Plotting

Motion through one minute of latitude around a great circle of latitude represents a movement of 1 Nm across the surface of the Earth. Motion across one minute of longitude at the equator represents an equal distance, 1Nm. The surface motion for a one minute change of longitude is reduced as latitude increases. The longitude scale on a conventional plotting sheet is designed to take account of this change of scale.

I used a conventional plotting sheet to solve Ex6 q1 of the RYA ocean yachtmaster course:

Plot the first position line (Noon latitude in this case)

Plot the second position line

From any point on the first PL draw a vector to show the run of the boat

Run a line parallel with the first PL to the end of the boat vector

The intersection with the second PL shows the new **Observed Position**

The longitude scale was defined by the grid at the bottom right of the plotting sheet. How to use draw software to solve the problem:

The radius of a circle of longitude varies with the cosine of the latitude. So the longitude scale is proportional to the cosine of the latitude.

We are at latitude 50S; cos 50 = 0.64. Draw a large square and set the height:width as 1:0.64; centre on this rectangle and sub divide the scale in one minute steps

**Using real graph paper the vertical
scale would be easy to define. You might chose 2cm to represent 1 minute
of latitude at 50 N. If so the horizontal scale for longitude would become 2*0.64
= 1.28 cm per minute.**

Solution for Ex6 Q2

And for Ex6 Q3 on a plotting sheet

Click here for a latitude 50 degrees plotting sheet

These bookish questions are interesting however for a sample of reality follow the link for Sun Run Sun where you will find log extracts and a plot for the afternoon of July 31 1988 when Tony Burris was on passage between Egypt and Crete.