Spatial Point Pattern Objects

DESCRIPTION:

Creates an object of class spp representing a Spatial Point Pattern. This object is a data frame with columns identifying the locations of the observations. It could contain other columns as well.

USAGE:

spp(data, x, y, boundary=bbox(data), drop=F) 
is.spp(x) 
as.spp(x) 

OPTIONAL ARGUMENTS:

data
a matrix, or data frame containing the locations that define the spatial point pattern. This could be contained in any two columns of the data frame. If arguments x and y are not used then: if data has columns named "x" and "y", these will be understood to contain the corresponding locational information; otherwise, the first two columns will be assumed to contain the necessary information. Use arguments x and y for other options. If data is not specified then x and y must contain the coordinates.
x
horizontal coordinate of the spatial point pattern. This argument could be a numeric vector or a character string for different options. If data is given, then x must be a character string or an integer denoting one of the named columns of data. If data is missing, then x must be a vector giving the x-coordinates of the point pattern.
y
vertical coordinate of the spatial point pattern. This argument could be a numeric vector or a character string for different options just as x. If data is given, then y must be a character string or an integer denoting one of the named columns of data. This must be different that the one indicated by x. If data is missing, then y must be a vector giving the y-coordinates of the point pattern.
boundary
a list with named components "x" and "y" denoting the vertices of a convex polygon containing the point pattern. The first and last points do not need to coincide. The default is to consider the pattern bounded by a rectangle, its bounding box.
drop
logical flag: should the rest of the information (other columns) be left in the resulting data object (which also inherits from class "data.frame") or should the location information be extracted from data? Default is FALSE, that is, leave all other columns intact.

VALUE:

the spp function returns an object of class "spp". This is essentially a data frame with two special attributes: attribute "coords" which tells corresponding methods which columns denote locational information; and attribute "boundary" which contains the same information as entered in the argument boundary.

The is.spp function returns TRUE if object is of class "spp" and FALSE otherwise.

The as.spp function coerces a data frame or a matrix with at least two columns to a spatial point pattern object, an object of class "spp".

DETAILS:

This function is provided to facilitate calls to methods that act on a spatial point pattern such as the plot method.

The list to be entered as argument boundary can be obtained in several different ways. Use scan to read information that may be available in ASCII files. Use chull or bbox to get the convex hull or bounding box for the point pattern. You may follow those calls by a call to the function poly.expand which will "expand" the polygon by a given fraction keeping its original shape. The list can also be obtained interactively from a graphics device by plotting the x and y coordinates and using the output from locator(type="l") to represent the vertices of the surrounding polygon. If using the latter method, the user must ensure that the resulting polygon is convex, use the function is.convex.poly to this end.

SEE ALSO:

, , , , .

EXAMPLES:

lansing.m <- spp(lansing[lansing$species=="maple",]) 
# Make an spp object from the easting and northing locations of 
#   the aquifer data frame: 
aquifer.xy <- spp(aquifer, drop=T) 
myunif <- spp(x=runif(100), y=runif(100))