quadratcount

Quadrat counting for a point pattern

Divides window into quadrats and counts the numbers of points in each quadrat.

Usage

quadratcount(X, ...)
  ## S3 method for class 'ppp':
quadratcount(X, nx=5, ny=nx, \dots,
               xbreaks=NULL, ybreaks=NULL, tess=NULL)
  ## S3 method for class 'splitppp':
quadratcount(X, \dots)

Details

Quadrat counting is an elementary technique for analysing spatial point patterns. See Diggle (2003).

If X is a point pattern, then by default, the window containing the point pattern X is divided into an nx * ny grid of rectangular tiles or `quadrats'. (If the window is not a rectangle, then these tiles are intersected with the window.) 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.

Alternatively, quadrats of any shape may be used. The argument tess can be a tessellation (object of class "tess") whose tiles will serve as the quadrats. The algorithm counts the number of points of X falling in each quadrat, and returns these counts as a contingency table.

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. To calculate an estimate of intensity based on the quadrat counts, use intensity.quadratcount.

To extract the quadrats used in a quadratcount object, use as.tess.

If X is a split point pattern (object of class "splitppp" then quadrat counting will be performed on each of the components point patterns, and the resulting contingency tables will be returned in a list. This list can be printed or plotted.

Marks attached to the points are ignored by quadratcount.ppp. To obtain a separate contingency table for each type of point in a multitype point pattern, first separate the different points using split.ppp, then apply quadratcount.splitppp. See the Examples.

Note

To perform a chi-squared test based on the quadrat counts, use quadrat.test.

References

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.

See Also

plot.quadratcount, intensity.quadratcount, quadrats, quadrat.test, tess, hextess, quadratresample, miplot

Aliases

• quadratcount
• quadratcount.ppp
• quadratcount.splitppp

Examples

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)

 # irregular window
 data(humberside)
 plot(humberside)
 qH <- quadratcount(humberside, 2, 3)
 plot(qH, add=TRUE, col="blue", cex=1.5, lwd=2)

 # multitype - split
 plot(quadratcount(split(humberside), 2, 3))
 
 # quadrats determined by tessellation:
 B <- dirichlet(runifpoint(6))
 qX <- quadratcount(X, tess=B)
 plot(X, pch="+")
 plot(qX, add=TRUE, col="red", cex=1.5, lty=2)