Plotting and reading a bearing using the Portland plotter

A Portland plotter is basically a ruler with a protractor or bearing compass dial device that can be rotated.

We are going to calculate the bearing from Cape Balshaw lighthouse near Cape Woodward on the Northern Territories on RYA Training Chart 3 , to the lighthouse on the west side on Guillemot Island (Fig 2.58 ‘A’ & ‘B’)

Line up the edge of the plotter so that the big green ‘course’ arrow is pointing in the direction we want to take the bearing (fig 2.59 ‘A’). Then draw a 2B pencil line between the two points and through the lighthouse on Gulliemot Island. The compass bezel is rotated so the two small north pointing arrows are pointing towards North towards the top of the chart and lined up with lines of longitude (Fig 2.59 ‘B’). It is worth while making sure that the parallel and vertical lines on the plotter compass bezel are in line with either a a black line of latitude or longitude on the chart which will make certain that it is aligned with a line of longitude and pointing towards true geographic north pole. We can then read the bearing off the plotter where the large ‘0’ is near the rotating compass bezel, along the midline of the plotter (Fig 2.59 ‘C’)
You should should have a bearing of 106° True. Remember all bearings taken from the chart are in relation to True geographical North as this is where all the lines of longitude are heading towards and what we lined the plotter dial to. Do not worry about the ‘Total Error East and West’ scale on the portland plotter for the moment. We will come back to this in a future module when we examine variation and magnetic errors in more detail.

It always worth double checking that you have the correct bearing is not wildly out. We can compare the line drawn between the two lighthouses with a 106° bearings from the adjacent compass rose. Both lines are parallel with each other which corroborates the accuracy of the plotter bearing (Fig 2.60)

Setting the compass to the distance being measured

Now we can measure the distance between the two lighthouses. All we need to do is place a point of the dividers on each of the lighthouses (Fig 2.61) and transfer this distance without adjusting the dividers to the adjacent latitude scale.

Adjacent latitude scale to measure the distance

We take the dividers to the latitude scale and place one point at a convenient scale mark and then read off the distance to where the next divider point touched the scale as in Fig 2.62.

When you transfer the dividers to the adjacent latitude scale you should place one end at a convenient mark. In this case i placed them at the 5’ mark (Fig 2.62 ‘A’). The other point of the dividers reached to the 10’.8 mark (Fig 2.62 ‘B’) and may be helpful to mark where the divider points lie with a pencil mark. We can either calculate the difference between the two latitudes or read off the distance between them. In this case it is 5’.8 minutes.

So the distance between Cape Balshaw and Guillemot Island lighthouses is 5.8 nautical miles.

1 minute of latitude = 1 nautical mile

So the distance between the two lighthouses is 5.8 nautical miles!

There are a couple things we have to try and remember here. On a mercator projection chart, we need to measure on the side axis of the chart at the same level as we are measuring on the chart. We never use the scales running along the top or the bottom of the chart, always on the sides of the chart. This is because when we are working off the vertical scales we are measuring from a great circle where one minute of arc is equal to one nautical mile. The horizontal scale, on the top and bottom of the chart, are not great circles and as we move further north on the chart all the parallels of latitude are getting concentrically smaller in diameter.

Due to the mercator projection and the oblate spheroid (ellipsoid) shape of the earth there are inherent distortions in our chart. If you measure 5’ minutes on the vertical scale at the top of the chart and check it at the bottom you will see that there is a small difference, therefore a nautical mile gets longer as you move northwards on a mercator projected chart.

This doesn’t really matter to us as long as we remember that when we make a measurement we take the dividers to the side of the chart in the area we are working. In reality the measurement errors we can make working off this scale of chart are minimal but may become significant when making passages of 600 nautical miles or more.