I am trying to workout trajectories from GPS tracking data to run in the FPT function. This is my code: id<-data.frame (FPT[,6]) xy<-data.frame (FPT[,4], FPT[,3]) datet <- as.POSIXct (datetime) x<-as.traj(id, xy, datet) And I get this error: Error in sort.list(y) : 'x' must be atomic for 'sort.list' Have you called 'sort' on a list? I have checked and x and y are both atomic so I don't understand the error. The sort command must be part of the function but I can't find out anything about it online - can anyone help with this error please? Thanks Sam [[alternative HTML version deleted]]