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6.4 Measuring distance
Because a chart is a scaled representation of the real world, it can be used to measure distances between objects. This is useful, among other things, for estimating sailing or motoring time between points. On a Mercator chart, the latitude scale on the sides of the chart serves as the distance scale. It's worth remembering that only the vertical, latitude scale on a Mercator chart can be used to measure distance, not the horizontal longitude scale. This is because each minute of latitude equals one nautical mile, while the distance between minutes of longitude decreases between the equator and the poles. Dividers are used to transfer the distance between two charted objects to the latitude scale on the side of the chart, or from the latitude scale to any points on the chart. Example: If you want to measure the distance between A and B you can open the divider in 2 miles and check how many times it fits between A and B (photo 6.4.1)
So, now we can measure distances and both plot and read out positions, but we also need directions. For example we need to find the course from "Akra Angalistros" to safe-water buoy Σ. To accomplish this we may use parallel rules as shown in this chart below (6.4.2).
First you line this instrument up with the two points. Then follows the intriguing part in moving the device to the compass rose without losing its alignment. Finally, when one of the rules is aligned with the heart of the compass card, you can read course A → B. In this example: 135°.
Besides the parallel rules there are other types of instruments available, notably the Breton plotter – also known as a Portland Course Plotter – which features an adjustable rose.