[R] help regarding permieter/boundary detection in spatial data set

[Erik Lehnhoff]

I have a large data set that consists of x,y
coordinates and number of plants counted at each
point.  The sampling grid consists of contiguous 1 m^2
quadrats placed over a patch of plants such that the
patch is competely within the grid and the perimeter
quadrats straddle the patch edge.  Each 1 m^2 quadrat
is further divided into 1/16ths, so some of the 1/16
sections are in the patch and some are outside of the
patch.  I need to remove from the data set any points
that have zero plants because they are outside of the
patch boundary.  Points having zero plants that are
within the boundary need to be retained.  

Is there package that can perform perimeter or
boundary detection, and then exclude any points
outside of the boundary?  Any help would be very much
appreciated.

Thank you,
Erik Lehnhoff