oce (version 0.9-17)

magneticField: Earth magnetic declination, inclination, and intensity

Description

Earth magnetic declination, inclination, and intensity

Usage

magneticField(longitude, latitude, time)

Arguments

longitude

longitude in degrees east (negative for degrees west). The dimensions must conform to lat.

latitude

latitude in degrees north, a number, vector, or matrix.

time

either a decimal year or a POSIX time corresponding to the longitude and latitude values, or a vector or matrix matching these location values.

Value

• A list containing declination, inclination, and intensity.

Caution

This code has not been adequately checked. The hope is to get some independent test values sometime in Oct 2013.

concept

tide

Details

Implements the International geomagnetic reference field, based on a reworked version of a Fortran program downloaded from a NOAA website. The code seems to have been written by Susan Macmillon of the British Geological Survey. Comments in the code indicate that it employs coefficients agreed to in December 2009 by the IAGA Working Group V-MOD. The code also suggests that the valid time interval is 1900.0 to 2015.0, with values up to 2020 having reduced accuracy. And, finally, values for dates 1945 to 2005.0 are said to be ``non-definitative,'' with other values in the range being declared ``definitative.''

References

1. The underlying Fortran code is from a NOAA website (http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html).

2. A website with a calculator for these fields is at http://www.geomag.bgs.ac.uk/data_service/models_compass/wmm_calc.html, and, as of October 7 2013, this site produces values that agree with those calculated by the present function to three digits after the decimal.

Examples

library(oce)
# Halifax NS
magneticField(-(63+36/60), 44+39/60, 2013)

## map of North American values
data(coastlineWorld)
mapPlot(coastlineWorld, longitudelim=c(-130,-55), latitudelim=c(35,60),
        projection="+proj=lcc +lat_0=20 +lat_1=60 +lon_0=-100")
lon <- seq(-180, 180, 1)
lat <- seq(-90, 90)
lonm <- rep(lon, each=length(lat))
latm <- rep(lat, times=length(lon))
## Note the counter-intuitive nrow argument
decl <- matrix(magneticField(lonm, latm, 2013)$declination,
               nrow=length(lon), byrow=TRUE)
mapContour(lon, lat, decl, col='red', levels=seq(-90, 90, 5))
incl <- matrix(magneticField(lonm, latm, 2013)$inclination,
               nrow=length(lon), byrow=TRUE)
mapContour(lon, lat, incl, col='blue', levels=seq(-90, 90, 5))