addCompass function

Add Compass Rose to Map

Add a compass rose to an existing map, similar to those found on nautical charts showing both true north and magnetic north.

addCompass(X, Y, rot="magN", useWest=TRUE, year,
   cex=1, col.compass=c("gainsboro","blue","yellow","black"), ...)

Arguments

• X: numeric -- longitude coordinate (degrees N) for centroid of compass rose.

• Y: numeric -- latitude coordinate (degrees W) for centroid of compass rose.

• rot: numeric|character -- Rotation (degrees) counterclockwise from 0 degrees (true North, see details).

• useWest: logical -- if TRUE, assumes longitude coordinates are centered on Greenwich extending westward to -180 and eastward to 180.

• year: numeric -- specific year to use from IGRF-13 (see details).

• cex: numeric -- character expansion to use in the display.

• col.compass: character -- colours for compass rose components (in order):

  1=background compass, 2=rotated arms, 3=central button, 4=pch (border).

• ...: dots -- additional parameters to pass to the text function.

Details

The basic idea comes from Jim Lemon (see References), but is modified here to reflect a compass rose used on BC nautical charts.

The default rotation ("magN") is a calculation of the initial bearing of a great-circle arc from the compass position to the north geomagnetic rot using the function calcGCdist. The default year is the current year, but the user can choose years from 1900 to 2025 for approximate rot locations using model output from NOAA's IGRF-13 (International Geomagnetic Reference Field).

The user can also specify a fixed rotation (e.g. rot=-30) or no rotation (either rot=0 or rot="trueN").

Returns

No value returned.

References

c("%", "\n", " [R-sig-Geo] How to display a compass rose on a map")

c("%", "\n", " Magnetic North, Geomagnetic and Magnetic Poles")

Author(s)

Rowan Haigh, Program Head -- Offshore Rockfish

Pacific Biological Station (PBS), Fisheries & Oceans Canada (DFO), Nanaimo BC

locus opus: Regional Headquarters, Vancouver BC

Last modified Rd: 2022-07-05

See Also

addBubbles, addLabels, addPoints, addStipples, calcGCdist

Examples

local(envir=.PBSmapEnv,expr={
  data(nepacLL,envir=.PBSmapEnv)
  par(mfrow=c(1,1),mar=c(3,4,0.5,0.5))
  plotMap(nepacLL, xlim=c(-134.5,-124.5), ylim=c(48,55), plt=NULL,
    col="lightyellow", cex.axis=1.2, cex.lab=1.5)
  addCompass(-132, 49.5, rot=-12, cex=1.5)
})