## table of contents

v.qcount(1grass) | GRASS GIS User's Manual | v.qcount(1grass) |

# NAME¶

**v.qcount** - Indices for quadrat counts of vector
point lists.

# KEYWORDS¶

vector, statistics, point pattern

# SYNOPSIS¶

**v.qcount**

**v.qcount --help**

**v.qcount** [-**g**] **input**=*name*
[**layer**=*string*] [**output**=*name*]
**nquadrats**=*integer* **radius**=*float*
[--**overwrite**] [--**help**] [--**verbose**] [--**quiet**]
[--**ui**]

## Flags:¶

## 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* **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

# DESCRIPTION¶

*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 two-dimensional 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. 590-591)[1] and in Ripley
(1981; p. 102-106)[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).

# NOTES¶

This program may not work properly with lat-long data. It uses
*hypot()* in two files: *count.c* and *findquads.c*.

# KNOWN ISSUES¶

Timestamp not working for header part of counts output. (2000-10-28)

# REFERENCES¶

**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*, 9:325-359, 1922.

[4] F. N. David and P. G. Moore. Notes on contagious distributions
in plant populations. *Annals of Botany*, 18:47-53, 1954.

[5] J. B. Douglas. Clustering and aggregation. *Sankhya B*,
37:398-417, 1975.

[6] M. Lloyd. Mean crowding. *Journal of Animal Ecology*,
36:1-30, 1967.

[7] M. Morista. Measuring the dispersion and analysis of
distribution patterns. *Memoires of the Faculty of Science, Kyushu
University, Series E.* *Biology*, 2:215-235, 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

# SEE ALSO¶

*v.random,* *v.distance,* *v.neighbors,*
*v.perturb*

# AUTHORS¶

James Darrell McCauley

when he was at: Agricultural Engineering Purdue University

Modified for GRASS 5.0 by Eric G. Miller (2000-10-28)

Modified for GRASS 5.7 by R. Blazek (2004-10-14)

# SOURCE CODE¶

Available at: v.qcount source code (history)

Accessed: Saturday Jul 27 17:08:51 2024

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