[R] Isocontour-lines of spatial data on a rectangular grid (not plots!)

[Dieter Menne]

Dear R-Listers,

I have spatial data on an equidistant rectangular grid, similar to
topographic data. I know that there are quite a few R-packages or base
functions that provide nice iso-contours plot, but I don't want a plot, just
the smoothed isocontour line of ONE level (e.g. 10 mm).

Data sets are large, so it would be preferable if the availability of
regular grid data could be exploited, and the fact that I only need ONE
isocontour line.  Polynomial fits are not adequate, but loess works nicely
for the plots. Akima/interp might be a little too general, and if I remember
Brian Ripley's comments correctly, the coefficients are "only for internal
use".

Can someone suggest me a package?


Dieter Menne