When recentering a world map, say to change an "Atlantic" view with longitude range -180 to 180, to a "Pacific" view, with longitude range 0 to 360, polygons crossed by the new offset, here 0/360, need to be clipped into left and right sub.polygons to avoid horizontal scratches across the map. The nowrapSpatialPolygons function performs this operation using polygon intersection, and nowrapRecenter recenters the output SpatialPolygons object.

nowrapRecenter(obj, offset = 0, eps = rep(.Machine$double.eps^(1/2.5), 2),
 avoidGEOS = FALSE)
nowrapSpatialPolygons(obj, offset = 0, eps=rep(.Machine$double.eps^(1/2.5), 2),
 avoidGEOS = FALSE)

Arguments

obj

A SpatialPolygons object

offset

offset from the Greenwich meridian

eps

vector of two (left and right) fuzz factors to retract the ring from the offset (2.5 root to accommodate rgeos precision rules)

avoidGEOS

default FALSE; use polyclip or gpclib code even if rgeos is available

Value

A SpatialPolygons object

See also

Examples

run <- FALSE if (require(maps)) run <- TRUE # \dontrun{ if (run) { world <- map("world", fill=TRUE, col="transparent", plot=FALSE) worldSpP <- map2SpatialPolygons(world, world$names, CRS("+proj=longlat +ellps=WGS84")) worldSpP <- worldSpP[-grep("Antarctica", row.names(worldSpP)),] # incomplete polygons worldSpP <- worldSpP[-grep("Ghana", row.names(worldSpP)),] # self-intersection mouth of Volta worldSpP <- worldSpP[-grep("UK:Great Britain", row.names(worldSpP)),] # self-intersection Humber estuary worldSpPr <- recenter(worldSpP) plot(worldSpPr) title("Pacific view without polygon splitting") }
if (run) { worldSpPnr <- nowrapRecenter(worldSpP) plot(worldSpPnr) title("Pacific view with polygon splitting") }
# } if (!rgeosStatus()) run <- FALSE if (run) { crds <- matrix(c(-1, 1, 1, -1, 50, 50, 52, 52), ncol=2) rcrds <- rbind(crds, crds[1,]) SR <- SpatialPolygons(list(Polygons(list(Polygon(rcrds)), ID="r1")), proj4string=CRS("+proj=longlat +ellps=WGS84")) bbox(SR) }
#> min max #> x -1 1 #> y 50 52
if (run) { SRr <- recenter(SR) bbox(SRr) }
#> min max #> x 1 359 #> y 50 52
if (run) { SRnr <- nowrapRecenter(SR) bbox(SRnr) }
#> min max #> x 5.5e-07 360 #> y 5.0e+01 52