The Modern Navigation Trap

[Mainebristol]

Ok, at the risk of getting pilloried, I have decided to take up a subject that has come up in several threads lately. Please bear with me, and I will try to be succinct.

Most of us know that the earth is not a sphere, but rather an ellipsoid. Further, it is an irregular ellipsoid that cannot be easily defined mathematically. This is a huge problem for cartographers because that irregularity makes it difficult to accurately locate a position. In order to solve this problem, governments develop models of the earth that are called datums. Where I live, in the U.S., our government uses a datum known simply as the 1989 datum (I think, could be off) and all of the NOAA charts are aligned to that datum, which very accurately represents the earth where our country is. GPS's also tend to use that datum and those charts, so that they TEND to be very accurate IN THIS COUNTRY.

There are traps. Even in the U.S., survey data can be old. I mean, really old. Well over a hundred years old. Further, the cartographers who created the chart did so on a scale that was meant to be of a fixed size, and kept that way by printing it on paper. They had margins of error to work with, that they believed to be acceptable, NEVER DREAMING that someday Joe Boater was going to come along and expand the scale dramatically on his plotter and try to navigate between two rocks whose location no one had measured in more than a hundred years!

More to follow.

[Travis McGee]

In 2006 I was transiting the Gulf of Tehuantepec in southern Mexico using the "one foot on the beach" method. On the north side, my old Garmin GPS showed my boat about 2 MILES inland! On the south side, instead of near the shore, it showed us about 3 miles off.

A very big problem comes in with folks trading favorite waypoints for anchorages and inlets. One guy might plot the "exact" lat/lon off his old admiralty chart. Another guy puts that lat/lon into his GPS and says "go there." Obviously, this could result in disaster. One must always know the source of anybody's lat/lons when plotting a near shore course, or threading inlets, passing rocks, etc.

(Addendum: It was 8 years ago, and maybe it wasn't 2 miles inland, but it was WAY inland, inland of the coastal road etc. Pretty freaky to see your boat "sailing" over dry land the other side of a highway.)

[Mainebristol]

With some knowledge and some caution, the typical Garmin user can navigate safely in this country, bearing in mind the effects of scale. I like paper back up, but this is a free country. You do what you want.

Once you leave our shores, however, I suggest that the wisest course is to look for the legend of the charts you are using. I consider it VITAL to check the datum. If the datum that the cartographer is using differs from the one that the GPS is using, then the GPS will probably be telling you that your location is in the wrong place on the chart! This is not theory. The difference could be feet, or it could be hundreds of yards. It could easily be enough to put a boat on the rocks.

What to do? If it's me, when I arrive at any place where I suspect that there may be problems, I take cross bearings with a hand bearing compass. But that's just me. A radar fix would be pretty good (if you know what you are doing) or whatever. The point is to use another redundant form of ground based navigation to check that the survey, the cartographer and the datum are all correct.

Seems anal? I suppose. But especially at night this simple check could save a deadly, deadly mistake. Sure, your chartplotter is your friend. But it pays to understand any system, no?

[Mainebristol]

Actually, I wrote both posts.

To expand; our country has invested enormous resources on surveys, and continues to do so. However, it spends those resources where the money is, which is to say on shipping channels. Other countries tend to do the same, and very little attention is given to shallow water, where we boaters tend to navigate. So survey data is often incomplete or obsolete, such as lead line data which can easily miss rocks between casts. On the remote shores of less wealthy countries who may have used older datums, I cannot imagine blindly following a GPS into a narrow channel. I mean, come on.

I love charts, and own many. But they were created by humans, and are an imperfect representation of what is out there. I like GPS, which can give an unbelievably accurate fix on one's position. The point is, the two often don't coincide.

[tomfl]

Originally Posted by Mainebristol

SNIP

The point is to use another redundant form of ground based navigation

SNIP

While I do wear glasses I have found looking at the things around me is a big clue to where I am.

I have also noticed that there are apps you can use (on a computer, tablet, or cell phone) that indicate the margin of error your GPS unit is currently experiencing. While most of the time it is quite small it does change, sometimes to values larger than I would expect.

[Dockhead]

This is all fairly elementary navigation, and something most sailors learned in their first (or second) navigation lesson.

But it's very important and certainly worth repeating.

The coordinate system you are thinking of is WGS84, which is the one used by GPS and nearly all modern charts. See: World Geodetic System - Wikipedia, the free encyclopedia.

You are incorrect that WGS84 is somehow skewed toward the U.S. -- by no means. The system is equally applicable to the whole world. The problems arise with:

1. Using charts with the wrong coordinate system, and failing to make corrections. Very rare, as charts with other coordinate systems almost no longer exist.

2. Mistakes in translating old surveys into WGS84 made according to other coordinate system.

3. Simple inaccuracies in the surveys themselves. A great part of the globe has not been surveyed in a 100 years or more, and techniques available then were not suited to position-finding which is accurate to a few meters. And the surveyors were not infallible, either. This is a very common problem in places like Mexico and South Pacific islands; less so in the U.S. and Western Europe, which has simply had the benefit of a great deal more intense surveying over the years.


As a result of which -- of course, you should never treat any one system of navigation as infallible, and you should never just "drive the dot" on your chart plotter, if you don't want to end up on the rocks. The killer app for double-checking the accuracy of your charts is radar. I always leave the radar on with chart overlay when doing pilotage in unfamiliar areas. A problem with the chart is immediately obvious -- the radar doesn't lie.

[Travis McGee]

Originally Posted by Mainebristol

Actually, I wrote both posts.

There was a snarky reply for a few minutes after your first post. I replied to it, and then the snarky post had been deleted.

[Mainebristol]

Thank you. I had hoped to learn something from this. I started this topic as a result of several posts in other related topics that gave me pause. Some boaters suggested that their plotters were capable of more than the are. I appreciate your input.

[Mainebristol]

Thanks, Travis.

[Pelagic]

Originally Posted by Mainebristol

I love charts, and own many. But they were created by humans, and are an imperfect representation of what is out there. I like GPS, which can give an unbelievably accurate fix on one's position. The point is, the two often don't coincide.

Nothing new in understanding the fixed and variable errors in ALL Navigational Aids plus the specific value that each offers.

Some offer realtime measurements off of physical realities, others like GPS provide the opportunity to return via a safe track to an exact position.

Relying on one aid only is amateurish, as is the ignorance in believing the accuracy of charted projections.

Continued use of Eyes/sounder/radar/ confirms electronic position... Not the other way around.

EDIT Plus what Dockhead said

[hpeer]

Nigel Calder covers this extensively in How to Read a Nautical Chart.

An, yes, it is not to be taken lightly.

http://www.amazon.com/Read-Nautical-.../dp/0071779825

But, once you understand the problem, and validate your charts, things re pretty good.

[GordMay]

Ninety-nine percent of NOAA nautical charts are on the North American Datum of 1983 (NAD 83), which, for charting purposes, is considered equivalent to the World Geodetic System of 1984 (WGS 84).
Most GPS receivers default to the World Geodetic System 1984 (WGS 84)

Much more ➥ DGPS & Your Chart

[bobofthenorth]

The importance of using the right datum was driven home years ago when I attended a GPS rally on quads that a friend had set up. He had just bought a handheld GPS and was wildly excited about using it. His enthusiasm exceeded his knowledge though because he set up the rally coordinates using some obscure datum that his unit happened to default to. That caused a whole lot of confusion at the start of the rally until we helped him sort out what datum we should be using.

I admit that I depend on GPS charts but I think I have adequate redundancy built in and in an absolute emergency we carry a variety of forms of paper backups. It is however very alarming to see your track moving resolutely through a headland when the GPS charts are just simply wrong. I use 2 depthsounders to confirm depths and frequently compare them to the charted depths whenever I am entering a doubtful chart area. I also use the radar set to a low range to help keep me in a narrow channel. The Mark I version eyeballs are useful but not always infallible, particularly at night or in fog.

[Dsanduril]

Originally Posted by Mainebristol

...our country has invested enormous resources on surveys, and continues to do so. However, it spends those resources where the money is, which is to say on shipping channels. Other countries tend to do the same, and very little attention is given to shallow water, where we boaters tend to navigate. ...

Originally Posted by Dockhead

...3. Simple inaccuracies in the surveys themselves. A great part of the globe has not been surveyed in a 100 years or more, and techniques available then were not suited to position-finding which is accurate to a few meters. And the surveyors were not infallible, either. This is a very common problem in places like Mexico and South Pacific islands; less so in the U.S. and Western Europe, which has simply had the benefit of a great deal more intense surveying over the years...

Which argues for crowd sourcing of exactly the information we are discussing. The weather forecasters have (to a certain extent) figured out how to do this and how to weed out bad data and/or correct suspect data. I've sent data to the British Admiralty which has subsequently been incorporated, and run depth transects in Glacier Bay where glaciers have receded (by miles) and no survey has since been made (privately distributed). Given that we are the people that go into this shallow water it may start to become incumbent on us to conduct the surveys. The same modern navigation tools that are causing the problems described in this thread can be used to allow many less skilled people to contribute useful data.

[senormechanico]

There's a bunch of rocks above and below the surface North of Port Ludlow in Puget Sound.
I've sailed past and through them lots of times, so when I got a G2 Vision card of the area for my Garmin chartplotter, I thought it would be fun to watch the underwater 3D chart as I sailed through there.
When passing one of the larger rocks which was covered in seals and birds, I had 20 feet of water under me, but the chart plotter showed the boat's icon disappearing INTO A ROCK !
It emerged on the other side perfectly intact, and I never saw less than 18 feet of depth.