Starpath Inland and Coastal Navigation Course
Plotting positions and courses
To plot a course line from a position...
Generally the line is our intended course away from a known position. The course could be specified in True or Magnetic terms, but in either case it is just one unique line on the chart. The only difference is which ring we use on the compass rose.
As with plotting positions, this is one of the key operations in chart plotting. We have a known position which we might have obtained from electronic equipment or from some piloting technique, and we have a known course we wish to sail which will take us to our destination. Or, we could be under sail, and we are simply plotting out our course over ground (COG) as given from the GPS.
Generally it is the COG that we need to plot here since this is the way we are actually moving. The compass heading of the boat is the way the boat is pointed but is not necessarily the way the boat is moving.
We use the dividers to help align the plotter. First use the divider tip to carefully count off the numbers on the magnetic ring on the compass rose. Again, we wish to confirm what the tick marks stand for since the bearings are only labeled every 30°. Once we find 230, we count out the next 8 degrees and then plant the point at 238M.
Next slide the plotter up to that point and then rotate it till the center is aligned along the edge. Then carefully roll it back down to the 1352 position and draw the line.