INTRODUCTION 
Background 
The Naval Oceanographic Office in conjunction with the Coast and 
Geodetic Survey, publishes the H. O. 1706 World Magnetic Variation Chart 
every five years. This chart is a smoothed representation of the earth's mag- 
netic field depicted by contours of equal values called isogonic lines. Such 
charts are indispensable, because from them are derived data necessary to 
accurately navigate both military and commercial ships and aircraft. 
For a number of years the cartographic technique used in compiling 
the world chart consisted of manually plotting and contouring smoothed survey 
data extrapolated to epoch. With the publication of the 1965 world chart, 
however, a more sophisticated approach in constructing the chart was 
employed. The Coast and Geodetic Survey, for the first time, constructed 
the world chart from a mathematical model of the geomagnetic field. An 
important step in the derivation was the analytic smoothing of survey data. 
The smoothing produced a mathematical model expressed by a set of spherical 
harmonic coefficients to degree and order 12 (Hurwitz et al., 1966). Chart 
isolines were computed from the mathematical model and automatically plotted 
to produce the world chart. 
Purpose of Report 
Because of the important role magnetic variation plays in navigation, 
its accurate portrayal is a significant goal. To this end, evaluations of 
previous world charts have been made with subsequent improvements incor- 
porated into the charts. A detailed evaluation, however, has not been 
practical prior to the 1965 chart. This has been due, in large part, to the 
formidable task of abstracting representative quantities of data from the 
charts for comparison with observed data. With coefficients of a mathema- 
tical model available for the first time, the difficulty of synthesis can be 
resolved by electronic computers. Utilizing this convenience, a direct com- 
parison of charted and observed values was made and an estimate of the 1965 
World Magnetic Variation Chart's reliability achieved. 
PRELIMINARY PROCESSING 
Division of World Chart 
To perform the evaluation in as much detail as practical, the World 
Aeronautical Chart quad-system was chosen to divide the world into small 
