X + 2(RMS). While this information is helpful in detecting anomalous areas, 
the differentation of true anomalies from other variables such as diurnal 
variation would be difficult. 
SUMMARY 
An evaluation of the 1965, H.O. 1706 World Magnetic Variation 
Chart was performed by direct comparison of charted values and survey data. 
Utilizing the World Aeronautical Chart quad-system, a mean and median 
value was computed for each chart providing an accuracy estimate of the 
isogonic lines appearing on these charts. Based on these values it was 
observed that the mathematical model used in constructing the 1965 world 
chart generally agreed with observed data to within 0.5°. An 80% confi- 
dence interval and root mean square were also compiled for each chart and 
provide indices of reliability and dispersion respectively. 
CONCLUSIONS 
It is generally conceded that present world charts are probably accur- 
ate to about 0.5° for the United States and Europe, but more in error in 
other areas, particularly at high latitudes (Chernosky et al., 1965). From 
the results of this study we have shown that 83% of the World Aeronautical 
Charts investigated have a median of less than 0.5°. This indicates that 
previous estimates of chart accuracies, while correct, are perhaps conserva- 
tive. We must realize, however, that a great deal of refinement to the world 
chart remains. 
Improvement of future charts depends, in large part, on the acquisition 
of additional data. As this study shows, over 67% of the areas considered are 
represented by less than 50 data points. This is particularly true for most of 
the southern hemisphere and the polar areas. Although many of these areas 
are inaccessible at present, additional data would result in considerable 
improvement of the world chart. 
The mathematical model derived by the Coast and Geodetic Survey 
is, for all practical purposes, as refined as is warranted by the available 
observed data. It is concluded, therefore, that the recent world chart 
drawn exactly from this model is equally as realistic. 
