Most boats have a compass on board. Most boaters never use them. That's sad. There is some confusion about exactly what that compass is pointing at and why it doesn't agree exactly with the chart or the COG displayed on your fancy GPS or Autopilot.
As the world turns: True North
This is simple. The world is a spinning thing and things that spin have an axis of rotation. It's that axis that defines True North (and south). Polaris (the north star) is exactly above the north pole of this axis - hence the name.
(Magnetic) Compasses don't care about rotation
The magnetic compass on your boat is aligned to Earth's magnetic field. Unfortunately the magnetic north pole didn't align conveniently to True North. Ask your geologist friend for the full explanation for this or read the wiki article below. That magnetic axis is not stable either. It wobbles and so the difference between True and Magnetic north varies slightly every year. Variation is also affected locally by the magnetic composition of the crust in different places.
Here is a WIKI page that explains in some detail why variation (declination) occurs. It even has a cool animation showing how it changes over time.
So wherever you are, there is some difference between what your compass says is north and what your chart or chartplotter says is north.
The difference between True and Magnetic North is called Variation.
Take a look at any nautical chart and look at one of the compass roses. It will show a Magnetic and a True ring. In the center will be some text like:
VAR 15.00 (2012)
Annual Decrease 4'
Because it's different everywhere and because it changes over time, it's not possible to just offset a compass to make it display True North.
So, just add or subtract Local Variation to my compass reading
Well... sort of. And in fact for coastal cruising, that's just fine. The problem is that your compass and it's installation are not perfect. Your compass has inaccuracies and these inaccuracies are different at each direction you point. This inaccuracy is known as Deviation (and that is what we aim to measure). Some of the deviation is due to the compass itself but more of it is probably due to how and where it is installed on your boat and what other magnetic junk is installed around it. Entertainment speakers are a good culprit but so is that big honkin' diesel engine.
True Virgins Make Dull Company (Add Whiskey)
This is the mnemonic (well, one of them) to remember how to convert fully from True Degrees to what your compass says. Linda and I were introduced to this topic in the ASA105 class we took at Black Rock Sailing School in Boston. Later, this same topic was covered - although not as comprehensively - by the Coast Guard Merchant Mariner (Captain's) License class and exam.
TVMDC (Add West)
T = True
V = Variation
M = Magnetic
D = Deviation
C = Compass.
From True, add Variation (add for west, subtract for east), this gets you to Magnetic.
Coastal cruisers can more or less stop here. Anyone who is using a compass to do really critical offshore navigation can definitely not stop here. They must account for deviation to get that last few degrees of conversion done. Missing Bermuda by 3 degrees from the mainland is... missing Bermuda!
Add Deviation (add for west, subtract for east) and this finally gets you to the Compass reading.
Determining the D
Deviation for a ship's compass is usually shown on a table or a graph. The deviation is different for every direction you point, so the number you plug in for D depends on your heading. There's lots of ways to measure this and there are professional services you can hire that will give you a nice deviation chart. I'm enough of a cheap geek to want to do it myself. You can also hire a service to correct your compass by adding compensating magnets to it. That all goes out the window when you install those new cockpit speakers.
Because Deviation changes with direction, it's important to measure it at many different points. Here's my very low-tech way of doing that with some off-the-shelf software and a spreadsheet.
- While Underway in smooth water or very still at anchor, point the boat at some distant fixed landmark.
- Using some GPS application (see below), measure the TRUE angle from your current position to that landmark on the chart
- At the same time (as soon as possible) read the ship's compass.
In the spreadsheet, I note what landmark I'm using, the TRUE angle and the MAGNETIC heading from the compas. The spreadsheet computes the Deviation at that heading (using TVMDC + w)
I used the Navionics Phone App but you could use something like OPEN CPN or anything else that can plot your GPS position and display angles to landmarks on nautical charts. If you have a chartplotter that lets you measure ranges and bearings, you can use that as well. Honestly, the phone app is faster and if you measure ranges to distant objects, it really doesn't matter if the GPS is 5 meters off.
Take measurements at as many directions as possible and plot them. It doesn't hurt to retake them at the same headings later as a check.
Here is what our spreadsheet looks like in Google Drive. Since I'm using the phone to take the measurements, I also enter the data directly into the Google Drive app in the phone. No pencil/paper involved.
Using Navionics, measure the TRUE angle from your current position to a fixed landmark. That is entered in Column B. Column C (Degrees Magnetic) is determined by adding (remember Add West) our local 15 degrees of Variation (from Column H). In the screenshot below, we're measuring 046T from our position to a water tower that we can see.(The boat really is pointing at that tower despite what our little arrow looks like)
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| Measuring a 046T bearing from our anchor in Provincetown Harbor to a water tank we could see visually and on the chart. |
Now, Read your magnetic compass (as accurately as possible) and enter that value into the PSC (per ship's compass) column. Argon has two compasses so we enter the reading for each in Columns F and G.
The Deviation then magically appears in the Deviation Column(s). It is found by subtracting the Ship's Compass reading(s) from the Degrees Magnetic. If you had a perfect compass and no metal objects anywhere near it, these two would be equal and your deviation would theoretically be 0. That's a big IF and lots of theory. In reality, you are most likely to see several degrees of deviation at many points of your compass.
What did we come up with?
Below is shown our actual table as measured in 2015. The Red and Green lines show the Port and Staboard compasses. We will certainly collect more points (especially around that spot at 150). Reading a magnetic compass on a bouncing boat is not that easy so it's a good idea to do the readings several times. It's possible that that anomaly at 150 will smooth out as we take more readings.
