Three Ways To Treat the IGRF on the Earth's Surface.
With the adoption of an International Geomagnetic Reference Field based on geocentric spherical coordinates, the problem will arise of how to use it on Earth's spheroidal surface. The most straightforward and accurate method is to synthesize the values of the field, using the correction factors for the shape of the earth. Some may wish to avoid these correction factors and use the geodetic coordinates directly. Another method is to use three different sets of coefficients for X, Y, and Z, each of which would accurately give the field value of one element on the earth's surface but would not be correct for the other two. The three methods are compared to find out how much work is involved and how accurate each is. It is shown that ignoring the correction causes unacceptably large RMS errors. The method which employs three sets of coefficients gives acceptable results but is somewhat awkward. 8 pp. Refs. (Author)