Compass and Charts

[S&S]

it is an old habit of mine not to rely on one method of positioning,

second, it keeps the kids occupied with the serious business of navigating.

I've been on too many old boats w/ dodgy electronics I guess. I tend to trust the paper charts more than the electronics.

[goboatingnow]

In practice its very hard to maintain an accurate magnetic compass on a small steel boat, deviatation varies all the time( vessel orientation, electrics onboard, etc and chnages significantly over time, The lack of proper compensating magnets and flinders bars etc only make th eproblem worse. It wide the teh main cimpass on ships is now the gyro compass

[s/v Jedi]

I used a pedestal mounted compass on a steel 30' sailboat for 17 years without any other trouble than having to use the deviation table that the adjuster left us after doing his magnet magic and we motoring the boat on different bearings for an hour or so.

Electronics interfere less with the compass on a steel boat than compared to a plastic or wooden boat.

cheers,
Nick.

[barnakiel]

I used magnetic compasses on steel fishing boats loaded with electronics. We adjusted for deviation and the deviation card was checked while in port (I believe once a year - this was done by retired skippers as their side-job). Can't see why the situation on a steel yacht would be worse ?

b.

[osirissail]

- - Also I rarely ever use the magnetic compass in my binacle - but - once ....
On a dark and overcast night off the north coast of D.R. in moderate seas while crewing for a friend on his 42 Nordic Tug - the 12VDC electrical system failed due to shorted out batteries. We had A/C but not DC. All the navigation and autopilot system went dark. There was only my flash light, a little 3 inch binacle compass and my lapbook navigation program working off the AC power. That little magnetic compass enabled us to point in the proper direction to avoid the cliffs and find the harbor entrance using my lapbook charting program. Trying to hand steer a power yacht in pitching seas using the little compass made a track line on my notebook charting system that I am embarrassed to show anybody. But we got back safely and I advised the owner to get a bigger, better magnetic compass.

[donradcliffe]

Just download the free SeaClear nav program and the free Noaa charts on your PC, hook it to the GPS, and forget the $20 paper charts. The autopilot will be steering 99.9% of the time and you use your GPS COG to tweak the autopilot setting for compass error and current, so the helm compass is just used to check the electronic equipment and for hand steering.

The nav program will give you your realtime position, a track of where you have been, the range and bearing to whatever you move the cursor to, and does it much more quickly and accurately than you can plot. We do have a few large scale paper charts for planning purposes, like the East Coast US.

For redundancy, we put waypoints into both fixed and handheld GPS, and carry a second computer.

[S/V Alchemy]

I have a large Ritchie Globemaster with compensator balls (go on, get the jokes out of the way early) at the pilothouse helm of my steel cutter. I also have a rudimentary plotter there (a Raymarine 420 with no cards, so I use it like a big GPS), and a KVH AC103 fluxgate compass.

At the outside helm, I use a handheld GPS.

I currently sail in Lake Ontario (the boat's actually on the hard at the moment), but will eventually be heading south and then west into the Pacific.

I use all of them about equally in conjunction with paper charts. I usually have a bearing in mind and a glance at the helm compass (the accuracy of which I know and the deviation of which is small due to those pair of shotputs) is a quicker, more intuitive confirmation of general heading than the GPS, or so I find.

Just as with my sextant practice, sometimes I will shut down all the electronics and work from DR plots, mag. compass bearings and, particularly, soundings in order to verify my position at night or in fog. The point of having these devices is not that they are critically necessary, but that you still know how to use them if they become so. I have yet to install an autopilot, and when I do, I will likely update the fluxgate compass to provide it with heading data.

My experience with GPS has included two episodes when it was obviously wrong: one was when I was at a known, charted physical waypoint and the GPS was just over a mile off (it corrected some ten minutes later) and the other was when I was using it for COG and speed, not lat/lon, and my six knot speed jumped for a few seconds to 60 knots and my lat/lon magically ended up 3/4 NM WSW of my assumed location.

These were using different GPSes. Obviously, "the system" is prone to technical hiccups or tweaking from the ground stations. A little investigation of the subject revealed that the "constellation" of GPS satellites is both aging and subject to failures from such natural events such as solar flares or even atmospheric drag (the atmosphere can expand due to heating and can "drag" at the satellites, requiring recalibrations).

So while GPS is indeed a blessing, I keep the compass handy, visible and in good working order. Intrinsically, unless I go into the Southern Hemisphere or sail over a unknown magnetic anomaly (many are charted and it's fun to sail over some giant chunk of iron just to watch the compass spin), the compass is less failure prone than either the GPS reliant on ship's power or batteries, or the GPS system which needs periodic adjustment. Also, despite political statements to the contrary, don't think that the military of the U.S. will not turn it off or make it less accurate if needed. It isn't actually for we sailor types: it's to aid one particular military establishment and their goals are always going to be concerned with finding the mooring before sunset so drinking may commence.

As a third party can't "turn off" the stars and the magnetic poles of the Earth (yet), I choose to keep my CN and pilotage skills in order, and that means a lot of peerage at compasses and plots on paper. I find the electronics make a nice back-up and a nice way of "looking down the road" to plan one's next steps, but I hesitate to place my self-interest and safety in something so easily crippled.

[osirissail]

- - I sense a huge misunderstanding amongst the users of GPS that is common throughout the world. They think the GPS is wrong and the "Nautical Chart" is correct. I hate to break the news to you, but most nautical charts are not correct. Add in the 140 different Datums used to make them around the world and it is rare indeed that the chart and the GPS position will ever agree.
- - Within the USA alone there are 40 different Datums in use on charts. Expected errors from 1/4 to 1.5 nm can be expected if the Chart and your GPS are not on the same Datum. Out on the Oceans the disagreement can exceed 10 nautical miles.
- - GPS's like any form of electronic positioning equipment can malfunction due to queer electrons or other things. But by and large the GPS is giving very good position information - it is just that it will not agree with what is drawn on the Chart.
- - Proof of this is to enable your ENC system to record your actual Tracks in and out of places you normally sail. For example Deshaies, Guadeloupe - I have been there many times and my Tracks are of "good clean water" experiences - When overlaid on some of the charts of Deshaies, my Tracks show me sailing right through the mountain on the north side of the harbor.
- - There is little or no money in the budgets of the governments who produce nautical charts - so as the disclaimer you must agree to to download NOAA charts states - the geographical and other information on most charts can be as much as 40 years or more old. That was before GPS's existed in common use.
- - Bottom line, don't totally trust anything when it comes to nautical navigation, but primarily the differences you will commonly see between Charts and GPS are the results of errors in the Chart. Only your "Mark 1, Mod 1" eyeballs and common sense/experience are the best and most reliable navigational tools. If it doesn't "look right" don't proceed until you have resolved the situation. - or have really "large compensator balls."

[s/v Jedi]

I have seen the same errors that Alchemy saw and I see the chart errors that Osiris describes all the time.

After a lot of experimenting, I was about to give up. My Raymarine RN300 GPS showed those "jumps" but before I could start up my handheld Garmin it was back to normal so I couldn't determine if both saw the jump. Then, I researched this problem "on Google" and found that I wasn't alone and many reports included this RN300 GPS. I bought a cheap Garmin 128 and hooked it up so that I always ran both and could switch the plotter between the two. It didn't take long before it happened and it turned out that the Garmin was steady while the Raymarine "jumped". I switched to use the Garmin as the primary and followed Raymarine's advise to update the firmware. That solved the problem but introduced other weirdness like the unit switching off instead of jumping. (the original problem was that the unit lost it's fix without reporting it for 5 minutes or more. Most of the time it could re-establish a fix before the alarm with jumps as the result, but sometimes it jumped for no reason to a wrong position and needed a reset to become sane again) Well, no reading is better than a wrong one but for a $700 unit there is no excuse so I ripped it out and didn't even want to sell it so it went into the dumpster. I ordered a Furuno GP-32 (at $300 less than half the price of the Raymarine unit), installed it and never had the problem again.

Now I only have the problem of inaccurate charts. For my current area (Colombia & Panama) I found great improvements with the C-Map 2006 edition so things do get corrected. Your GPS unit is important and must earn your trust and the Raymarine units were flawed. I also had two of those "sensors" (the 120 model I think) and they showed the same symptoms as the RN300 units. Raymarine gave up too and took these flawed designs off the market. This whole deal was what put me really off the Raymarine brand.

I think in general, Garmin is okay. But I can't keep up with the new models and all of them have plotters and charts and stuff. The Furuno GP-32 is the unit I really trust. It even establishes and keeps a fix with the antenna upside down!

cheers,
Nick.

[donradcliffe]

As mentioned above, the prudent navigator will take all inputs into account, and slow down or stop the boat if necessary if they don't agree. Steering compasses are usually as reliable as the GPS units, but I have seencompass errors over 15 degrees, like once after a lightning near-miss in Vanuatu, and several times when handhelds VHFs or screwdrivers have been placed near them.

[David M]

Originally Posted by lorenzo b

We went to West Marine and got a good compass and binoculars with a gift certificate we've been sitting on, but still haven't figured out the charts. I've been looking at the NOAA site and downloading 8 by 11s and it's obvious that a big chart is the way to go. We're going to be leaving La. and heading east and south down the coast of Fl. and then up the east coast and I really don't want to buy 60 charts at $20 each for areas I'm just passing through and not coming back to.
What's the solution?

Three electronic charting systems. One powered by the boats batteries and two that are powered by "Duracell" type disposable batteries. I cant believe I am saying this, but I think the time has arrived where electronic charting systems with independent adequate backups are reliable enough. I still also believe that having a sextant, almanac, plotting sheets, chronometer and sight reductions tables on board for crossing large bodies of water is a good idea.

[S/V Alchemy]

Originally Posted by osirissail

<snip>
- - Bottom line, don't totally trust anything when it comes to nautical navigation, but primarily the differences you will commonly see between Charts and GPS are the results of errors in the Chart. Only your "Mark 1, Mod 1" eyeballs and common sense/experience are the best and most reliable navigational tools. If it doesn't "look right" don't proceed until you have resolved the situation. - or have really "large compensator balls."

I agree with this entirely. Most chart veterans know to keep a critical eye on their paper charts, particularly in less-travelled areas, because the information is either stale or frequently "off". GPS navigators may not always recognize when the lat/lon and the plotter aren't agreeing with the reality, or how to compensate for that reality with (generally) a nudge in the direction of the error.

Others, of course, make their own sketches of distant anchorages with new GPS-derived waypoints, which is why so many swear by cruiser-penned guidebooks.

[s/v Jedi]

And this is where radar helps. The radar shows you the real situation, with high precision. Modern gadgets, like a radar overlay on the chart plotter are very useful. Some plotters even allow you to offset the chart so that you can match it with the radar, correcting 90% of the errors in the chart.

cheers,
Nick.

[osirissail]

Originally Posted by s/v Jedi

And this is where radar helps. The radar shows you the real situation, with high precision. Modern gadgets, like a radar overlay on the chart plotter are very useful. Some plotters even allow you to offset the chart so that you can match it with the radar, correcting 90% of the errors in the chart. cheers, Nick.

- - I have been running dilberately separate systems - Furuno Radar and Digital Charting Program - physically side by side. The problem with the original Chart Plotters with radar overlay was the "shadow" of the radar never matched the outlines on the charts. The reason is the charts are not registered correctly or just incorrect.
- - If there was a way to shift the chart or radar overlay to manually "line up" the two - that would be the answer to a great system. Radar cannot "see" what is behind a closer target so coastal/harbor chart outlines would show the missing stuff the radar cannot "see."
- - Do you know any names of systems/brands/programs that do this?

[S/V Alchemy]

Originally Posted by s/v Jedi

And this is where radar helps. The radar shows you the real situation, with high precision. Modern gadgets, like a radar overlay on the chart plotter are very useful. Some plotters even allow you to offset the chart so that you can match it with the radar, correcting 90% of the errors in the chart.

My only caveat with this is to avoid multi-function displays. Given my choice, I would always choose to have a stand-alone radar display that can output to a plotter or a PC for the magical overlay function, rather than an MFD on which plots, radar, depth and the evening news can be shown. Yes, it makes for a more crowded helm, but it doesn't all have to be at the helm, and it avoids the situation where the display itself is on the fritz, leaving you with five nav devices and no way to see them.

This is why I say "no thanks" to some of the wonderful "name that anchored tanker while you monitor your exhaust temperature" setups...too many eggs in too little a basket.