On Thu, 14 Oct 2010, Karl Ove Hufthammer wrote:
Dear list members
The GSHHS map data uses longitudes from 0 to 360, which causes problems
fetching maps around longitude 0. Example:
library(maptools)
gpclibPermit()
gshhs.c.b <- system.file("share/gshhs_c.b", package="maptools")
xl=c(-30,30)
yl=c(50,80)
rmap=Rgshhs(gshhs.c.b, xlim=xl, ylim=yl)
plot(rmap$SP)
There are no error messages, but some countries are just missing, e.g.,
Ireland, Iceland and Greenland. To get a map containing them, we have to use
xlim=c(330,360). But what if we want to have both countries to the east and
to the west of long = 0 on the same map (i.e., SpatialPolygons)?
Rgshhs takes a ‘shift’ argument, but this only changes the *output*
coordinates from 0,360 to -180,180; it doesn’t change the accepted input
variables.
Is it necessary to fetch the map twice? If so, which xlim values should be
used, and how should the two maps be merged into one SpatialPolygons? Note
that if I change ‘xlim’ to c(0,30), Great Britain is missing ... :-(
As things stand, I suspect that there is no alternative, but will take a
look (and encourage others to do so too!). Merging with
unionSpatialPolygons() as usual.
Roger
--
Karl Ove Hufthammer
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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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