Rhumb Line

So I woke up in the middle of the night wondering whether ‘about 1900 nautical miles’ was really the distance from Steeple Point to Quirpon Island off the northern tip of Newfoundland or whether I had been sloppy in using the Google Earth ruler to measure it. This got me to thinking about rhumb lines.

The Wikipedia definition of Rhumb Line is:

  • “In navigation, a rhumb line . . . is a line crossing all meridians of longitude at the same angle, i.e. a path derived from a defined initial bearing. That is, upon taking an initial bearing, one proceeds along the same bearing, without changing the direction as measured relative to true north.”

My measurement wasn’t “a line crossing all meridians of longitude at the same angle” but a line crossing all meridians of longitude at the same latitude.

There will be a difference in the lengths of these lines. How much longer will depend how far north the lines are drawn, because of the spherical nature of the earth. The question for me was whether the Google Earth ruler measurement differs from other means of measuring the distance.

So, I reviewed the measurements.

Firstly, for my latitude line, I needed to be more accurate in my landfall in Newfoundland.

The coordinates for Steeple Point are: 50 52 34N  004 33 39W

For Cape Bauld Lighthouse on Quirpon Island the northern tip of Newfoundland: 51 38 24N  055 25 36W

If we move 55 miles south as the crow flies, we come to the harbour at Conche – the coordinates  here are: 50 53 09N  55 53 22W – very nearly on the same latitude as Steeple Point.

The Google Earth ruler gives a length from Steeple Point to Conche of 1,915 nautical miles.

I looked for a site that calculates the Rhumb line and came across this one Movable Type Scripts.

Entering the coordinates for Steeple Point and Conche gave me:

  • Distance:            3,536 km (to 4 SF*) – 1,908 nautical miles
  • Initial bearing:     290°02′46″
  • Final bearing:     249°13′06″
  • Midpoint:            53°38′41″N, 030°17′27″W

You will notice that, whereas the distance is fairly accurate, this isn’t strictly a rhumb line because the bearings are not the same.

Best I can do for the moment.

By the way, Conche looks a good place to visit in summer.