Swinging the compass against the GPS?

[PhillM]

I was wondering if I could use my GPS to swing my steering compass? My thinking is that I would steer to 0 degrees by the GPS, then record what the compass says, then do the same for 45 degrees, 90, 135, 180, 225,270 and 315.

I was on a short boat handling course and asked the instructor, but he said that would not work. In Tom Cunliffe’s Expert Sailing book, he suggested using a hand bearing compass for comparison. So why not GPS?

I should say that I am not planning on crossing oceans just yet (or even getting out of the Solent by far) so deviation is not overly important right now, but I am interested in learning and practicing for a time when I might want to go that bit further.

What does the forum think?

 

[status]

The first issue I would see is that a GPS tells you the direction you are traveling in whilst the compas tells you the direction you are pointing, the two do not always coincide.

If you can find a bit of water with no tides or currents then on a windless day I cant see the two would be sufficiently out to cause a problem.

As you suggest, I do most of my sailing along the coast using Mk 1 eyeball so having checked there are no glareing errors (using gps and handheld) on the steering compass I am not bothered.

For the 'what if' doomsayers, I have three GPS on board (not planed, just happened) one of which is handheld, paper charts and a handheld compass. On totting up the risks, if the main compass is a little out I will live with it!

Stuart

 

[maby]

The trouble with doing it the way you describe is that there is a long lag in the GPS calculating your course and it is very difficult to get accurate readings. Think about it - the GPS is receiving a series of locations for your boat and having to calculate the course by applying basic trigonometry to them to work out the direction you are travelling. But in the first place, each individual fix is not particularly accurate - with standard GPS electronics, an individual fix is only accurate to a few tens of yards - your plotter determines a more accurate position by averaging a series of fixes taken over a period of several seconds - fine to give you an accurate static position, but not so good if you are moving. Add to that the difficulty of holding a course to sufficient accuracy for a long enough period of time - the course displayed on your GPS is based on the smoothed out fixes collected over the last several seconds, fed into the trigonometry and may differ significantly from the course you are steering now.

Your better solution is to do it by using clearly identifiable landmarks a moderate distance away. Most plotters (and good handheld GPSs) will allow you to move the cursor to a point on the map and give you a bearing for that location from your current position. Choose a time with minimal current and wind, preferably in a bay or estuary where there are landmarks in most directions, identify one that is on the map and also easy to pick out by eye, put the plotter cursor on it, steer the boat to point directly at it, keeping the speed as low as possible while retaining control, and give the plotter time to settle down, then read the bearing off.

 

[stuarta]

I think in theory it should be do-able. I say in theory as the practice may not be so simple.

Your GPS will give COG (T) so there will be variation to consider, albeit a pretty simple and consistant consideration. The bigger problem is that any latteral effect on the boat will alter your COG. i.e. Wind, tidal stream. The effect of these components will not be consistant and will vary depending on which course you are steering. There will also be other local effects which will change how they affect your COG.

Having said that if you find suitable conditions it will give you a rough stab which is probably all you need for your current purposes.

From a quick Google I see that it has been discussed on here before - including discussion on not using COG:

http://www.ybw.com/forums/showthread.php?t=202492

 

[Chalker]

You'll need to make sure that there is NO tidal flow making your heading different to your track. Compass will show the heading but the GPS will give the track - COG.

The problem is how to know that this is no tidal flow. Tidal stream atlases are predictions, not absolutes.

1/2 a knot of cross tide flow whilst you make 5 knots will give an estimated 6 degree error which may be more than your present compass error!

In the past I have located my position by GPS on a chart, measured the heading to an identifiable fixed point ashore (not a buoy), headed directly at that point and noted the compass reading. You do not get fixed angle intervals but can draw a graph of the headings that you can measure.

 

[Chalker]

I see that 4 of us were busy writing responses at the same time!

 

[VicS]

Not only tidal flow but the wind will affect it.

In theory in perfectly calm conditions and still water its possible.

Better to use transits with fixed charted marks or to stand well away from magnetic influences and use a hand bearing compass

 

[prv]

In the past I have located my position by GPS on a chart, measured the heading to an identifiable fixed point ashore (not a buoy), headed directly at that point and noted the compass reading. You do not get fixed angle intervals but can draw a graph of the headings that you can measure.

That's exactly what I was going to suggest. If the targets are a reasonable distance away then any small error in the position has a negligible effect on the angles. And on Lake Solent, you ought to be able to find targets in any direction you need.

Pete

 

[onesea]

The problem is your GPS gives COG so you would have to maintain exact heading, without the GPS Ariel moving due to rolling etc there is also the GPS built in smoothing to consider.

You would probably achieve a degree or 5? accuracy at best. Which is probably as accurate as you can get? although how accurate is it?

Another way of doing it that is not quite by the book but easy? and more accurate?

I see your Solent based so:

Position yourself in the Solent where you are surrounded by KNOWN positions (not buoys) as far away as possible.

Slow moving (less than 5 knots but depending on distance this is insignificant)

Point boat exactly at known object example:

EG Fawley Power Station Chimney press MOB (on GPS ) to record your position.
Note:
Object: Power Station Chimney
Position:
Boats Compass heading:

Then do same for all headings:
Spinnikar tower, Hurst castle Light house, Bramble pearch, calshot light, end of Ryde pier, the forts etc etc

Once you have done this for enough headings you will be able to draw up a deviation chart.

Calculating Magnetic for Variation etc then use GPS or way-point program to calculate bearing between to "known" positions.

The advantage of this is you can do this anytime any place and current.

You can also do similar when running in on known transit bearings.

Accuracy??

Well assuming both positions are 20m out 40m total error over a mile or 2? Probably more accurate than you can read your compass...

Awaiting the RYA method

 

[maby]

I see that 4 of us were busy writing responses at the same time!

and we all got it right!

 

[Chalker]

Four consecutive answers with agreement, no disputes!

We claim a record on YBW forums. (please, no arguments regarding forums vs fora!)

 

[Neil_Y]

If you have a phone with a compass app that should work, but I've no idea how accurate they are.

 

[savageseadog]

The problem is your GPS gives COG so you would have to maintain exact heading, without the GPS Ariel moving due to rolling etc there is also the GPS built in smoothing to consider.

You would probably achieve a degree or 5? accuracy at best. Which is probably as accurate as you can get? although how accurate is it?

Another way of doing it that is not quite by the book but easy? and more accurate?

I see your Solent based so:

Position yourself in the Solent where you are surrounded by KNOWN positions (not buoys) as far away as possible.

Slow moving (less than 5 knots but depending on distance this is insignificant)

Point boat exactly at known object example:

EG Fawley Power Station Chimney press MOB (on GPS ) to record your position.
Note:
Object: Power Station Chimney
Position:
Boats Compass heading:

Then do same for all headings:
Spinnikar tower, Hurst castle Light house, Bramble pearch, calshot light, end of Ryde pier, the forts etc etc

Once you have done this for enough headings you will be able to draw up a deviation chart.

Calculating Magnetic for Variation etc then use GPS or way-point program to calculate bearing between to "known" positions.

The advantage of this is you can do this anytime any place and current.

You can also do similar when running in on known transit bearings.

Accuracy??

Well assuming both positions are 20m out 40m total error over a mile or 2? Probably more accurate than you can read your compass...

Awaiting the RYA method

There's once place in the NW of England or N Wales to do something like this and that's Holyhead, currents are small enough inside the harbour and there is the space to do it. A windless period is uncommon there though. Other than that nothing wrong with the idea, 2-3 degrees is accurate enough for small craft, without putting a figure on it I find COG settles very quickly and accurately. The only place similar on the South coast would be Portland Harbour? Ramsgate and Dover might be OK too.

 

[status]

Four consecutive answers with agreement, no disputes!

We claim a record on YBW forums. (please, no arguments regarding forums vs fora!)

I am just waiting for the OP to thank you so I can claim a Lakesailor!

 

[lw395]

The best way to use the GPS to swing a compass is to use it to provide a true bearing to a distant visible object, then record the compass reading when that object is directly abeam, ahead etc.
Some sort of pelorus to give a sighting line that can be set at intervals helps.

Two dowels and some holes in a bit of plywood that sits on the cabin top is one way.

If you have a helper, you slowly turn the boat and call 'now' as the distant object is in line, the helper records the compass reading, together with the bearing to object on the gps.
you need to be far enough from the object that the true bearing is not changing quickly.

Put the results in a spreadsheet, reduce it to deviation vs heading, graph it and check you have enough data points to give a smooth enough curve.

 

[Talulah]

Surprised no one has mentioned the Sun yet for checking the Compass.
Required as part of the RYA Ocean ticket.

On a side note.
Our main ships compass is a Silva 150T binnacle mounted compass circa 1989.
Silva in Sweden have stopped making spares.
At the last count there were only 3 sets of adjusting bars/magnets left in Sweden. (The adjustment magnets that sit under the compass.) AllSpars in Plymouth are the agents if you need a new set. Ours are all corroded so have just ordered a set. Hence only probably 2 sets left now in existance.

 

[PhillM]

Wow, I am overwhelmed with the responses and advice. Thank you all.

I think I’ve got it, COG shows where I am moving to but the compass shows where I’m heading to. Wind and tide conspire to make these sufficiently different to prevent use of the GPS for swinging the compass (and of course why we use course to steer and try to calculate leeway, doh! )

There is a lot of advice in all your posts so I am going to go through it all a bit later. Meantime, thank you all very much. I really do love the forums

 

[prv]

Surprised no one has mentioned the Sun yet for checking the Compass.
Required as part of the RYA Ocean ticket.

I've done it once, on Stavros, probably between Gibraltar and Madeira.

No need for the mirrors, the tables and the maths if you're in the Solent though

Pete

 

[ex-Gladys]

Wow, I am overwhelmed with the responses and advice. Thank you all.

I think I’ve got it, COG shows where I am moving to but the compass shows where I’m heading to. Wind and tide conspire to make these sufficiently different to prevent use of the GPS for swinging the compass (and of course why we use course to steer and try to calculate leeway, doh! )

There is a lot of advice in all your posts so I am going to go through it all a bit later. Meantime, thank you all very much. I really do love the forums

...time to do a Day Skipper Theory over the winter????

 

[PhillM]

...time to do a Day Skipper Theory over the winter????

Actually have that and passed practical. Just couldn't figure this one out without some help. Somtimes it takes talking to others to aid understanding...... So thanks again to everyone.