How to use dividers
Using dividers and the latitude scale on your nautical chart, you are able to measure distance in nautical miles. (Remember, do not use longitudes to measure distance. Longitude lines converge at the poles and the distance between them changes relative to your position on the earth.)
One minute equals one nautical mile. Examine the scale of your chart. In these examples, our chart is in degrees and minutes.
Place one point of the dividers at position A and the other point at at position B. Then, maintaining the spread, measure the distance using the latitude scale. In this case, the distance is 15 minutes or 15 nautical miles. Always use the latitude scale located in the same horizontal region that you are measuring.
Walking the dividers enables you to measure distances greater than the span of your dividers.
You can either draw the line or use a straight edge to guide the dividers.
To measure, set the dividers to a whole number, in this case we use 12, and walk the dividers along the line.
The distance between the buoys is 24 nautical miles (two steps of 12 miles each). But it doesn’t always work out so evenly.
Here we adjust the last step of the dividers, measure the distance, and add it to the cumulative total of previous steps. We only had one previous step in this example so the distance measured is 12 plus 7.75=19.75 miles. If we had stepped the dividers two times before adjusting for the final measurement, the distance measured would have been 12 plus 12 plus 7.75 or 31.75 miles.