R/OSM_scale_lookup.R

In ggmap: Spatial Visualization with ggplot2

#' Look up OpenStreetMap scale for a given zoom level.
#'
#' Look up OpenStreetMap scale for a given zoom level.
#'
#' @param zoom google zoom
#' @return scale
#' @details The calculation of an appropriate OSM scale value for a given zoom
#'   level is a complicated task.  For details, see
#'   \url{https://wiki.openstreetmap.org/wiki/FAQ}
#' @author David Kahle \email{david@@kahle.io}
#' @export
#' @examples
#'
#' OSM_scale_lookup(zoom = 3)
#' OSM_scale_lookup(zoom = 10)
#'
#' \dontrun{
#' # these can take a long time or are prone to crashing
#' # if the osm server load is too high
#'
#' # these maps are were the ones used to tailor fit the scale
#' # the zooms were fixed
#' ggmap(get_map(zoom =  3, source = 'osm', scale = 47500000), extent = "device")
#' ggmap(get_map(zoom =  4, source = 'osm', scale = 32500000), extent = "device")
#' ggmap(get_map(zoom =  5, source = 'osm', scale = 15000000), extent = "device")
#' ggmap(get_map(zoom =  6, source = 'osm', scale = 10000000), extent = "device")
#' ggmap(get_map(zoom =  7, source = 'osm', scale =  5000000), extent = "device")
#' ggmap(get_map(zoom =  8, source = 'osm', scale =  2800000), extent = "device")
#' ggmap(get_map(zoom =  9, source = 'osm', scale =  1200000), extent = "device")
#' ggmap(get_map(zoom = 10, source = 'osm', scale =   575000), extent = "device")
#' ggmap(get_map(zoom = 11, source = 'osm', scale =   220000), extent = "device")
#' ggmap(get_map(zoom = 12, source = 'osm', scale =   110000), extent = "device")
#' ggmap(get_map(zoom = 13, source = 'osm', scale =    70000), extent = "device")
#' ggmap(get_map(zoom = 14, source = 'osm', scale =    31000), extent = "device")
#' ggmap(get_map(zoom = 15, source = 'osm', scale =    15000), extent = "device")
#' ggmap(get_map(zoom = 16, source = 'osm', scale =     7500), extent = "device")
#' ggmap(get_map(zoom = 17, source = 'osm', scale =     4000), extent = "device")
#' ggmap(get_map(zoom = 18, source = 'osm', scale =     2500), extent = "device")
#' ggmap(get_map(zoom = 19, source = 'osm', scale =     1750), extent = "device")
#' ggmap(get_map(zoom = 20, source = 'osm', scale =     1000), extent = "device")
#'
#' # the USA
#' lonR <- c(1.01,.99)*c(-124.73,-66.95)
#' latR <- c(.99,1.01)*c(24.52, 49.38)
#' qmap(lonR = lonR, latR = latR, source = 'osm', scale = 325E5)
#'
#' }
#'
#'
OSM_scale_lookup <- function (zoom = 10){
  df <- data.frame(
    z = 2:20,
    s = c(175000000, 47500000, 32500000, 15000000, 10000000, 5000000,
    2800000, 1200000, 575000, 220000, 110000, 70000, 31000, 15000,
    7500, 4000, 2500, 1750, 1000)
  )
  with(df, s[z == zoom])
}