spam (version 0.20-3)

nearestdist: Distance Matrix Computation

Description

This function computes and returns specific elements of distance matrix computed by using the specified distance measure.

Usage

nearest.dist( x, y=NULL, method = "euclidean",
             delta = 1, upper = FALSE,
             p=2, miles=TRUE, R=NULL,
             eps = NULL, diag = NULL)

Arguments

x

Matrix of first set of locations where each row gives the coordinates of a particular point. See also Details.

y

Matrix of second set of locations where each row gives the coordinates of a particular point. If this is missing x is used. See also Details.

method

the distance measure to be used. This must be one of "euclidean", "maximum", "minkowski" or "greatcircle". Any unambiguous substring can be given.

delta

only distances smaller than delta are recorded.

upper

Should the entire matrix (NULL) or only the upper-triagonal (TRUE) or lower-triagonal (FALSE) values be calculated.

p

The power of the Minkowski distance.

miles

For great circle distance: If true distances are in statute miles if false distances in kilometers.

R

For great circle distance: Radius to use for sphere to find spherical distances. If NULL the radius is either in miles or kilometers depending on the values of the miles argument. If R=1 then distances are of

eps

depreciated. Left for backwards consistency.

diag

depreciated. Left for backwards consistency. See details.

Value

• A spam object containing the distances spanned between zero and delta. The sparse matrix may contain many zeros (e.g., collocated data). However, to calculate covariances, these zeros are essential.

Details

For great circle distance, the by 2 matrices x and y contain the degrees longitudes in the first and the degrees latitudes in the second column. eps and delta are in degrees. The distance is in single precision (I am still not sure where I loose the double in the Fortran code) and if calculating the entire matrix upper=NULL (instead of adding its transpose) it may not pass the symmetry checks, for example. Default value of Earth's radius is 3963.34miles (6378.388km). The arguments eps and diag do not have any effect and are left for compatibility reasons. x and y can be any object with an existing as.matrix method. A quick scan revieled distance functions in at least 7 packages. The argument names should be as general as possible and be coherend with many (but not all) available distance functions. The Fortran code is based on a idea of Doug Nychka.

Examples

# Note that upper=T and using t(X)+X is quicker than upper=NULL;
#     upper=T marginally slower than upper=F.

# To compare nearest.dist with dist, use diag=FALSE, upper=TRUE
nx <- 4
x <- expand.grid(as.double(1:nx),as.double(1:nx))
sum( (nearest.dist( x, delta=nx*2, diag=FALSE, upper=TRUE)@entries-
              c(dist(x)))^2)

# Create nearest neighbor structures:
par(mfcol=c(1,2))
x <- expand.grid(1:nx,1:(2*nx))
display( nearest.dist( x, delta=1))
x <- expand.grid(1:(2*nx),1:nx)
display( nearest.dist( x, delta=1))