[R-sig-Geo] Interpolating along an XY path]

[Michael Sumner]

Rob Schick wrote:
> I have a series of x,y coordinates that represent a movement path of a
> tagged animal. I want to regularize the track, but approx interpolates over
> the range of x's as opposed to treating the track as an ordered sequence.
> I've tried coercing it into a time series, but this only seems to work when
> the X's constantly increase. The following dummy track provides a glimpse of
> the problem with approx:
>   

If you treat the x's and y's separately with approx you can get what you 
want (although I'm sure some careful thought would produce a better use 
of approx):

x <- c(1,3,5,5,5,4,2,1.5)
y <- c(1,1,1,2,3,3,3,3)
xypair <- cbind(x,y)
xypair
rownames(xypair) <- 1:length(x)
plot(x,y,type='l')
points(xypair)
text(x,y,rownames(xypair),offset=.5,pos=2)
points(approx(xypair[,1],n=30)$y, approx(xypair[,2],n=30)$y,col=3,pch="*")

BTW, I have a package for animal track data that contains a function 
called equalTime - it interpolates points in space (euclidean) based on 
an (approximation to an) equal time interval to create regular spaced 
points in time, assuming straight motion between fixes. You might find 
it useful. It's never really been finished, but I use it especially for 
the S4 validation of trajectory with "trip", and for quick speed 
filtering and time spent gridding.

http://staff.acecrc.org.au/~mdsumner/Rutas/

HTH, Mike.