Quick approximation for map projections

Instead of reprojecting everything, we just set the aspect ratio of the plot to the ratio between the length of one degree of lat over one degree of lon.
tidyverse · hadley · Mar 25, 2014 · Apr 29, 2013 · Feb 27, 2014 · Mar 17, 2014
commit 7637548616f7fdf1a20a8239c9b0b91e5458595d
diff --git a/DESCRIPTION b/DESCRIPTION
@@ -71,6 +71,7 @@ Collate:
     'coord-map.r'
     'coord-munch.r'
     'coord-polar.r'
+    'coord-quickmap.R'
     'coord-transform.r'
     'facet-.r'
     'facet-grid-.r'

diff --git a/NAMESPACE b/NAMESPACE
@@ -191,12 +191,14 @@ export(borders)
 export(calc_element)
 export(continuous_scale)
 export(coord)
+export(coord_aspect)
 export(coord_cartesian)
 export(coord_equal)
 export(coord_fixed)
 export(coord_flip)
 export(coord_map)
 export(coord_polar)
+export(coord_quickmap)
 export(coord_trans)
 export(cut_interval)
 export(cut_number)
@@ -289,6 +291,7 @@ export(position_identity)
 export(position_jitter)
 export(position_stack)
 export(qplot)
+export(quickmap)
 export(quickplot)
 export(rel)
 export(resolution)

diff --git a/R/coord-quickmap.R b/R/coord-quickmap.R
@@ -0,0 +1,65 @@
+#' Cartesian coordinates with an aspect ratio approximating mercator projection.
+#'
+#' The represenation of a portion of the earth, wich is approximately spherical,
+#' onto a flat 2D plane requires a projection. This is what
+#' \code{\link{coord_map}} does. These projections account for the fact that the
+#' actual length (in km) of one degree of longitude varies between the equator
+#' and the pole. Near the equator, the ratio between the lengths of one degree
+#' of latitude and one degree of longitude is approximately 1. Near the pole, it
+#' is tends towards infinity because the length of one degree of longitude tends
+#' towards 0. For regions that span only a few degrees and are not too close to
+#' the poles, setting the aspect ratio of the plot to the appropriate lat/lon
+#' ratio approximates the usual mercator projection. This is what
+#' \code{coord_quickmap} does. With \code{\link{coord_map}} all elements of the
+#' graphic have to be projected which is not the case here. So
+#' \code{\link{coord_quickmap}} has the advantage of being much faster, in
+#' particular for complex plots such as those using with
+#' \code{\link{geom_tile}}, at the expense of correctedness in the projection.
+#'
+#' @export
+#' @inheritParams coord_cartesian
+#' @examples
+#' # ensures that the ranges of axes are equal to the specified ratio by
+#' # adjusting the plot aspect ratio
+#' 
+#' if (require("maps")) {
+#' # Create a lat-long dataframe from the maps package
+#' nz <- map_data("nz")
+#' # Prepare a plot of the map
+#' nzmap <- ggplot(nz, aes(x=long, y=lat, group=group)) +
+#'   geom_polygon(fill="white", colour="black")
+#'
+#' # Plot it in cartesian coordinates
+#' nzmap
+#' # With correct mercator projection
+#' nzmap + coord_map()
+#' # With the aspect ratio approximation
+#' nzmap + coord_quickmap()
+#' }
+#' 
+#' # Resize the plot to see that the specified aspect ratio is maintained
+coord_quickmap <- function(xlim = NULL, ylim = NULL) {
+  coord(limits = list(x = xlim, y = ylim),
+    subclass = c("quickmap", "cartesian"))
+}
+
+#' @export coord_aspect quickmap
+coord_aspect.quickmap <- function(coord, ranges) {
+  # compute coordinates of center point of map
+  x.center <- sum(ranges$x.range) / 2
+  y.center <- sum(ranges$y.range) / 2
+
+  # compute distance corresponding to 1 degree in either direction
+  # from the center
+  x.dist <- dist_central_angle(x.center+c(-0.5,0.5), rep(y.center,2))
+  y.dist <- dist_central_angle(rep(x.center,2), y.center+c(-0.5,0.5))
+  # NB: this makes the projection correct in the center of the plot and
+  #     increasingly less correct towards the edges. For regions of reasonnable
+  #     size, this seems to give better results than computing this ratio from
+  #     the total lat and lon span.
+
+  # scale the plot with this aspect ratio
+  ratio <- y.dist / x.dist
+
+  diff(ranges$y.range) / diff(ranges$x.range) * ratio
+}
diff --git a/man/coord_quickmap.Rd b/man/coord_quickmap.Rd
@@ -0,0 +1,51 @@
+% Generated by roxygen2 (4.0.0): do not edit by hand
+\name{coord_quickmap}
+\alias{coord_quickmap}
+\title{Cartesian coordinates with an aspect ratio approximating mercator projection.}
+\usage{
+coord_quickmap(xlim = NULL, ylim = NULL)
+}
+\arguments{
+  \item{xlim}{limits for the x axis}
+
+  \item{ylim}{limits for the y axis}
+}
+\description{
+The represenation of a portion of the earth, wich is approximately spherical,
+onto a flat 2D plane requires a projection. This is what
+\code{\link{coord_map}} does. These projections account for the fact that the
+actual length (in km) of one degree of longitude varies between the equator
+and the pole. Near the equator, the ratio between the lengths of one degree
+of latitude and one degree of longitude is approximately 1. Near the pole, it
+is tends towards infinity because the length of one degree of longitude tends
+towards 0. For regions that span only a few degrees and are not too close to
+the poles, setting the aspect ratio of the plot to the appropriate lat/lon
+ratio approximates the usual mercator projection. This is what
+\code{coord_quickmap} does. With \code{\link{coord_map}} all elements of the
+graphic have to be projected which is not the case here. So
+\code{\link{coord_quickmap}} has the advantage of being much faster, in
+particular for complex plots such as those using with
+\code{\link{geom_tile}}, at the expense of correctedness in the projection.
+}
+\examples{
+# ensures that the ranges of axes are equal to the specified ratio by
+# adjusting the plot aspect ratio
+
+if (require("maps")) {
+# Create a lat-long dataframe from the maps package
+nz <- map_data("nz")
+# Prepare a plot of the map
+nzmap <- ggplot(nz, aes(x=long, y=lat, group=group)) +
+  geom_polygon(fill="white", colour="black")
+
+# Plot it in cartesian coordinates
+nzmap
+# With correct mercator projection
+nzmap + coord_map()
+# With the aspect ratio approximation
+nzmap + coord_quickmap()
+}
+
+# Resize the plot to see that the specified aspect ratio is maintained
+}
+