Lines crossing Mercator projection map

Dear Mike,

Thank you for your answer.

I get an error with the polygons intersection:

wrld.clip <- gIntersection(clipPoly, wrld_simpl)

Error in RGEOSBinTopoFunc(spgeom1, spgeom2, byid, id, "rgeos_intersection")
:

 TopologyException: side location conflict at -91.437497778887078
17.241108073444931

library(rgeos)

sessionInfo()

The problem seems to be a bad polygon in the wrld_simpl database

gIsValid(wrld_simpl)

[1] FALSE

Warning message:

In RGEOSUnaryPredFunc(spgeom, byid, "rgeos_isvalid") :

 Ring Self-intersection at or near point -95.902496339999999
66.946641920000005


Any way to fix it?

BTW, I am sorry for the packages call. Here it comes:
library(maps)
library(maptools)
library(rgdal)

/Gis



On 12 November 2012 12:08, Michael Sumner <mdsumner at gmail.com> wrote:

This is happening because the maps data has data that is out of the
range of [-180, 180], either from the source data itself or from the
clipping somehow:

 range(world.map$x, na.rm = TRUE)
[1] -179.9572  190.2908


You would have to carefully clean this up to make it useable, but I
would just avoid it and use the clean data set in maptools (or import
your own from some other source.

Here you can intersect a polygon with wrld_simpl to clip it from the
infinities at the poles before reprojecting:

library(maptools)
library(rgdal)
library(rgeos)

data(wrld_simpl)

poly <- Polygon(cbind(c(-180, 180, 180, -180, -180), c(-80, -80, 80, 80,
-80)))
clipPoly <- SpatialPolygons(list(Polygons(list(poly), ID = "1")),
proj4string = CRS(proj4string(wrld_simpl)))

wrld.clip <- gIntersection(clipPoly, wrld_simpl)

wrld.merc <- spTransform(wrld.clip, CRS("+proj=merc"))

plot(wrld.merc)



Also, please declare the packages you have in use with library() or
require() so that code is reproducible.

Cheers, Mike.

On Mon, Nov 12, 2012 at 11:06 AM, Giuseppe Bianco <giubi78 at gmail.com>
wrote:

Dear List,

I have been struggling all the day long with this issue but I didn't find
any workaround.

I would like to have exactly the map that come out from the code below

but

without the ugly line crossing one side to the other.

world.map <- map(fill=T, plot=F)
IDs <- sapply(strsplit(world.map$names, ":"), function(x) x[1])
poly.map <- map2SpatialPolygons(world.map, IDs=IDs,
proj4string=CRS("+proj=longlat"))
merc.map <- spTransform(poly.map, CRS("+proj=merc"))

# Set the map to -80 to 80
bb <- cbind(c(-180, 180), c(-80, 80))
bb.poly <- SpatialPoints(bb, proj4string = CRS("+proj=longlat"))
bb.merc <- spTransform(bb.poly, CRS("+proj=merc"))
bb.merc <- as.data.frame(bb.merc)

# Here the ugly plot
plot(merc.map, col="gray", border="gray", xlim=bb.merc[,1],
ylim=bb.merc[,2])

Any help is very appreciated,
/Gis

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--
Michael Sumner
Hobart, Australia
e-mail: mdsumner at gmail.com

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