Add Contours on a Existing map

Draw contour lines to an existing map, using mapLines(). Note that label placement in mapContour is handled differently than in contour().

Usage

mapContour(
  longitude,
  latitude,
  z,
  nlevels = 10,
  levels = pretty(range(z, na.rm = TRUE), nlevels),
  labcex = 0.6,
  drawlabels = TRUE,
  underlay = "erase",
  col = par("fg"),
  lty = par("lty"),
  lwd = par("lwd"),
  debug = getOption("oceDebug")
)

Arguments

longitude

numeric vector of longitudes of points to be plotted, or an object of class topo (see topo), in which case longitude, latitude and z are inferred from that object. Importantly, the longitude system must match that of the mapPlot() call that made the underlying plot. If not, the contours can have spurious lines that run across the plot. See “Dealing with longitude conventions” for a method of handling conflicting longitude conventions between mapPlot() and mapContour().

latitude

numeric vector of latitudes of points to be plotted.

z

matrix to be contoured. The number of rows and columns in z must equal the lengths of longitude and latitude, respectively.

nlevels

number of contour levels, if and only if levels is not supplied.

levels

vector of contour levels.

labcex

cex value used for contour labelling. As with contour(), this is an absolute size, not a multiple of par("cex").

drawlabels

logical value or vector indicating whether to draw contour labels. If the length of drawlabels is less than the number of levels specified, then rep() is used to increase the length, providing a value for each contour line. For those levels that are thus indicated, labels are added, at a spot where the contour line is closest to horizontal on the page. First, though, the region underneath the label is filled with the colour given by par("bg"). See “Limitations” for notes on the status of contour labelling, and its limitations.

underlay

character value relating to handling labels. If this equals "erase" (which is the default), then the contour line is drawn first, then the area under the label is erased (filled with white 'ink'), and then the label is drawn. This can be useful in drawing coarsely-spaced labelled contours on top of finely-spaced unlabelled contours. On the other hand, if underlay equals "interrupt", then the contour line is interrupted in the region of the label, which is closer to the scheme used by the base contour() function.

col

colour of the contour line, as for par("col"), except here col gets lengthened by calling rep(), so that individual contours can be coloured distinctly.

lty

type of the contour line, as for par("lty"), except for lengthening, as described for col.

lwd

width of the contour line, as for par("lwd"), except for lengthening, as described for col and lty.

debug

an integer specifying whether debugging information is to be printed during the processing. This is a general parameter that is used by many oce functions. Generally, setting debug=0 turns off the printing, while higher values suggest that more information be printed. If one function calls another, it usually reduces the value of debug first, so that a user can often obtain deeper debugging by specifying higher debug values.

Sample of Usage

library(oce)
data(coastlineWorld)
if (requireNamespace("ocedata", quietly=TRUE)) {
    data(levitus, package = "ocedata")
    par(mar = rep(1, 4))
    mapPlot(coastlineWorld, projection = "+proj=robin", col = "lightgray")
    mapContour(levitus$longitude, levitus$latitude, levitus$SST)
}

Dealing with longitude conventions

Suppose a map has been plotted using longitudes that are bound between -180 and 180. To overlay contours defined with longitude bound between 0 and 360 (as for the built-in coastlineWorld dataset), try Clark Richards' method (https://github.com/dankelley/oce/issues/2217, as below.

# Start with z=z(lon,lat), with lon bound by 0 and 360
z2 <- rbind(z[lon > 180, ], z[lon <= 180, ])
lon2 <- lon + 180
mapContour(lon2, lat, z2)

See also

A map must first have been created with mapPlot().

Other functions related to maps: formatPosition(), lonlat2map(), lonlat2utm(), map2lonlat(), mapArrows(), mapAxis(), mapCoordinateSystem(), mapDirectionField(), mapGrid(), mapImage(), mapLines(), mapLocator(), mapLongitudeLatitudeXY(), mapPlot(), mapPoints(), mapPolygon(), mapScalebar(), mapText(), mapTissot(), oceCRS(), oceProject(), shiftLongitude(), usrLonLat(), utm2lonlat()

Author

Dan Kelley