R/util-point_offset.r

In jsta/glatos: A package for the Great Lakes Acoustic Telemetry Observation System

#' @title Identify new location based on distance and bearing from another
#'
#' @description Calculates latitude and longitude for new point that is x meters
#'   away at bearing y from a geographic location (Longitude, Latitude). uses
#'   "destPoint" function from "geosphere" package and calculations are based on
#'   great circle distances.
#'   
#' @param lon vector of longitudes (dd) to calculate offset points
#' @param lat vector of latitudes (dd) to calculate offset points
#' @param offsetDist vector of distances to calculate offset point (meters or
#'   feet)
#' @param offsetDir vector of directions to calculate point from starting point.
#'   Options are NA,"N", "NNE", "NE", "ENE", "E", "ESE", "SE", "SSE", "S",
#'   "SSW", "SW", "WSW", "W", "WNW", "NW", "NNW"
#' @param distUnit specify meters or ft ("m" or "ft")
#'
#' @examples
#' lat <- rep(44.0, 17)
#' lon <- rep(-83.0, 17)
#' 
#' offsetDir <- c(NA,"N", "NNE", "NE", "ENE", "E", "ESE", "SE", "SSE", "S",
#' "SSW", "SW", "WSW", "W", "WNW", "NW", "NNW") 
#' 
#' offsetDist <- seq(100, 1700, by = 100) 
#' distUnit <- 'm'
#' 
#' point_offset(lon, lat, offsetDist, offsetDir, distUnit)
#' 
#' @export

point_offset <- function(lon=NA, lat=NA, offsetDist=NA, offsetDir=NA, 
                         distUnit="m"){
 #require(geosphere) #for spatialPoints

  if(distUnit == "ft") offsetDist <- 0.3048*offsetDist #conver to m if needed
  if(!(distUnit) %in% c("ft","m")) 
    stop ("Input attribute 'dirUnit' must be 'm' (meters) or 'ft' (feet).")

 dirKey <- data.frame(
    txt=    c(NA,"N", "NNE", "NE", "ENE", "E", "ESE", "SE", "SSE", "S", "SSW", 
              "SW", "WSW", "W", "WNW", "NW", "NNW"),
    deg=c(NA, 0, 22.5, 45, 67.5, 90, 112.5, 135, 157.5, 180, 202.5, 225, 247.5, 
          270, 292.5, 315, 337.5),
    stringsAsFactors=F)

 bearing <- dirKey$deg[match(offsetDir, dirKey$txt)]

pos <- geosphere::destPoint(cbind(lon,lat), bearing, offsetDist)
return(pos)
}