Originally Posted by daddle
You cannot calibrate the ship's compass against an uncalibrated fluxgate compass (instrument, iPhone, autopilot
If using GPS and a careful helmsperson one must use the heading not the course. I'm not so sure reversing the course between marks is very effective or convenient. A trustworthy (calibrated) heading cannot be obtained with the typical GPS or chartplotter
. A sight thru the mast and forestay to a landmark plotted on a known good chart is about the only way. At night one can use Polaris aligned with the mast and forestay. For the EW calibration you will need to choose another star, landmark, or boat feature.
If variation is large one should use a magnetic reference rather than a true one, perhaps.
Don't attempt it in a harbor. Or where the chart indicates any underwater issues like pipelines, powerlines or magnetic anomalies. Or near a bridge.
It's a good thing to fiddle with on long calm nights at sea.
If you don't think that a differential/WAAS GPS can be used (of course correctly is the operative term) and ANY other method is better on a small boat
attempted by one or two "unqualified" people then go ahead and spread the word...
On a calm day with little or no current
(in comparison to boat speed)...I dare anyone to calibrate the average small boat
compass with something other than a GPS and get better results...go ahead and waste your time....the average vsmall boat compass isn't even marked in increments big enough to notice the change...
Here's right off the Ritchie Compass site-
Ritchie Navigation - Compass Compensation
Method 2. (Requires the use of GPS or Loran)
Step one. While at sea, with the compass in its intended position, but not finally secured, obtain the Loran/GPS bearing to a visual buoy or landmark that is within 10o of a North/South line.
Step two. Position your boat along that line and steer your boat directly at that mark. Turn the port/starboard compensator until the compass heading matches the Loran/GPS bearing.
Step three. Check the Southerly course by steering
away from the mark, to a bearing 180° from Step 2. The compass heading should be bearing from or bearing to +180 degrees. If any error is present, it is an alignment error. Rotate the compass itself to correct for one half of this error. Repeat Steps 1 & 2 and then recheck this Step 3.
Step four. Simply repeat the procedures of Steps 1, 2 and 3 for the East/West course, using the fore/aft compensator, although, at this time, any alignment error should have been eliminated, and no alignment correction is required.
Step five. Upon completing the procedure, secure the compass in its final position.
Note: When performing this method, always use the bearing "To or From" mode on the Loran/GPS. Do not use the Heading Information because it is inaccurate in real time.
I will add that with a WAAS GPS and a decent boat speed, you CAN just use the heading (in real time).... just make sure that it is staying within one degree or so with the bearing to/from. It's easier and no need for a visual landmark)