Create N-S and E-W grid lines over a geographic region

Create N-S and E-W grid lines over a geographic region; create and plot corresponding labels

gridlines(x, easts = pretty(bbox(x)[1,]), norths = pretty(bbox(x)[2,]),
 ndiscr = 100)
gridat(x, easts = pretty(bbox(x)[1,]), norths = pretty(bbox(x)[2,]),
 offset = 0.5, side = "WS")
# S3 method for SpatialLines
labels(object, labelCRS, side = 1:2, ...)
# S3 method for SpatialPointsDataFrame
text(x, ...)

Arguments

x

object deriving from class Spatial-class
easts

numeric; east-west values for vertical lines
norths

numeric; north-south values for horizontal lines
ndiscr

integer; number of points used to discretize the line, could be set to 2, unless the grid is (re)projected
offset

offset value to be returned, see text
object

SpatialLines-class object, as returned by gridlines
labelCRS

the CRS in which the grid lines were drawn and labels should be printed; if missing, the CRS from object is taken
side

for labels: integer, indicating side(s) at which gridlines labels will be drawn: 1=below (S), 2=left (W), 3=above (N), and 4=right (E); for gridat: default “WS”, if “EN” labels placed on the top and right borders
...

for labels: ignored; for text: arguments passed on to text, see below for example use of adj

Value

gridlines returns an object of class SpatialLines-class, with lines as specified; the return object inherits the projection information of x; gridat returns a SpatialPointsDataFrame with points at the west and south ends of the grid lines created by gridlines, with degree labels.

The labels method for SpatialLines objects returns a SpatialPointsDataFrame-class object with the parameters needed to print labels below and left of the gridlines. The locations for the labels are those of proj4string(object) the labels also unless labelCRS is given, in which case they are in that CRS. This object is prepared to be plotted with text:

The text method for SpatialPointsDataFrame puts text labels on its coordinates, and takes care of attributes pos, labels, srt and offset; see text.

See also

spTransform; llgridlines in rgdal (recent versions) for plotting long-lat grid over projected data

Examples

data(meuse)
coordinates(meuse) = ~x+y
plot(meuse)
plot(gridlines(meuse), add = TRUE)
text(labels(gridlines(meuse)))
title("default gridlines within Meuse bounding box")

proj4string(meuse) <- CRS("+init=epsg:28992")
#> Warning: Discarded datum Amersfoort in CRS definition
crs.longlat <- CRS("+init=epsg:4326")
meuse_ll <- spTransform(meuse, crs.longlat)
grd <- gridlines(meuse_ll)
grd_x <- spTransform(grd, CRS("+init=epsg:28992"))
#> Warning: Discarded datum Amersfoort in CRS definition

# labels South and West:
plot(meuse)
plot(grd_x, add=TRUE, lty=2)
grdat_ll <- gridat(meuse_ll)
grdat_x <- spTransform(grdat_ll, CRS("+init=epsg:28992"))
#> Warning: Discarded datum Amersfoort in CRS definition
text(grdat_x)

# labels North and East:
plot(meuse)
plot(grd_x, add=TRUE, lty=2)
grdat_ll <- gridat(meuse_ll, side="EN")
grdat_x <- spTransform(grdat_ll, CRS("+init=epsg:28992"))
#> Warning: Discarded datum Amersfoort in CRS definition
text(grdat_x)

# now using labels:
plot(meuse)
plot(grd_x, add=TRUE, lty=2)
text(labels(grd_x, crs.longlat))

# demonstrate axis labels with angle, both sides:
sp = SpatialPoints(rbind(c(-101,9), c(-101,55), c(-19,9), c(-19,55)), crs.longlat)
laea = CRS("+proj=laea +lat_0=30 +lon_0=-40")
sp.l = spTransform(sp, laea)
plot(sp.l, expandBB = c(0, 0.05, 0, .05))
gl = spTransform(gridlines(sp), laea)
plot(gl, add = TRUE)
text(labels(gl, crs.longlat))
text(labels(gl, crs.longlat, side = 3:4), col = 'red')
title("curved text label demo")

# polar:
pts=SpatialPoints(rbind(c(-180,-70),c(0,-70),c(180,-89),c(180,-70)), crs.longlat)
polar = CRS("+init=epsg:3031")
gl = spTransform(gridlines(pts, easts = seq(-180,180,20), ndiscr = 100), polar)
plot(spTransform(pts, polar), expandBB = c(.05,0,.05,0))
lines(gl)
l = labels(gl, crs.longlat, side = 3)
l$pos = NULL # pos is too simple, use adj:
text(l, adj = c(0.5, -0.5))
l = labels(gl, crs.longlat, side = 4)
l$srt = 0 # otherwise they end up upside-down
text(l)
title("grid line labels on polar projection, epsg 3031")
if (FALSE) {
if (require(maps)) demo(polar) # adds the map of the antarctic
}