[R-sig-Geo] spsample and small sample size

[Patrick Giraudoux]

Dear listers,

I met a trouble with spsample which has been solved by Roger Bivand off 
list. I though it was general enough to be of interest for other users. 
In brief I needed to get 50 random points on a SpatialLinesDataFrame 
called "hydntline" and got:

spsample(hydntline,50,type="random")
Error in function (classes, fdef, mtable)  :
unable to find an inherited method for function "coordinates", for 
signature "numeric"

Roger's response is the following:

The reason is that some lines are assigned no sample points, and the 
function tries to create a SpatialPoints object with no points. The 
workaround is to increase n, then reduce the output by sampling back down.

z <- spsample(hydntline,4000,type="random")
zz <- z[sample(nrow(coordinates(z)), 50)]
plot(hydntline, axes=TRUE)
points(zz, pch=3)


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