Add a Longitude and Latitude Grid to an Existing Map

Plot longitude and latitude grid on an existing map. This is an advanced function, requiring coordination with mapPlot() and (possibly) also with mapAxis(), and so it is best avoided by novices, who may be satisfied with the defaults used by mapPlot().

Usage

mapGrid(
  dlongitude = 15,
  dlatitude = 15,
  longitude,
  latitude,
  col = "darkgray",
  lty = "solid",
  lwd = 0.5 * par("lwd"),
  polarCircle = 0,
  longitudelim,
  latitudelim,
  debug = getOption("oceDebug")
)

Arguments

dlongitude: increment in longitude, ignored if longitude is supplied, but otherwise determines the longitude sequence.
dlatitude: increment in latitude, ignored if latitude is supplied, but otherwise determines the latitude sequence.
longitude: numeric vector of longitudes, or NULL to prevent drawing longitude lines.
latitude: numeric vector of latitudes, or NULL to prevent drawing latitude lines.
col: color of lines
lty: line type
lwd: line width
polarCircle: a number indicating the number of degrees of latitude extending from the poles, within which zones are not drawn.
longitudelim: optional argument specifying suggested longitude limits for the grid. If this is not supplied, grid lines are drawn for the whole globe, which can yield excessively slow drawing speeds for small-region plots. This, and latitudelim, are both set by mapPlot() if the arguments of the same name are passed to that function.
latitudelim: similar to longitudelim.
debug: a flag that turns on debugging. Set to 1 to get a moderate amount of debugging information, 2 to go two function levels deep, or 3 to go all the way to the core functions. Any value above 3 will be truncated to 3.

Value

A data.frame, returned silently, containing "side", "value", "type", and "at". A default call to mapPlot() ensures agreement of grid and axes by using this return value in a call to mapAxis().

Details

This is somewhat analogous to grid(), except that the first two arguments of the latter supply the number of lines in the grid, whereas the present function has increments for the first two arguments.

Author

Dan Kelley

Examples

# \donttest{
if (utils::packageVersion("sf") != "0.9.8") {
    # sf version 0.9-8 has a problem with this projection
    library(oce)
    data(coastlineWorld)
    par(mar = c(2, 2, 1, 1))
    # In mapPlot() call, note axes and grid args, to
    # prevent over-plotting of defaults.
    mapPlot(coastlineWorld,
        type = "l", projection = "+proj=ortho",
        axes = FALSE, grid = FALSE
    )
    mapGrid(15, 15)
}

# }