On Thu, 8 Jan 2009, Matt Oliver wrote:
Hans,
Take a look at the proj4 package, specifically project()
Or rather rgdal, which is a fully featured implementation with a proper
interface to the PROJ.4 library. Unfortunately, there does not seem to be
a way of shifting the prime meridian in a helpful way - and the original
question was not about projection, but about joining +180 to -180, which
is obvious in spherical but not in planar terms. rgdal provides methods
for projection and datum transformation for objects defined in the sp
package, as well as for projection of matrices of coordinates.
Roger
PS:
This gives something, but at the cost of meaningless coordinates and
artefacts for Greenland and Iceland, which could be subsetted out:
wrld_simpl_nA <- wrld_simpl[wrld_simpl$NAME != "Antarctica",]
library(rgdal)
wrld_simpl_nA_22k <- spTransform(wrld_simpl_nA,
CRS("+proj=merc +datum=WGS84 +x_0=22000000"))
proj4string(wrld_simpl_nA_22k) <- CRS("+proj=merc +datum=WGS84")
wrld_simpl_nA_22k_ll <- spTransform(wrld_simpl_nA_22k,
CRS("+proj=longlat"))
plot(wrld_simpl_nA_22k_ll, axes=TRUE)
The alternative is as given before - dividing the polygons into ones to be
recentered and ones to be left as is.
see http://trac.osgeo.org/proj/
There are many projections that allow you to recenter a map on a specific
longitude
For example
x <- seq(-180, 180)
y <- seq(-90, 90)
xy <- expand.grid(x, y)
plot(xy)
mer <- cbind(0, y) #prime meridian
points(mer[,1], mer[,2], col = "white")
proj.xy <- project(xy, "+proj=eck4") ###project grid into eckert IV
proj.mer <- project(mer, "+proj=eck4") ###project meridian into eckert IV
plot(proj.xy)
points(proj.mer[,1], proj.mer[,2], col = "white")
proj.xy <- project(xy, "+proj=eck4 +lon_0=90W") ###project grid into eckert
IV with central meridian at 90W
proj.mer <- project(mer, "+proj=eck4 +lon_0=90W") ###project meridian into
eckert IV with central meridian at 90W
plot(proj.xy)
points(proj.mer[,1], proj.mer[,2], col = "white")
On Thu, Jan 8, 2009 at 4:07 AM, Hans-J?rg Bibiko <bib...@eva.mpg.de> wrote:
Hi,
I wonder if there's a workaround to generate seamless maps based on
SpatialPolygons objects. E.g. to produce a map showing Australia at the left
edge and America at the right one ( xlim := min: 110?E , max: 30?W ).
Or the way around, is there a function to re-center a given map by defining
the median longitude? (I know the function 'recenter' which produce a map
from 0? to 360?)
Kind regards,
--Hans
_______________________________________________
R-sig-Geo mailing list
R-sig-Geo@stat.math.ethz.ch
https://stat.ethz.ch/mailman/listinfo/r-sig-geo
--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: roger.biv...@nhh.no
_______________________________________________
R-sig-Geo mailing list
R-sig-Geo@stat.math.ethz.ch
https://stat.ethz.ch/mailman/listinfo/r-sig-geo
