addCompass {PBSmapping}

Add Compass Rose to Map

Description

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

Usage

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").

Value

No value returned.

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

References

[R-sig-Geo] How to display a compass rose on a map

Magnetic North, Geomagnetic and Magnetic Poles

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)
})