Starpath Inland and Coastal Navigation Course
Plotting positions and courses
To measure the direction of a line on the chart...
(1) Align parallel rulers with the line in question.
(2) Move the parallel rulers without slipping to the nearest compass rose and align one edge with the center of the rose.
(3) Read true orientation of the line from the outer ring of the compass rose, or read the magnetic orientation from the middle ring of directions.
"Compass rose" is an unusual term for this diagram on a chart, but it is used universally, even on high latitude charts that do not include any compass directions at all on the diagram. On typical compass roses, however, you have 3 scales, an outer scale showing true directions relative to true North at 000 and a middle scale which is rotated by the local magnetic variation so it reads magnetic headings, and then a third inner ring that shows compass directions marked off in compass points (each point is 11.25°).
If you want to be double sure that you did not move or slide on the way to the compass rose, you can walk the dividers back to the line after reading or marking the direction on the rose. If the parallels do not line up when you return, you will have to repeat the whole process, since you do not know if you moved coming or going.
Place a tip on the line and then slide the parallels up against the tip. Then rotate the parallel rulers back down to align their edge with the line on the chart. This method is a good way to get a careful alignment even when bouncing around in a seaway.
Then use one hand to hold down the aligned ruler, and use the other hand to move the other part of the rulers away from it. This is taking the first step with the rulers. Then change your pressure to hold down the one you moved, and bring the back one up to it.
When the ruler edge will finally reach the center of the compass rose, align it with the center of the rose and read the appropriate ring where the parallel ruler edge crosses it. If the light is poor or when bouncing around, it may be necessary to draw a line along the ruler edge crossing the scale and then read the scale.
To read the scales, we must do just as when reading the latitude scale or any other scale. Count the tick marks in between the labeled ones to be sure we know what they stand for. A common mistake can be to count them as 1° each when they are actually 2° each, or vice versa.
In this example, there is 1° per tick mark on the True scale and the direction we want is 320° T. Notice that it is difficult to read along the edge of the clear plastic rulers. Drawing a line will help in a case like this.