On Thu, 8 Jan 2009, Hans-Jörg Bibiko 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°)
Not without recentering, I think. As you've probably found out:
library(maptools)
load(url("http://spatial.nhh.no/R/etc/TM_WORLD_BORDERS_SIMPL-0.2.RData";))
rc <- recenter(world_simp)
gives nasty artefacts where polygons cross the Prime Meridian, so subsets
of countries crossing 180° and west of but not crossing the Prime Meridian
need to be recentered, and added back into the object. That is a good deal
of work. Do you need the country polygons, or can you manage with just the
shoreline (as polygons):
gshhs.c.b <- system.file("share/gshhs_c.b", package="maptools")
Px <- c(110, 330)
Py <- c(-62, 90)
P <- Rgshhs(gshhs.c.b, xlim=Px, ylim=Py, level=1)
plot(P$SP)
A coarse borders file is also available, but only as lines, and varies in
levels of detail between countries see ?Rgshhs.
If you do need the country polygons, we'd need to list the blocks
of polygons needing recentering. The maps package also has Pacific view
databases for shorelines and some borders.
Hope this helps,
Roger
Kind regards,
--Hans
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--
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
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