quadratcount {spatstat} | R Documentation |
Divides window into quadrats and counts the numbers of points in each quadrat.
quadratcount(X, nx=5, ny=nx, xbreaks, ybreaks)
X |
A point pattern
(object of class "ppp" ).
|
nx,ny |
Numbers of quadrats in the x and y directions.
Incompatible with xbreaks and ybreaks .
|
xbreaks |
Numeric vector giving the x coordinates of the
boundaries of the quadrats. Incompatible with nx .
|
ybreaks |
Numeric vector giving the y coordinates of the
boundaries of the quadrats. Incompatible with ny .
|
Quadrat counting is an elementary technique for analysing spatial point patterns. See Diggle (2003).
The window containing the point pattern X
is divided into
an nx * ny
grid of rectangular tiles or `quadrats'.
The number of points of X
falling in each quadrat is
counted. These numbers are returned as a contingency table.
If xbreaks
is given, it should be a numeric vector
giving the x coordinates of the quadrat boundaries.
If it is not given, it defaults to a
sequence of nx+1
values equally spaced
over the range of x coordinates in the window X$window
.
Similarly if ybreaks
is given, it should be a numeric
vector giving the y coordinates of the quadrat boundaries.
It defaults to a vector of ny+1
values
equally spaced over the range of y coordinates in the window.
The lengths of xbreaks
and ybreaks
may be different.
The algorithm counts the number of points of X
falling in each quadrat, and returns these counts as a
contingency table. The [i,j]
entry in the contingency table
is the point count for the quadrat with coordinates
(xbreaks[i],xbreaks[i+1])
by (ybreaks[i], ybreaks[i+1])
.
The return value is a table
which can be printed neatly.
The return value is also a member of the special class
"quadratcount"
. Plotting the object will display the
quadrats, annotated by their counts. See the examples.
To perform a chi-squared test based on the quadrat counts,
use quadrat.test
.
A contingency table containing the number of points in each
quadrat.
The table is also an object of the special class "quadratcount"
and there is a plot method for this class.
Adrian Baddeley adrian@maths.uwa.edu.au http://www.maths.uwa.edu.au/~adrian/ and Rolf Turner rolf@math.unb.ca http://www.math.unb.ca/~rolf
Diggle, P.J. Statistical analysis of spatial point patterns. Academic Press, 2003.
Stoyan, D. and Stoyan, H. (1994) Fractals, random shapes and point fields: methods of geometrical statistics. John Wiley and Sons.
X <- runifpoint(50) quadratcount(X) quadratcount(X, 4, 5) quadratcount(X, xbreaks=c(0, 0.3, 1), ybreaks=c(0, 0.4, 0.8, 1)) qX <- quadratcount(X, 4, 5) # plotting: plot(X, pch="+") plot(qX, add=TRUE, col="red", cex=1.5, lty=2)