R/get_border_lon_lat.R

In subinjo92/navermap:

get_border_lon_lat <- function (center_lon, center_lat, zoom, xpix, ypix)
{
  zoom_res <- c(65536, 32768, 16384, 8192, 4096, 2048, 1024, 512, 256, 128,
                64, 32, 16, 8, 4, 2, 1, 0.5, 0.25, 0.175, 0.0875)

  # It operates at a level of 5 or higher.
  width_meter <- zoom_res[zoom] * xpix
  height_meter <- zoom_res[zoom] * ypix
  ur.lon <- geosphere::destPoint(c(center_lon, center_lat),
                                 b = 90, d = width_meter/2)[1]
  ur.lat <- geosphere::destPoint(c(center_lon, center_lat),
                                 b = 0, d = height_meter/2)[2]
  ll.lon <- geosphere::destPoint(c(center_lon, center_lat),
                                 b = 270, d = width_meter/2)[1]
  ll.lat <- geosphere::destPoint(c(center_lon, center_lat),
                                 b = 180, d = height_meter/2)[2]
  c(ll.lon, ll.lat, ur.lon, ur.lat)
}