R/mapDistance.R

In rossholmberg/pkgButterfly: Functions to calculate relevant local oceanographic variables based on Afan et. al

#' Convert either latitude or longitude distances between angles (degrees) and distances (km)
#'
#'
#' @param degrees Numeric value, degrees for distance
#' @keywords maps gis
#' @export
#' @return numeric value

latDegToKm <- function( degrees ) {
  return(
    degrees * 110.574
  )
}



#' Convert either latitude or longitude distances between angles (degrees) and distances (km)
#'
#'
#' @param km Numeric value, distance in km
#' @keywords maps gis
#' @export
#' @return numeric value

latKmToDeg <- function( km ) {
  return(
    km / 110.574
  )
}



#' Convert either latitude or longitude distances between angles (degrees) and distances (km)
#'
#'
#' @param degrees Numeric value, degrees for distance
#' @param lat Numeric value, latitude position
#' @keywords maps gis
#' @export
#' @return numeric value

lonDegToKm <- function( degrees, lat = 39.32633 ) {
  return(
    degrees * 111.32 * cos( lat / ( 180 / pi ) )
  )
}



#' Convert either latitude or longitude distances between angles (degrees) and distances (km)
#'
#'
#' @param lat Numeric value, latitude position
#' @param km Numeric value, distance in km
#' @keywords maps gis
#' @export
#' @return numeric value

lonKmToDeg <- function( km, lat = 39.32633 ) {
  return(
    km / ( 111.32 * cos( lat / ( 180 / pi ) ) )
  )
}



#' Take two latlon coordinates, and return the distance between them.
#'
#'
#' @param lat.1 latitude position, point 1
#' @param lon.1 longitude position, point 1
#' @param lat.2 latitude position, point 2
#' @param lon.2 longitude position, point 2
#' @param unit distance units desired for output, default "km"
#' @keywords maps gis
#' @export
#' @return numeric value, km between the two points

mapDistance <- function( lat.1, lon.1, lat.2, lon.2, unit = "km" ) {

  # run a couple of pre-flight checks
  # if( !{ length( lat.1 ) == 1L || length( lat.2 ) == 1L || length( lat.1 ) == length( lat.2 ) } ) {
  #     stop( "Either coordinates 1 and 2 must be the same length, or one of them must be length 1." )
  # }
  # if( length( lat.1 ) == length( lon.1 ) && length( lat.2 ) == length( lon.2 ) ) {
  #     stop( "Lat vectors must match the length of their corresponding lon vectors." )
  # }

  # get the latitudinal distances
  lat.dist <- latDegToKm( abs( lat.1 - lat.2 ) )

  # get a latitude to use as a reference point
  if( length( lat.1 ) == 1L && length( lat.2 ) == 1L ) {
    lat.ref <- mean( lat.1, lat.2 )
  } else if( length( lat.1 ) == 1L ) {
    lat.ref <- lat.1
  } else if( length( lat.2 ) == 1L ) {
    lat.ref <- lat.2
  } else {
    lat.ref <- ( lat.1 + lat.2 ) / 2
  }

  # and the longitudinal distance
  lon.dist <- lonDegToKm( abs( lon.1 - lon.2 ), lat = lat.ref )

  # calculate a distance in km
  dist <- sqrt( lat.dist ^ 2 + lon.dist ^ 2 )

  # convert units if required
  dist <- switch( unit,
                  "km" = dist, "kilometres" = dist,
                  "m" = dist * 1000, "metres" = dist * 1000,
                  "cm" = dist * 1e5, "centimetres" = dist * 1e5,
                  "miles" = dist / 1.609344, "mi" = dist / 1.609344,
                  "ft" = dist * 3280.844, "feet" = dist * 3280.844,
                  "yards" = dist * 1093.61, "yrds" = dist * 1093.61, "yrd" = dist * 1093.61,
                  "inches" = dist * 39370.1, "inch" = dist * 39370.1, "in" = dist * 39370.1
  )

  # return the result
  return( dist )

}