The Modern Navigation Trap

[noelex 77]

Originally Posted by s/v Beth

You guys arguing about this stuff is really funny. Seeing if you can navigate through a rock field by just using GPS or google earth or paper+plastic is all a flawed conversation.

Maps and resources such as Google earth can have significant errors which makes "blind" navigation impractical. However retracing a recently recorded GPS track eliminates these "mapping errors". We are still left with GPS errors which are increased because there are errors in both the position when recorded on the track and errors in the GPS position when following the track.

Navigation should never be about using one piece of information and there is always other important data, but there are (thankfully rare) times when retracing your GPS track becomes the primary source of information.

An example is dragging anchor while on outer reef anchorage at night.

You need to have careful understanding of the errors involved and also record and preserve the track. Having a GPS track that will allow you to re anchor, or escape the anchorage is often useful as supplemental information. Occasionally it can be important primary information.

There is nothing new about these sort of tactics. In the days before GPS I always mentally noted the best compass course to escape an anchorage (I still do). In conditions with very poor visibility (such as a thunderstorm at night) it was/is good information. The GPS does the job much better.

This does not stop you keeping a visual lookout and using all the other sources of information. Boats only move slowly and even men can multitask.

[Stu Jackson]

Originally Posted by hpeer

Cursor is in bottom right, to the right of the bouy.

Also, I just went back to verify this as it has been a few years since I looked at it. And it is.

However, I have now noticed something additional. What I see now is that the area at Francouis Harbor (and also at Grey River where there is also a problem) someone/somehow did an overlay on the GE presentation. Since I new there was a problem there, that is the place I went to look. (And at Grey River.) I "assumed" that the chart overlay would be the same everywhere. It is not.

I now think that "they" (Fugawi??) have "fixed" the error by adding some "adjusted" chart info to the GE view.

They are out for a long weekend so I can't ask them until next week.

Just a question: If you've been through there before, have you considered using TRACKS? That way the database of the shoreline and rocks become meaningless and you could follow it in and out like many do at anchorages for bailouts in the dark & stormy...

[hpeer]

The small scale paper chart for this area carries this note:

HORIZONTAL DATUM: Chart is drawn on an unknown datum. Positions on NAD 83 or WGS 84 must be moved "C" seconds northward/southward and "D" seconds eastward/westward to agree with this chart.

TABLE OMITTED HERE. But for 56 40 the adjustment is 11.9 southward and 46.7 westward

SOURCES: Reproduction of USNOO chart 2416. Surveyed by the British Admiralty, 1872 -1885. The depths in sloped figures are from smaller scale charts.

Because the horizontal datum for this chart is unknown, positions plotted from navigation systems such as GPS and Loran-C may be in error by 0.3 miles, even after incorporating the aforementioned corrections. Positing methods such as range and bearing should therefore be used.

I don't have the more detailed chart in Fugawi, perhaps that is why they align?

[Jammer Six]

Originally Posted by Dsanduril

I agree wholeheartedly, but as we get more accurate it does bring up the modern trap question of "highways" between waypoints.

Or the one I've seen on the Sound, four cruisers in five square miles of open water trying to drive through the same Maptech point.

Fortunately, I was the stand on vessel.

[valhalla360]

If you are trying to pass within 30' of a rock, you have other problems than the accuracy of the GPS and chart. If you aren't going dead slow and making visual contact with the rock before approaching, I don't care how good you GPS is, I'm betting you eventually hit it.

Crowd sourcing is commonly used and probably more accurate than 1 obsessive master chart plotter.

[hpeer]

Originally Posted by Stu Jackson

Just a question: If you've been through there before, have you considered using TRACKS? That way the database of the shoreline and rocks become meaningless and you could follow it in and out like many do at anchorages for bailouts in the dark & stormy...

The track was on the chart plotter image.

But it is not a place a guy gets to but once or twice a lifetime.

[Jammer Six]

Originally Posted by Stu Jackson

That way the database of the shoreline and rocks become meaningless and you could follow it in and out like many do at anchorages for bailouts in the dark & stormy...

Depths change. Twice (or four times) a day.

Good luck.

Originally Posted by valhalla360

Crowd sourcing is commonly used and probably more accurate than 1 obsessive master chart plotter.

I'm not interested in either "probably" or "commonly" where rocks are concerned, because folks who run into rocks are both probably and commonly amateurs.

[goboatingnow]

Originally Posted by Jammer Six

Depths change. Twice (or four times) a day.

Good luck.


I'm not interested in either "probably" or "commonly" where rocks are concerned, because folks who run into rocks are both probably and commonly amateurs.

Hmmm you regard the US navy as amateurs, fighting talk there

Dave

[noelex 77]

Originally Posted by Jammer Six

I'm not interested in either "probably" or "commonly" where rocks are concerned, because folks who run into rocks are both probably and commonly amateurs.

Most terrestrial maps show areas you can explore, but marine charts are a little different, their job is primarily to show the no go areas.

Multiple map sources are therefore very useful. Even if they are less than 100% reliable like the consumer input layer on electonic maps, (which on occasion I have found very helpful), or a smudge on google earth. Taking the most pessimistic of the sources is safer than hoping the "official" chart has all the answers.

The bottom line is that false positives on marine maps are not much of a concern, it is the false negatives that will cause you grief.

[noelex 77]

Here is an example of some consumer information on an electronic chart.

The green icon contains a warning that the area is "shallower than charted"
This is added as an overlay (that can be turned on and off) to the chart by a private individual.
I anchored near here about and watched several yachts run aground at or near this spot. The official depth is listed as 10-11m.

With this sort of amateur added information some caution needs to be exercised. It has not got the rigorous verification of a proper marine survey, but providing you are aware of the information it is easy to play it safe and give the point a wider berth than the official charts would indicate.

This sort of "local knowledge" added to an official chart is only in its infancy, but it has a promising future.

[Andrew Troup]

Originally Posted by noelex 77

Most terrestrial maps show areas you can explore, but marine charts are a little different, their job is primarily to show the no go areas.

Multiple map sources are therefore very useful. Even if they are less than 100% reliable like the consumer input layer on electonic maps, (which on occasion I have found very helpful), or a smudge on google earth. Taking the most pessimistic of the sources is safer than hoping the "official" chart has all the answers.

The bottom line is that false positives on marine maps are not much of a concern, it is the false negatives that will cause you grief.

I hope I'm not the only one thinking this is a characteristically wise observation from N77.


I didn't fully realise this myself until a minor challenge in the mid 1980s.
On checking the actual charts on a boat I'd recently been flown in to join as navigator (because their Satnav had failed!), I found we didn't have the chart for Suva harbour, out next port of call. (It was listed in the Rolodex as being on board)

We were about to retrieve our docklines in Nuku'alofa, and if we delayed more than an hour or two, we would lose the narrow timeslot during which the sun would be from the right angle to be able to con safely through the nearby reefs, whose nav aids were in some disarray. So it would cost us an extra day.

I normally carried large sheets of tracing paper for such contingencies but on this occasion I think I had a limited baggage allowance, so I seem to recall plundering the galley for baking paper which I joined with masking tape.

I took off in the dinghy and found a friendly sailor on a nearby boat, who was happy for me to copy his chart. When I saw just how much information it contained, I quickly realised I wasn't going to have time to transcribe sufficient detail for a daytime entry to be viable, so I had a brainwave, and the realisation mentioned above lodged in my consciousness.

First, though, I concentrated on carefully capturing the locations and characteristics of the various lights. This took a while. However, because they can be unambiguously identified at night, and only three are needed at any one time for an unambiguous fix, this was the key to the success of my idea of a night entry using limited information. (Even if, as usual in this part of the world, some of the lights were not lit on any particular night)

-- I've since found that German merchant naval captains used similar reasoning to draft usable charts from the "List of lights", or just from the Pilot, when they found themselves having to divert to neutral or friendly ports - eg South America - when various world wars broke out --

I then marked in the details pertaining to the channel (including approaches) and the quarantine anchorage, and plotted any particularly tall landmarks or peaks which would stand out against the sky. These tasks took hardly any time at all.

Armed with the above realisation, all I had to do then was to draw a bold line around everywhere the water started to get too thin for our draft, areas containing moorings etc, and put a similarly bold circle around any isolated NO GO areas, like rocks, coral heads, piles, and such. I didn't need to know why not to go there.

So the 'chart' showed nothing of the shoreline, the shore- and dock- side facilities, or any extraneous info of any sort; most would be of zero interest when entering at night even if time had permitted.

We arrived in the vicinity in the early afternoon, so we hove to off the beautiful island to the south of Suva whose name I forget, and swam and snorkelled until dusk. The entry was straightforward and uneventful - the rudimentary, but purpose-built chart made it a doddle. It also didn't hurt that the entire crew kept their eyes peeled for things not to hit (which of course they should always do when entering crowded waters at night)

I suppose the short version of this story would be "IF you know where you are, and where you must NOT be, that's all you strictly need to know"

[Seaworthy Lass]

Originally Posted by btrayfors

.....took me a while to determine why the latitude lines were accurate, but the longitude lines were not....because the zero degree longitude line ran thru Paris, not Greenwich!

Never was able to determine exactly where in Paris....perhaps the home of the chartmaker's favorite whore in Montmartre??

Bill

Close, Montparnasse .

The meridian runs through the Paris observatory in the 14th arrondissement, almost due south of the nearby Luxembourg gardens.

[StuM]

FWIW, if you come across any more old french charts ( pre WW1), the Paris meridian is 2°20′14.03″ E of the Greenwich meridian.

[wdkester]

[QUOTE=hpeer;1468898]Screen shot of chart plotter. Lets see if this is clear. Presuming it is, you can see my track through the headlands.

But, when using Fugawi 4.5 in split screen with GE, and doing an overlay, the headlands and harbor line up.
......Damn.....
Image is not attaching, says "Rotate Image"
Started to happen a couple of days ago.

Try this PhotoBucket link to the same image.

Francois Bay Photo by hpeer | Photobucket[/QUOTE

I don't have Fugawi and can't comment directly on the coincidence of GE overlayed on the chart.

I'm wondering what would happen if you were to import the track into GE? I think it would plot right on top of the channel into Francis Bay. This is why I think so; I just plotted the cursor position (from your screen shot) on OpenCPN using CM93 charts. It plots about .5 nm SW of the apparent Fugawi cursor and is about .2NM due East of West point (and lands right on the ferry track). I mentally moved moved your track .5nm West and a little South and it looks like it might be OK. Which says the CM93 chart might be OK.

I then moved the cursor position to GE and it plotted .2nm East of West point. Which says that for that point CM93 and GE were similar.

[RobertLeask]

I had charts for British Columbia which credited Capt. Cook and the Endeavour for some of the soundings. I wouldn't be surprised if the same data is used on the electronic versions.