variation was sorted with respect to both sign and magnitude into the appro- 
priate interval and a count of residuals falling in each interval tabulated. 
The vertical axis of the histogram is the percentage of the total number of 
residuals contained in each interval and is graduated in steps of 10%. 
Statistics 
1. World Aeronautical Chart 
The histogram and associated statistics for each World Aero- 
nautical Chart (WAC) are presented in the numeric sequence 
shown in Figure 1. WAC numbers are listed consecutively 
except for those gaps created by charts containing no data. 
2. Number of Points 
The total number of data points (NO PTS) within each 
chart is likewise tabulated and presented with each illustra- 
tion, 
3. Mean 
The mean for each chart was then computed and is defined 
as follows: 
Mean (X;) = 2:%e = Xo) : 20%) 
Where 
Xp = mean of the residual variation values 
X_ = charted variation value (H.O. 1706, epoch 1965.0) 
Xo = _ observed variation value (all available observations 
since 1900 extrapolated to epoch 1965.0) 
Xr = residual variation value 
N total number of data points within the chart 
This value represents a type of average that divides a distribution 
into two equal parts in terms of the total values of the residuals. 
