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Old 20 July 2007, 09:17   #11
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All a bit worrying really. It's not the first time my mapsource chart has not been as clear in identifying shallows, rocks and ledges as a true chart does. Whilst I accept that a chartplotter is by no means a substitute for local knowledge or a good Admiralty chart, and there's always the disclaimer about not using the plotter for navigation (I can't see this as fair - why else would you buy one?) surely Garmin could make them emulate the paper versions a bit more closely. Especially since their data source is the HMHO charts in the first place!

K
Couldn't agree more - especially in your area where the islands almost double in size at low tide. I suspect the OS maps show things up far better.
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Old 20 July 2007, 09:21   #12
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I think the problem with charts is what they were designed for - shipping. Obviously with a largish ship if you see an area that is too shallow for your draft you give it a very wide berth.

It is very different in a small boat like a RIB. People enjoy picking their way through channels etc that no yachtie would go near.
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Old 20 July 2007, 09:59   #13
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For a start my comments were "tongue in cheek".

Secondly the OS map seems to be far more accurate than even the best charts - just look at the satellite/aerial view - the OS maps are made from those.

I have noticed many times in our area(Gower) that the OS maps are much better for close in work.
Codders, if I may be so bold....

I see them to be somewhat tongue in cheek, but as a respected contributor to the forum, others less experienced on the seamanship front MAY have taken the point seriously. They may have got an OS of the S of England and gone to sea. (Remember the guy who took his narrow boat out from Fleetwood, aiming for the Isle of Man. With a road atlas (where the Isle of Man was an insert on the next page!!!!)

On close inspection of the arial photo you CAN clearly see the rocks that extend across The Run that are not shown on the OS map. I believe OS don't work from Chart Datum (lowest Astro) but from some other Datum, hence this sort of omission?

Your comment about the charts being for ships is a fair one, but much of the survey work was carried out close in by RN boats with lead lines, so, where there has not been much shift they are still the best reference. Likewise OS maps aren't for navigators at sea. Prehaps a happy medium, but we should still encourage newbies to learn with a principal method, from which they can enhance and refine their skill.
Surely the skill of navigation is to use all points of reference, and have the good sense to interpret them to build a picture.
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Old 20 July 2007, 10:02   #14
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It is very different in a small boat like a RIB. People enjoy picking their way through channels etc that no yachtie would go near.
PS I have taken a yacht in The Run off bembridge, and some of the Ribsters on here say they stay as far off as the cardinal which is in >10m of water!
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Old 20 July 2007, 10:06   #15
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Surely the skill of navigation is to use all points of reference, and have the good sense to interpret them to build a picture.
Couldn't agree more - but how many boaters poo poo OS maps when they are an equally valuable additional source?

I carry OS maps and aerial photos on my PDA along with the charts on my Raymarine. I admit I have neglected paper charts because a RIB is not the ideal envirnoment for them but seeing how lacking in detail some of the electronic charts are I will carry some in future.

Having said that in my home waters there is not much point in carrying charts at all. The sandbanks change so much the place can be totally different 6 months later - yes really!!!
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Old 20 July 2007, 10:16   #16
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Having said that in my home waters there is not much point in carrying charts at all. The sandbanks change so much the place can be totally different 6 months later - yes really!!!
Which I think comes back to the point about using "multiple methods to determine your position and relative to any hazards."

What methods of navigation do you use to dodge them and stay safe if charts are not representative of the reality?
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Old 20 July 2007, 10:17   #17
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A key issue on admiralty charts is when the survey took place. If it’s something like 1890 you can treat it with a healthy degree of scepticism! However one advantage of the admiralty charts is the ability to work out how much water to expect in a given location.

IMHO most rocks only extend maybe max 0.5 meters above the sea bed and if you are trolling around at 18kts in 0.5m thinking it is ok because there are no rocks on the chart then you really need your head examining. Larger ledges are generally well charted and so are large rocks.
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Old 20 July 2007, 10:20   #18
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PS I have taken a yacht in The Run off bembridge, and some of the Ribsters on here say they stay as far off as the cardinal which is in >10m of water!
Better safe than sorry if ones not an expert on a particular area.

In my case my main transit from Chichester to Culver Bay made sense to go round the marker. And when you in a 4 meter RIB in rough seas I see no sense in picking up worse water going over the ledge.

I have cut it but I wouldn't recomend it as a matter of course to someone who didn't know the area.

NR.
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Old 20 July 2007, 10:29   #19
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Better safe than sorry if ones not an expert on a particular area.


NR.
Yep. Absolutely. Everytime

Wasn't havin' a go at the ribsters who play safe. It was the slight at us yachties not doing things cos we got a big lump underneath!

The point is, if the conditions are right and you can read and interpret good information, you can go many places that the unprepared can't.
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Old 20 July 2007, 10:41   #20
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Which I think comes back to the point about using "multiple methods to determine your position and relative to any hazards."

What methods of navigation do you use to dodge them and stay safe if charts are not representative of the reality?
Local knowledge and the MK1 eyeball as they say - it's the only way around here!!!
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