read.gps cannot read a file, or when the
data have been manipulated.
as.gps(longitude, latitude, filename = "")latitude and
longitude, in which case these values are extracted from the data
frame and the second argument is ignored.gps-class.
gps data: [[,gps-method,
[[<-,gps-method, gps-class,
plot,gps-method, read.gps,
summary,gps-method