4 different course readings which one is best to follow

Jobs_a_ good_ un

New member
28 Jan 2005
home bristol boat where ever we end up
Ive got a Simrad cp 34 which gives me a course via GPS
Ive got a roberttson auto pilot which gives another course about 3 degrees difference in the solent magenetic i presume
Ive also got a fixed compass which gives a course about 5-10 degrees difference from both of the above
and ive got a hand held compass which is different from all of them
Help !!!!!!!!!!


To solve that one you first will really need a Compass Adjuster to work his wizardry on the Ship's fixed magnetic compass.

Remember that the GPS will show your course "Over the Ground", not the way the boat is pointing owing to cross currents, tidal stream and any leeway caused by the wind.

My Autohelm (Raymarine) handbook gives the method for correcting the autopilot so that is reads the same as the Ship's fixed compass so get that corrected and the deviation card on prepared first. I presume that the Simrad handbook would be similar.

To check the hand bearing compass hop over the side into your dinghy (without any o/b fixed}, after doing all the above and, being slowly towed behind the yacht, the see if it reads, within reason, the same as the Ship's Corrected Fixed Compass. The reason for being in the dinghy is to avoid any magnetic deviation. Alternatively, if the yacht is on a mooring, stand well clear of any magnetic effect and check the hand bearer against the fixed (corrected) compass.

Hope this helps,


Deleted User YDKXO

Agree with MEPQ7. You should get the fixed compass swung especially if you've added electronic gizmos to the helm since the boat was built
This is something you should do on a regular basis anyway, like once a year at the beginning of the season. Find 3 or 4 transits (2 or more fixed objects in a line) on a chart preferably in substantially different directions. Work out from the chart what the bearing of those transits really is and then line up your boat with each of the transits and write down the various compass bearings you are getting. You will soon see how accurate each one is and if your fixed compass is out by more than 1-2 deg in any direction, get it swung.
Hand held compasses can be badly affected by other magnetic fields so take it well away from any metal or electrical source before you take a reading. Worthwile finding positions on your boat where the hand held compass is least affected for future reference
Also check that the fluxgate compass driving the autopilot is not close to anything metal or electrical. I once had a boat with an autopilot which was all over the place. It took me weeks to realise that the carton of beer cans I had stowed under the dinette seat was affecting the compass. Doh!


Active member
25 Jun 2001
I'll just keep out the way - you clearly don't know if you're coming or going! /forums/images/graemlins/ooo.gif


14 Jun 2004
Swansea Marina
Might also be worth checking that your GPS is set to the same datum as your chart. Probably WGS84 but might be British Ordnance. That can cause a discrepancy of a few degrees.


4 May 2004
London & Cape Town
Sami, you def need to get this checked out.

Compass, GPS, auto pilot on mine are all agree to within 1-2 deg.

Your hand held is not to be relied on for absolute bearings, too much human error... but if your ships compass is out by 10 deg it needs swinging.

Ps don't confuse COG with heading as stated above.

We've all made that mistake.


New member
16 May 2001
S. Yorkshire / Devon
Forget the GPS, that doesn't tell you where you are pointing.

You can set the autopilot compass head easily.

1. Find it.

2. Release it so it can be rotated.

3. Stand on the flybridge and take a reading along the centreline of the boat with the handbearing compass, and shout to a pal to move the fluxgate compass head slightly so Autopilot display matches your handbearing compass reading.

4. Tighten it all up.

5. Follow proceedure for 'swinging' auto pilot compass (usually involves motoring round in a constant circle whilst pressing buttons).

6. Get a specialist in to swing steering compass and produce deviation card, unless you can adjust it yourself.


New member
31 Aug 2002
Stratford upon Avon & S.Devon
I'm with you Trevor. With the movement you get in any sort of sea it is often impossible to read the compass to anything better than the nearest 5 degrees. Well it has been with the compasses on the last 3 boats I've had anyway. OK I understand that if you're on a long leg of a trip and power goes then you could be embarrassed if your compass was 5 degrees out but for most peoples' boating that level of inaccuracy is livable with. IMHO.

Sea Devil

Well-known member
19 Aug 2004
Boulogne sur mer & Marbella Spain
My last boat was exactly the same - and I had swung the compass -
I really think it is normal.

Assuming you are confident about the way-point then use the GPS - (except when crossing the channel because the tidal effect is uneven so you should use the compass except for the last part) Use the compass to set the boat roughly on the course you want then use the GPS for fine tuning...