Function that applies another function to nearest neighbors of a specified location?

Question

I'm sure this is quite basic, but I just couldn't find anything appropriate: I am looking for a function that applies another function to variables of a SpatialPointsDataFrame, but only for locations that are the nearest neighbors of a specified location.

Alternatively, how can I subset a SpatialPointsDataFrame by distance to a given location?

For example, in the meuse data set (gstat package) I would like to calculate mean, variance and quantiles of zinc concentration for the 20 closest locations that are at most 0.1km away from location x=179997, y=331662.

I found the function applynbd() in the spatstat package, but it applies the given function to all points in the data frame insteat of a single one. Moreover, only a number of nearest neighbors OR a distance (max. radius) can be specified instead of both.

Answer 1

The following code will get you a list of distances from a point to a set of points by using the spDistsN1 function from the sp package. Let's assume that the point you are looking for is not part of the meuse SpatialPointsDataFrame:

require(sp)
data(meuse)
coordinates(meuse) = c("x","y")
dist_vector = spDistsN1(meuse, c(179997, 331662))

...or if it is one of the existing points in the SPDF, let's say the tenth point:

dist_vector = spDistsN1(meuse, meuse[10,])
# or even better, add it as a column to meuse
meuse[["distance"]] = spDistsN1(meuse, meuse[idx,])

and then you can select the 20 closest points:

meuse_dat = as.data.frame(meuse)
meuse_dat = meuse_dat[order(meuse_dat[["distance"]]),]
meuse_dat[["close"]] = FALSE
meuse_dat[["close]][1:20] = TRUE
meuse_subset = meuse_dat[close && distance < 1000,]

and calculate some stats:

require(plyr)
summarise(meuse_subset, mn = mean(zinc), vr = var(zinc))