If points fall exactly on polygon vertices, the default NULL gives arbitrary assignments. If TRUE, then all points identical in machine numerical representation terms are set as within the polygon, if FALSE, outside.
Value
A vector of logical values. True means the point was inside the
polygon, False means the point was outside. Note that "inside" is an arbitrary concept for points precisely on the polygon boundary.
References
Rowlingson, B. and Diggle, P. 1993 Splancs: spatial point pattern analysis
code in S-Plus. Computers and Geosciences, 19, 627-655;
the original sources can be accessed at:
http://www.maths.lancs.ac.uk/~rowlings/Splancs/. See also Bivand, R. and
Gebhardt, A. 2000 Implementing functions for spatial statistical analysis
using the R language. Journal of Geographical Systems, 2, 307-317.