R/CSquare2LonLat.R

In ices-tools-dev/sfdQC: Functions to produce quality control reports and plots for ICES WGSFD VMS data and products

#' @export
CSquare2LonLat <- function (csqr, degrees = 0.05) {
  ra <- 1e-06

  # decode the c_square string
  chars <- nchar(csqr)
  tensqd <- as.numeric(substr(csqr, 1, 1)) + ra
  gqlat <- round(abs(tensqd - 4) * 2/10) * 10/5 - 1
  tenslatd <- as.numeric(substr(csqr, 2, 2)) + ra
  gqlon <- (2 * round(tensqd/10) - 1) * -1
  tenslond <- as.numeric(substr(csqr, 3, 4)) + ra
  unitsqd <- as.numeric(substr(csqr, 6, 6)) + ra
  iqulat <- round(unitsqd * 2/10)
  unitslatd <- as.numeric(substr(csqr, 7, 7)) + ra
  iqulon <- (round((unitsqd - 1)/2, 1) - floor((unitsqd - 1)/2)) * 2
  unitslond <- as.numeric(substr(csqr, 8, 8)) + ra
  tenthsqd <- as.numeric(substr(csqr, 10, 10)) + ra
  iqtlat <- round(tenthsqd * 2/10)
  tenthslatd <- as.numeric(substr(csqr, 11, 11)) + ra
  iqtlon <- (round((tenthsqd - 1)/2, 1) - floor((tenthsqd - 1)/2)) * 2
  tenthslond <- as.numeric(substr(csqr, 12, 12)) + ra
  hundqd <- as.numeric(substr(csqr, 14, 14)) + ra
  iqhlat <- round(hundqd * 2/10)
  hundlatd <- as.numeric(substr(csqr, 15, 15)) + ra
  iqhlon <- (round((hundqd - 1)/2, 1) - floor((hundqd - 1)/2)) * 2
  hundlond <- as.numeric(substr(csqr, 16, 16)) + ra

  # check supplied resolution
  reso <- 10^(1 - floor((chars - 4)/4)) - 
             ((round((chars - 4)/4, 1) - floor((chars - 4)/4)) * 10^(1 - floor((chars - 4)/4)))
  if (degrees < reso[1])
      stop("Returning degrees is smaller than format of C-square")

  # calculate lat and long
  if (degrees == 10) {
    lat <- ((tenslatd * 10) + 5) * gqlat - ra
    lon <- ((tenslond * 10) + 5) * gqlon - ra
  } else
  if (degrees == 5) {
    lat <- ((tenslatd * 10) + (iqulat * 5) + 2.5) * gqlat - ra
    lon <- ((tenslond * 10) + (iqulon * 5) + 2.5) * gqlon - ra
  } else
  if (degrees == 1) {
    lat <- ((tenslatd * 10) + unitslatd + 0.5) * gqlat - ra
    lon <- ((tenslond * 10) + unitslond + 0.5) * gqlon - ra
  } else
  if (degrees == 0.5) {
    lat <- ((tenslatd * 10) + unitslatd + (iqtlat * 0.5) + 0.25) * gqlat - ra
    lon <- ((tenslond * 10) + unitslond + (iqtlon * 0.5) + 0.25) * gqlon - ra
  } else
  if (degrees == 0.1) {
    lat <- ((tenslatd * 10) + unitslatd + (tenthslatd * 0.1) + 0.05) * gqlat - ra
    lon <- ((tenslond * 10) + unitslond + (tenthslond * 0.1) + 0.05) * gqlon - ra
  } else
  if (degrees == 0.05) {
    lat <- ((tenslatd * 10) + unitslatd + (tenthslatd * 0.1) + (iqhlat * 0.05) + 0.025) * gqlat - ra
    lon <- ((tenslond * 10) + unitslond + (tenthslond * 0.1) + (iqhlon * 0.05) + 0.025) * gqlon - ra
  } else
  if (degrees == 0.01) {
    lat <- ((tenslatd * 10) + unitslatd + (tenthslatd * 0.1) + (hundlatd * 0.01) + 0.005) * gqlat - ra
    lon <- ((tenslond * 10) + unitslond + (tenthslond * 0.1) + (hundlond * 0.01) + 0.005) * gqlon - ra
  }

  # return  
  data.frame(latitude = lat, longitude = lon)
}