How to Measure Distance
On Earths surface, 1° of latitude may be considered 60 nautical miles in
length; whereas, the length of 1° of longitude varies with latitude.
Therefore, the latitude scale must be used for measuring distance.
Although this scale is expanded on a Mercator chart, the expansion
exactly equal to the expansion of distance at the same latitude.
Therefore, in measuring distance on a Mercator chart, one must be
careful to use the latitude and longitude scale in the area one is
measuring. NEVER use the longitude scale.
How to Measure
Use the table and figure 1-12 to learn how to measure distance on a
Place one point of the dividers at the beginning of the area
to be measured.
Open the dividers to the desired distance to be measured.
Move the dividers over to the closest latitude scale; do not
open or close the dividers!
Place one point of the dividers on a whole degree of
Place the other point on the latitude scale.
Determine the distance between the two points. (In figure
1-12, the measurement indicates 10 nmi.)
Figure 1-12. How to measure distance on a chart.