[R-sig-Geo] Strange lines when projecting a world map

Roger Bivand Roger.Bivand at nhh.no
Thu Dec 5 21:22:58 CET 2013


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.Bivand at nhh.no



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