Break polygons at meridian for recentering

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

Author

Roger Bivand

See also

recenter-methods, nowrapSpatialLines

Examples

run <- FALSE
if (require(maps)) run <- TRUE
if (FALSE) {
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)
}
#> Warning: GEOS support is provided by the sf and terra packages among others
#> Warning: GEOS support is provided by the sf and terra packages among others
#>       min max
#> x 5.5e-07 360
#> y 5.0e+01  52