Compass setting up

Simple answer, No, the GPS direction output, is not accurate enough, you need to have the compass swung onboard and adjusted as ness.

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Dave,

Scotty's right of course, GPS derives heading from movement over the ground, no good for swinging a compass.

what type of compass is it? do you have an idea of how far out it is? do yuou know where the adjustment screws are

I have 'swung' a compass many times, its not hard, all you need is a handbearing compass, pencil and paper to make up a deviation card, Robinson the compass adjuster/nautical antique seller in Hamble does it or we could meet up and I'll do it but not for at least 2 weeks as I'm away with work.



<hr width=100% size=1>Sod the Healey - I think I'll buy an E-Type.
 
Hi Ben,

Thanks for the reply. The compass I have at the moment is just a small Danforth. It does have adjustments so maybe using my hand bearing compass as a guide I will give it a try. If not I may take you up on your kind offer.


<hr width=100% size=1>Take care.

Dave

Dave Knowles
Southampton - UK

http://www.MyCleopatra.co.uk
 
How is it that your handheld compass is immune from deviation but your steering compass isn't?

Unless one does it properly using a pelorus and fixed features known to be accurately charted one may as well just find a low current area and use the GPS (remembering to allow for variation). In the end, unless you are driving a ship around the steering compass is usually only graduated in 5 degree increments.

Our boat is steel and it is still simple to set up the steering compass against the GPS, then in comparison it is within a degree or two of the autopilot electronic compass at the worst point... but can't tell exactly as my steering compass is in 5 degree increments and I can't steer that well either.

John

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your handbearing compass is normally used away and to the outer side of the boat, and is smaller, so much less affected by the surroundings than one nailed in amonsgt all the fabric. Unless you are wearing chain mail or have a metal arm.

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I agree (most unusual, but I do /forums/images/icons/laugh.gif )

The real killer here is how accurately you can read the various compasses involved and to what extent they are being affected by local boat induced errors.
I personally think that a comparison against GPS on a steady heading during a slack water period will probably be as good as, and probably better than,anything else, other than a professional compass swinging session.

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Small boat compasses are nigh impossible to set up with any degree (no pun intended) of accuracy - not least because they are seldom mounted on the centre line and therefore do not produce a sine curve of deviation, so any adjustment on one heading will only increase the problem elsewhere. Your GPS will probably give you as good idea as anything else, provided its taken over a long enough course in good weather. As mentioned elsewhere you probably have only five degree marks and this is probably OK so long as you keep track of your position, so you know where you are, and where you're going. I find a good long reach of river where the course can be taken off the chart is a good enough guide for adjustment, or a head on transit to steer at. Doing a 'swing' on a small vessel with an 'off centre' compass is IMHO a waste of time, best just to get familiar with your compass through careful use. The variation around UK is too small to worry about for the degree of accuracy which can be expected from a small compass.

<hr width=100% size=1>Trev
 
Hand bearing compass is much smaller and hence less susceptible to deviation, also, when swinging the comapss yoiu have the option to move the handbearing compass away from things like engines etc.

As you say there is much more accurate ways of doing it but hardly neccessary for a smallish boat.

I learnt this method whilst working for Westerly Sea school in the 1980's, a master mariner and yachtmaster examiner by the name of John Williams used to come and do it for us, its a two person job and I have done many of them with him.

As rightly pointed out by others as well, anyone that can steer within less than 5 or so degress is a better helm than me....

<hr width=100% size=1>Sod the Healey - I think I'll buy an E-Type.
 
Trev, yes I see your variation is only around 5 degrees or so. We have over 20 degrees here so get plenty of practice with the "East is best, west is worst, or um , ah... oh stuff its thataway" navigation.

GPS gets horribly wrong here with tide information, of course, if you don't tell it about it also.

John

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Having just done this for three days, I can say it is possible to steer to a degree or less, but takes a level of concentration you would not apply to normal everyday boating. It also depends on the compass.

On one boat the compass would swing dramatically, on another boat (a Rule compass I believe) it was possible to do accurate sector searches and arrive back at the drifting target with just a few meters in it.

The quality of the compass seems to have a lot to do with it. A smaller boat with the Rule compass had been accurately and frequently swung, and once on a course, didn't 'hunt' at all, just responded accurately to heading. The larger boat, though easier to keep in a straight line had a compass which 'hunted' all over the place, and we used a gps in Compass only mode to do the dead reckoning bits. Some compasses just don't have enough dampening to be of any use to a motorboater.

For a laugh, try using the GPS on compass setting , and steering a course, and you'll find it is possible to steer a course to a degree or so, even in roughish waters.

Over longer distances, we were using a plotted route using a constant speed and course, and then triangulating at each 'waypoint'. No electronic aids, just compass and speedo, then hand held compass to triangulate.. We were bang in the middle of the 'cocked hat' each time.

<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide
 
Hi Brendan. I think what you say is a compass hunting is the effects of one that is not damped as much as another so it responds quickly (and overshoots) making steering to it difficult.

I would say it is impossible in a small vessel to maintain a heading always within 1 degree of your course in anything other than a dead flat sea. What I think you will find is happening is that you actually wander further than 1 degree off course, the dampened compass slowly responds, you recognise its response and correct, and it slowly (in its damped way) returns but before it has time to drift around to what your true off course heading was. It can be expected in small seas (by that I mean say under say those in Force 5 for small vessels), your meanderings will likely average out to give the effect of having steered a heading within 1 degree all the time, but in fact what has happened is that the average of the headings steered is less than 1 degree off the course. I think that in large seas this becomes less so, as when the sea disturbs the boats heading the correction is not always fully counteracting (for example taking seas on the port bow means that your average heading becomes one that starboard biased).

GPS is damped also, according to the rate of update in your receiver.

John

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I agree, anyone who can steer to with one or two degrees especially in a small boat is a better man than me. On a personal note I would be very reluctant to Swing a Compass based on a hand held, real Compass Adjusters have a verified unit and are expert at their trade even to knowing by sight what "looks wrong" rather like an Accountant looking at a row of figures and knowing there's an error or a Printer looking at a page and 'seeing' the mistakes without reading the copy.

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I agree that it's impossible to actually hold the course accurately at any moment in time..and that you are constantly adjusting. The idea on the exercises we were doing though is to arrive at a planned destination with as few cumulative errors as possible. The overall effect is that you arrive within a degree or so.

We were working hard on these exercises.F4/F5 on a 6 mile run with no landmarks to aim at on the horizon, and 20 feet off the target. I wouldn't want to do this in normal boating, the concentration is too intense.

The 'damped' compass was not damped to the extent it would respond slowly, in fact it was very responsive to small turns, it just didn't swing violently all the time.

The 'damping' on most GPS and plotters is not simply down to the rate of update (this is variable on some modern sets) but down to the software algorithms that average readings (again, on some sets, you can change the level of averaging). As well as damping the course reading, it also allows for errors in satellite reception, or even complete loss of satellite reception.

You can test this by taking a GPS in a car, then driving through a tunnel. Watch the position and speed readings - different manufacturers and individual sets have different levels of sophistication so the results vary but usually the set in the absence of a signal, will calculate where it thinks you should be based on the last position speed and course reading and project your course forward as though there is still a signal.

<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide
 
Thanks for the comment Brendan; I agree with your comment on GPS's, I was just being economical with my description.

There is also a processing delay through GPS's and some time back I checked a number of receivers for the delay in processing and displaying time (I have never checked it for position which would be harder to do). Using UTC (ex WWV monitored by radio ie essentially no error, unlike on the internet where there is a significant and indeterminate error of around 0.5 secs) the difference was typically the negative difference between GPS time and UTC (currently GPS time is around 13 seconds behind - I haven't checked) and typically about another 2 seconds rx processing time behind. I have not made any recent checks, I assume it is less than 2 seconds now in modern receivers - I would be interested if anyone knows or has measured it.

Just a further thought comes to mind on the average track often resulting in the correct course being sailed is this may be the basis of the recommendation that when helming one should not when finding one is off course purposely counteract by sailing off course for a while in the other direction (people do it).

John

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Ah, getting of into realms of esoteria now. UTC and GPS time vary, but that delta is very accurately known, and the time shown on a GPS receiver is actually corrected for that difference and the information to encode that is automatically transmitted to the receiver when the almanac data is updated which happens whenever you turn the set on.

Typically GPS receivers display time accurate to about 1 second of UTC (though the internal clock is accurate to nano seconds of UTC, it's just the software to display the time introduces a delay). If when you checked against WWV, it may have been that your set had not received or stored the latest leap second updates, as this can create an offset of about 12seconds. For more info see <A target="_blank" HREF=http://gpsinformation.net/main/gpstime.htm>http://gpsinformation.net/main/gpstime.htm</A> ....

"Start of Garmin quote>

Provided the unit has collected current leap second count from the navigation message, (current leap second difference from GPS time is only broadcast once in a 12.5 minute Nav. message), or current leap second has not changed since the last time the unit collected this variable, the time displayed on the front of the unit should be accurate to within 1 second of UTC.

>end of Garmin Quote

Joe Mehaffey comments:
This means that IF your GPS does not have (or does not save) the leap second offset from last time it was operated, your time may be off by perhaps 12 seconds until the complete NAV MESSAGE is received by the GPS. Jack and I have observed that "typically" Garmin GPS receivers display time which is delayed from about 1/2 to 1 second behind UTC. Lowrance GPS receivers are usually between 1 and 2 seconds delayed behind UTC. In both cases, this is a result of the display driver subroutine having low priority as the "GPS internal clock" is within a few nanoseconds of correct.

Similarly, the NMEA time output on the serial link is typically delayed a second or two depending on various factors"


<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide
 
I do know enough to have done it properly, allow for the ephemeris to be updated, etc, and I do understand the propagation delays through bus/wire protocols (which is of course a cause of what I was saying if rx is remote from the display) including all the things about the NMEA baud rate (and that many use non NMEA buses- Seatalk as one common example), the transmission time cycle being dependant on the number of sentences sent, and if the display is multifunctional the muxing of other sentences in with the GPS rx ones sent to it eg wind added to the sentences sent to a plotter, etc etc etc). I thought it irrelevant to give a detailed description of those things when all I said was I had made some measurements and gave the results.

I realise GPS time is known accurately and that it is accurate within the rx, which is no surprise seeing it is an essential element of position determination, but I also do know that GPS time is not always corrected to UTC for display (it may be on some or all Garmin, but there are plenty of other manufacturers apart from Garmin) nor is it necessarily corrected for processing delays (read into that everything in the way between antenna and ones eyes! In fact, to avoid the case of my being corrected, also usually between satellite and ones eyes but that extra error is insignificant, of course, from the point of view of display accuracy).

Interestingly, after telling me that I may have been doing it wrong, it seems you corroborate I came up with the right answers, as I take it from your comments that your Garmin rx displays time about 1 second slow and some Lowrance ones 1-2 secs slow, which is in complete accord with my observations, now made some years ago over a selection of rx.

John



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Sorry if I misunderstood, but your comment.....

"the difference was typically the negative difference between GPS time and UTC (currently GPS time is around 13 seconds behind - I haven't checked) and typically about another 2 seconds rx processing time behind"

....suggested you saw a time delay of about 15 seconds between UTC and displayed GPS time. I didn't mention anything about you doing it wrong, just gave a possible reason why your measurements were 13 seconds adrift of UTC before allowing for processing time

Anyway, as I said, esoteria, and probably boring to most forumites, though it's an area I love to delve into.

<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide
 
No hard feelings Brendan. Thanks for the clarification as to how you read my post - I'll try to be more careful next time. It is sometimes hard to both explain and if on the receiving end actually understand what is meant given the necessary brevity of posts.

Best thing of the lot, always, is you seem to be enjoying boating!

Regards

John

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