MAPPING CONTOURS 
The positions of the contours are determined by proportion 
between any two adjacent levels, thus the 80 contour would pass 
half way between elevations 79.5 and 80.5 or it would pass through 
494 Z 
AZ. 
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Fig. 21. Contours INTERPOLATED on Cross-SEecTION SURVEY 
a point two-fifths of the way from 79.6 to 80.6. There would be 
three (one-foot) contours between 24.5 and 27.8 and five between 
65.2 and 70.7. 
All the contours passing between any two levels should be lo- 
cated at one operation rather than to follow out each contour separ- 
ately. In interpolating on a cross-section survey it is well to fol- 
low along a row of squares rather than to work without any definite 
order. 
Frequently two-foot contours only are interpolated, in which 
case they should always be the even contours as 70, 72, 74, etc. 
On small scale plans sometimes the contour interval is five or ten 
feet as 70, 75, 80, 85 or 70, 80, 90, etc. In any case interpolating is 
done in the same manner. 
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