user defined point pattern departs from a complete
spatial random point pattern.
Points are distributed following a complete spatial randomness (CSR) pattern if
events are equally likely to occur anywhere within an area. There are two
types departure from a CSR: regularity and clustering. Figure 1 gives an
example of a complete random, regular and a clustered pattern.
Figure 1: Realization of twodimensional Poisson processes of 50 points
on the unit square exhibiting (a) complete spatial randomness, (b)
regularity, and (c) clustering.
Various indices and statistics measure departure from CSR. The
v.qcount
function implements six different
quadrat count indices that are
described in Cressie (1991; p. 590591)[1] and in Ripley (1981; p. 102106)[2]
and summarized in Table 1.
Table 1: Indices for Quadrat Count Data. Adapted from Cressie [1],
this table shows the statistics computed for the quadrats in Figure
2.
These indices are computed as follows:
v.qcount chooses
nquadrads
circular quadrats of radius
radius such that they are completely within
the bounds of the current region and no two quadrats overlap. The number of
points falling within each quadrat are counted and indices are calculated to
estimate the departure of point locations from complete spatial randomness.
This is illustrated in Figure 2.
Figure 2: Randomly placed quadrats (n = 100) with 584 sample points.
The number of points is written as category to the
output map (and not to
an attribute table).
This program may not work properly with latlong data. It uses
hypot() in
two files:
count.c and
findquads.c.
v.random, v.distance, v.neighbors, v.perturb
General references include:
[1] Noel A. C. Cressie.
Statistics for Spatial Data. Wiley Series in
Probability and Mathematical Statistics. John Wiley & Sons, New York, NY,
1st edition, 1991.
[2] Brian D. Ripley.
Spatial Statistics. John Wiley \& Sons, New
York, NY, 1981.
References to the indices include:
[3] R. A. Fisher, H. G. Thornton, and W. A. Mackenzie. The accuracy of the
plating method of estimating the density of bacterial populations.
Annals
of Applied Biology, 9:325359, 1922.
[4] F. N. David and P. G. Moore. Notes on contagious distributions in plant
populations.
Annals of Botany, 18:4753, 1954.
[5] J. B. Douglas. Clustering and aggregation.
Sankhya B, 37:398417,
1975.
[6] M. Lloyd. Mean crowding.
Journal of Animal Ecology, 36:130, 1967.
[7] M. Morista. Measuring the dispersion and analysis of distribution patterns.
Memoires of the Faculty of Science, Kyushu University, Series E.
Biology, 2:215235, 1959.
A more detailed background is given in the tutorial:
[8] James Darrell McCauley 1993. Complete Spatial Randomness and Quadrat Methods
 GRASS Tutorial on v.qcount
Timestamp not working for header part of counts output. (20001028)
James Darrell McCauley
when he was at: Agricultural Engineering Purdue University
Modified for GRASS 5.0 by Eric G. Miller (20001028)
Modified for GRASS 5.7 by R. Blazek (20041014)
Last changed: $Date: 20180930 19:02:53 +0200 (Sun, 30 Sep 2018) $
Available at: v.qcount source code (history)
Main index  Vector index  Topics index  Keywords index  Graphical index 
Full index
© 20032018 GRASS Development Team, GRASS GIS 7.4.3 Reference
Manual