GPS and compass variation

[nickjaxe]

Hi all never quite got my head around compass variation, all I know is for my area its 4deg west at the moment, what I normally do is subtract this from true, I am right with this, all my boating is to do with diving off ribs we go offshore maybe 6miles max, anyway our club have just got a new gps an MLR312, now the one on my boat is a garmin 126 and you can set it to auto correct the variation, looked at it today its 4deg west, but our new set is a manual adjustment and marked + or - the number of degs, to set it at 4west am I right to set it -4deg, just to clarify about the saying I think it goes east is least west is best, the thing I did not understand about that saying, is that referring to eastern hemisphere, western hemisphere, and am I correct when in my local area just to subtract -4 no matter which direction I am going. hope I am making sense.

Nick in cheshire.

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[SydneyTim]

"Variation East Compass Least" means that with an easterly variation the compass bearing will be lower than the true one, conversely West= Best therefore the compass will read higher than true.

Therefore to correct for a 4 degree west variation if you are going from compass to true subtract it, and if going from true to compass add it.

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[Norrsken]

There is an old rule called the CADET rule which goes

Ccmpass
Add
East for
True

Therefore Compass subtract West for true, the hemisphere you are in makes no difference.

Since I guess you will be steering by a magnetic compass, the compass will read 004 degrees when you are steering North. If the instruction book does not say which is plus and which is minus, set your correction to +4 and go out and steer into the current. If the GPS agrees with your comapss heading we've got it right. If the GPS reads 8 degrees high, then we've got it wrong so set it to -4 and try again.

Best of luck


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[VicS]

I have never really managed to get my head around the east, west, least, best business.
I suggest you draw yourself just a crude compass rose, mark true north and the magnetic north a bit to the west of true north and you will see that the angle between magnetic north and any other point (going clockwise round the circle) is more than the angle between true north and that point. For example 90 T = 94 M.
Hence magnetic bearings or courses are greater (by 4 degrees) than the true bearing or course. If you keep this fact in your mind you cannot go wrong.

Regarding the correction to the GPS an anomaly has been reported in the way in which the correction is applied to MLR instruments. It was on the 2 March on scuttlebut. Search for MLR over the last 2 weeks and you will find it.


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[LadyInBed]

You could do a bit of online learning <A target="_blank" HREF=http://www.sailingissues.com/index.html> here </A>
Select Nav Course (Ch 3 is compass)

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[nickjaxe]

Ok guys 1st thank you for taking the time to help me I do appreciate it, all interesting stuff "so am I correct to think if I set our MLR gps to +004deg it should read the same as our compass as long as the compass is giving correct readings" this is all I want to do with it really, all my navigation is on mag headings off the chart I do not use true, now to complicate things, vic had a look at the post you mentioned, that seems to make maters worse, trust us to get an MLR unit, I dont know if ours will be affected the same, I will e-mail mailspeed thats were we got it.

Regards nick in cheshire.

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[dickh]

Be careful - I seem to remember another thread recently with MLR kit that the variation is entered the "wrong way" to convention - so you could end up 8º out....
Check carefully and compare with another known GPS

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I'd rather be sailing... /forums/images/icons/smile.gif

 

[VicS]

Sorry to have thrown a bit of a spanner in the works but I thought it worth bringing it to your attention. I think what I would do is either contact MLR, (part of Thales Navigation aren't they now?) or enter in the correction blindly following the instructions and then go out for a spin and see if compass and GPS agree or disagree by 8 degrees.

I would not feel inclined to trust a mail order firm. They may be very helpful but you do need to be confident that you are talking to someone with detailed and expert knowledge of the equipment concerned.

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[graham]

Personally I think its better to use the GPS in True.

There is no confusion then just add 4° in our neck of the woods plus any deviation to give a compass course .

If the GPS is set for True you can apply distance and bearing directly to the chart with no further allowances.

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[nickjaxe]

Ok all noted thanks guys, I suppose there is a lot to be said for setting it for true, I think I may have sorted it today, got my garmin 126 mounted that on the boat along side the clubs MLR gps, put them both in simulator mode and set them both up to the same waypoint 70 miles away, my garmin is set to 4west,I set the MLR to +4 on that setting they both showed exactly the same CTS 273 distance 74.3mls, so I think I am sorted, but I have leaned something by it to also pass on to my diving buddys about magnetic variation.

Thanks for all your help guys, Nick.

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[alant]

If you are in a RIB, using a compass to steer can be hit & miss anyway, considering the bumping around at speed, so its unlikely you will be able to steer within 4 degrees variation.
Any 'high speed' nav is a much more a matter of pre-planning, with known 'courses to steer' including times for each leg & going for the waypoints in most cases. Applying 'anything' to a chart en-route is impossible unless the boat is stopped.
I'm in agreement with the others regarding GPS, I prefer to keep it on TRUE, saves any further confusion particularly in any emergency.

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[nickjaxe]

Thanks for the comments, all noted got this reply from mailspeed re the MLR gps with the variation prob as follows.



Nick/Elaine,

The problem to which you refer only effects the MLR SP24 and SP24XC hand-held GPS's, where compass variation has to be entered as a "correction". So, for example, a local variation of 4 degrees West is entered as 4 degrees East.

With the FX312 and FX412 fixed GPS's, compass variation is entered in the more usual + or - format. Hence to display bearings in degrees magnetic, using the example above, you would add 4 degrees to the true bearing.

Regards,

Peter Holmes
Mantsbrite Electronics Ltd.


So its sorted thanks for all your input guys going to give some thought to going with true set, very pleased with the above responce.

Nick.

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