nowrapRecenter: Break polygons at meridian for recentering

Description Usage Arguments Value Author(s) See Also Examples

View source: R/nowrapRecenter.R

Description

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.

Usage

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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(s)

Roger Bivand

See Also

recenter-methods, nowrapSpatialLines

Examples

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run <- FALSE
if (require(maps)) run <- TRUE
## Not run: 
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")
}

## End(Not run)
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)
}
if (run) {
SRr <- recenter(SR)
bbox(SRr)
}
if (run) {
SRnr <- nowrapRecenter(SR)
bbox(SRnr)
}

Example output

Loading required package: sp
Checking rgeos availability: FALSE
 	Note: when rgeos is not available, polygon geometry 	computations in maptools depend on gpclib,
 	which has a restricted licence. It is disabled by default;
 	to enable gpclib, type gpclibPermit()
Loading required package: maps

maptools documentation built on Sept. 7, 2021, 5:09 p.m.