R/selpos.R

In geo: Draw and Annotate Maps, Especially Charts of the North Atlantic

#' Geographical point selection.
#' 
#' Select geographical data points by arbitrary criterion.
#' 
#' The normal way of working with geographical data is to store positions as a
#' list with names lat and lon. This is easier for most applications, except
#' selection of subsets, where it is essential to access individual elements.
#' The purpose of this routine is merely to ease the selection process.
#' 
#' @param lat Latitude of points or list containing lat,lon.
#' @param ind Selection criterion.
#' @param lon Longitude of data points. If missing, this must be part of the
#' lat argument.
#' @return Returns list with elements lat,lon which satisfy the criterion.
#' @seealso \code{\link{geoplot}},
#' @examples
#' 
#' \dontrun{
#'              subs<-selpos(pos,,z>6)# Select positions where z>6
#' 
#'        The Function is trivially defined as
#'        function(lat, lon = NULL, ind)
#'        {
#'                       if(is.null(lon)) {
#'                            lon <- lat$lon
#'                            lat <- lat$lat
#'                       }
#'                       lat <- lat[ind]
#'                       lon <- lon[ind]
#'                       return(lat, lon)
#'        }
#' }
#' @export selpos
selpos <-
function(lat, lon = NULL, ind)
{
	if(is.null(lon)) {
		lon <- lat$lon
		lat <- lat$lat
	}
	lat <- lat[ind]
	lon <- lon[ind]
	return(lat, lon)
}