On Thu, 5 Dec 2013, Francesco Carotenuto wrote:
Dear list, as in
the title, I found that when projecting a world map by mollweide projection and
setting a?? longitude value, some strange
parallels are drawn. The real problem is that these lines ???interact??? with a
raster when performing the ???over??? function.
How can I manage
it?
The underlying question is what you want to do with the objects after
projection. The problem in your example is that some polygons/lines cross
-180+66 degrees, so get reflected back across. A possible solution is to
clip the polygons before projection at -180+66 degrees +/- a small number,
but before looking at this in more detail, I need to know what the
projected object is intended for. When +lon_0=0, there is no such effect,
as this data set is clipped at -180 already.
Roger
Here I provide a
simulated example to let you understand what I mean
Thanks.??
#Example of a
world map mollweide projection with a specific longitude value
library(sp)
library(maptools)
library(rgdal)
data(wrld_simpl)
w_pol<-wrld_simpl
w_pol<-spTransform(w_pol,
CRS("+proj=moll +lon_0=66"))
plot(w_pol)
??
# Intersecting a
grid with the projected world map
gr<-expand.grid(x
= seq(-180, 180, length.out = 100), y = seq(-90, 90, length.out = 100))
coordinates(gr)<-~x+y
proj4string(gr)<-CRS("+proj=longlat
+ellps=WGS84 +datum=WGS84 +no_defs")
gr<-spTransform(gr,
CRS("+proj=moll +lon_0=66"))
plot(gr[w_pol,])
??
??
Dr. Francesco Carotenuto (Ph.D.)
Department of Earth Sciences
Federico II University
Largo S. Marcellino 10,
80138 NAPLES (ITALY)
http://francesco-carotenuto.blogspot.it/
http://3dpaleontology.blogspot.it/??
[[alternative HTML version deleted]]
--
Roger Bivand
Department of Economics, Norwegian School of Economics,
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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