The IGRF is a mathematical model that describes the field and its secular changes as a spherical harmonic expansion. It is updated every five years, and later versions may re-define the field at earlier times. This is important to remember if you are comparing old maps to new ones. The IGRF is a product of the International Association of Geomagnetism and Aeronomy (IAGA), and the original version was defined in 1968.
Every five years, the IAGA issues a contemporary main field model that predicts the field for the next five years. These models have names that are prefixed with "IGRF." Each new model updates the model that was used to predict the previous five (or more) years. Updated models are called DGRF for Definitive Geomagnetic Reference Field. Major updates since 1980 use data from MAGSAT, consisting of measurements of vector components and total intensity of the geomagnetic field between 350 and 560 km altitude.
To correct data sets which had older versions of reference fields removed, add (F0 - Fn) to each data point, where the two parameters are total intensity values computed from the old and new reference fields respectively. See Peddie N.W. 1982, 1983, and 1986 for details. Charts of many types are available on-line, as downloadable postscript files, and for sale (less than $5.00 each) from the USGS, NOAA, GSC, and just about any other government geoscience agency. For example, you could use either the NOAA Geomagnetism page, or the Canadian National Geomagnetism Program's home page.