Chartplotter compass v fixed boat's compass

[Memphis_Chung]

Hi,

Went to the boat today to inspect my lovely new Garmin GPS Map 750 chartplotter. It's a great unit. Unfortunately, the new binnacle pod means that the boat's compass is not visible when standing at the helm. On the whole, it shouldnt be a problem as I tend to be sitting when I helm, but on the occasions when I am standing, I thought i would use the Chartplotter's compass. It reads differently to the boat's compass (am assuming as a consequence of no deviation?). Can anyone advise me on whether navigating off a chartplotter's compass would be any different than than a ship's compass?

Thanks all.

Dave

 

[philip_stevens]

A small ship I do some work on, only had a magnetic compass when it was bought (40yo and 2nd hand).

A GPS compass has been fitted to allow autopilot use, and has effectively pushed the magnetic compass into standby. All nav is now by the GPS compass.

(The GPS compass aerial array is made up of 3 aerials in triangular setting about 15cm apart. Very stable.)

So I would say that your GPS compass from your plotter or GPS would be quite adequate. You always have your magnetic compass if you have an electrical/plotter/GPS failure.

I only use the plotter compass, but have the magnetic if all else fails. (I also have a separate GPS).

 

[Stork_III]

Hi,

Went to the boat today to inspect my lovely new Garmin GPS Map 750 chartplotter. It's a great unit. Unfortunately, the new binnacle pod means that the boat's compass is not visible when standing at the helm. On the whole, it shouldnt be a problem as I tend to be sitting when I helm, but on the occasions when I am standing, I thought i would use the Chartplotter's compass. It reads differently to the boat's compass (am assuming as a consequence of no deviation?). Can anyone advise me on whether navigating off a chartplotter's compass would be any different than than a ship's compass?

Thanks all.

Dave

Question 1 : Is the ship's compass correctly adjausted.
Question 2. Is the plotter set on mag or true.

 

[Pye_End]

Are you reading course over the ground on the chartplotter, or do you have it linked up to a fluxgate compass somewhere in the system?

If the former, then it is two different pieces of information for steering. If you have longish passages that go acorss the tide - eg a trip over to France, then you really need to steer to a compass heading rather than COG.

 

[Fascadale]

The bulb on my steering compass went out on a night passage so I was forced to rely on the plotter compass.

I found it quite difficult to hand steer the course using the plotter, something to do I think with the lag in digital instrument being different from the responses of the magnetic compass. I was correcting late and often overcorrecting.

If you have the autopilot on then there is not such a problem

 

[Playtime]

A GPS compass has been fitted to allow autopilot use, and has effectively pushed the magnetic compass into standby. All nav is now by the GPS compass.

(The GPS compass aerial array is made up of 3 aerials in triangular setting about 15cm apart. Very stable.)

So I would say that your GPS compass from your plotter or GPS would be quite adequate. You always have your magnetic compass if you have an electrical/plotter/GPS failure.

GPS compass? I think you are confusing two types of heading information.

A GPS derived course over the ground (COG) is computed by the GPS from successive position fixes. There is no reference to the earth's magnetic field and therefore, arguably, it is not a compass.

An electronic or fluxgate compass uses the earth's magnetic field and replaces the analogue needle with electronic sensors positioned in the 3 axes of rotation. It computes a 'real' (magnetic) compass heading. A fluxgate compass is fitted to all autopilots and heading data derived from it can (generally) be displayed on a chart plotter.

Some GPS-type devices also include a fluxgate compass but it is confusing, to me at least, to call such a device a GPS compass.

As stated by Pye-End, it is not recommended to steer by the GPS COG, especially when going cross-tide. It is fine to steer by the fluxgate compass derived heading data, however displayed, provided that the fluxgate compass has been calibrated and checked against an accurate magnetic compass.

 

[electrosys]

GPS compass? I think you are confusing two types of heading information.

A GPS derived course over the ground (COG) is computed by the GPS from successive position fixes. There is no reference to the earth's magnetic field and therefore, arguably, it is not a compass.

Indeed. And it's even slightly worse than that ...

Some NMEA $trings give both True and Magnetic directional information, giving the illusion that GPS is somehow able to sense and compensate for local magnetic variation, whereas it cannot (how could it ?). This calculation is actually performed using a complex look-up table algorithm within the GPS receiver.

But - the rate and direction of the Earth's local magnetic variations change over time, whereas the table remains fixed at the time of manufacture. Some manufacturers claim that their algorithms have adequate long-term compensation, but I'd err on the side of caution and not rely too heavily on this.

 

[Lady Campanula]

"....And a star to steer her by!"

 

[Searush]

"And a star to steer her by . .

Provided you are aware that the stars (except, effectively, Polaris) are moving & how the one you are using is moving.

 

[boaterbaz]

I find it very difficult to steer to a digital type compass display.I think its because the "old fashioned" compass card gives you a visual indication of the rate of turn and when to correct the vessels swing. On the accuracy theme, how accurately can you steer a course in any sea. I,ve had many a fishing boat where the compass was at best a guide to your actual course.

 

[philip_stevens]

GPS compass? I think you are confusing two types of heading information.

I am confusing nothing -
GPS Compass

I maintain the electrical installation on the boat, and the GPS Compass was installed by a local firm.

 

[Playtime]

I am confusing nothing -
GPS Compass

I maintain the electrical installation on the boat, and the GPS Compass was installed by a local firm.

OK - I blame Furuno for the confusion .

The blurb from the link you provided states the following -

This hybrid integration of GPS receiver technology and a solid-state rate sensor provide accurate and steady heading information, even when bridges or tall buildings block satellite signals.

The SC50 offers highly accurate GPS WAAS Data for SOG, COG, ROT, and L/L.

In other words it appears to be a GPS providing COG heading data (not magnetic) and a rate sensor that will continue to provide heading info when the GPS signal is lost.

In strong cross tides it should be noted that COG can be very different to the true (magnetic) heading that an analogue or fluxgate compass would provide.

 

[Stu Jackson]

Compass heading is the way your bow is pointing and not necessarily the way the boat is moving which would be shown on your GPS or autopilot fluxgate heading.

Many have mentioned tides, and they're a good example.

A better one is a river. River current is going due west. So's your boat. So both the compass and GPS heading are the same.

You get caught in a back eddy on the river, the boat's still going west but the bow of the boat has been tossed 90 degrees to due north.

The compass shows N (0 degrees) while the GPS heading is still 270.

This is basic pilotage material that should be well understood.

Steering to a compass course or a GPS bearing is also difficult, because the desire to maintain a tight course while watching a compass takes away from sailing the boat with small variations in wind speed and direction while the boat moves around. Try getting the sails set properly, and yaw around five degrees each way and you'll be doing just fine.

 

[earlybird]

OK - I blame Furuno for the confusion .

I think that the OP rather sowed the seeds of this confusion by saying that he was using the compass of a newly installed Garmin 750 chartplotter.
He didn't have £3K of Furuno kit!

 

[fireball]

GPS compass? I think you are confusing two types of heading information.

I have seen (what I believe to be) a GPS compass - on the fastcat between Poole & the CI's .
They have 4 mushrooms on a diamond - about 1m across.
The mushrooms give their position (which must be more accurate than my GPS) back to a processor which then decides - based on the relative postion of each mushroom - what direction the ship is pointing in ...

That was what I understood it to be anyway - and I did ask!

 

[alan_d]

Confusion is arising because people are interpreting "GPS Compass" to mean one of two things, either (1) an expensive device which uses satellite position data to compute the vessel's heading or (2) the standard compass rose which most GPS devices (including chartplotters) will display on request, which in fact shows COG. The first of these devices will tell you which way your vessel is pointing (as will a magnetic compass), the second will tell you which way it is going (or to be precise, the direction it has just come from).

 

[Stu Jackson]

Confusion is arising because people are interpreting "GPS Compass" to mean one of two things, either (1) an expensive device which uses satellite position data to compute the vessel's heading or (2) the standard compass rose which most GPS devices (including chartplotters) will display on request, which in fact shows COG. The first of these devices will tell you which way your vessel is pointing (as will a magnetic compass), the second will tell you which way it is going (or to be precise, the direction it has just come from).

I beg to differ. The ONLY information the GPS has is the boat's heading, which is COG. All the different "views" on a GPS come from only ONE source, the COG from the boat's movement. Whether it's the digital display of heading vs. bearing, the cute compass thingie, or the "Highway" view, it's all the same. Like my last post about the bow being 90 degrees off from the boat's heading. There's one and only one chip inside the GPS and it doesn't care which way your bow is pointing. The only thing it can do is tell you where you are, what direction (bearing) and distance to the next waypoint, and, if using tracks, where you've been.

 

[Porthandbuoy]

On the accuracy theme, how accurately can you steer a course in any sea.

Say, half a point or thereabouts. Bring back points I say and scrap these new-fangled degrees.

 

[Playtime]

I have seen (what I believe to be) a GPS compass - on the fastcat between Poole & the CI's .
They have 4 mushrooms on a diamond - about 1m across.
The mushrooms give their position (which must be more accurate than my GPS) back to a processor which then decides - based on the relative postion of each mushroom - what direction the ship is pointing in ...

That was what I understood it to be anyway - and I did ask!

Umm - was it something like this?

I'm starting to believe that there really is a genuine GPS compass. It looks from the Marconi link that it uses DGPS to give 'really' accurate fixes for 3 (or may be 4) points about a metre apart. From this it computes the true (non magnetic!) orientation of the vessel relative to the 'earth' and hence an accurate true (non magnetic!) heading.

If this is correct I am amazed that such accuracy (to a claimed 0.5 degrees) can be obtained with such a small separation of reference points. I guess that the Differential part of DGPS compensates equally for the 3 (or 4) reference points to give this amazing accuracy.

Can anyone confirm this?

 

[alan_d]

I beg to differ. The ONLY information the GPS has is the boat's heading, which is COG. All the different "views" on a GPS come from only ONE source, the COG from the boat's movement. Whether it's the digital display of heading vs. bearing, the cute compass thingie, or the "Highway" view, it's all the same. Like my last post about the bow being 90 degrees off from the boat's heading. There's one and only one chip inside the GPS and it doesn't care which way your bow is pointing. The only thing it can do is tell you where you are, what direction (bearing) and distance to the next waypoint, and, if using tracks, where you've been.

You may or may not be confused yourself, but you certainly risk confusing other people. You use the term "GPS heading" to mean COG. Most people would restrict the use of "heading" to "the way the bow is pointing", which you refer to as "compass heading". This caveat apart, I would agree with what you say as regards a standard GPS set or chartplotter, but there is this other piece of equipment, called "a GPS Compass" which uses the positional information from three or more onboard GPS antennae to compute the direction the vessel is pointing.