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Old 19 July 2007, 19:09   #1
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< Edit: split from this thread JK >
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Originally Posted by MikeCC View Post
Don't want to make light of it but green bits 1 or 2m above chart datum are usually a give away.
The point I was TRYING to make is that the chart clearly shows a uniform flat area that dries - in reality there are rocks all over the place at different heights so whilst the area is covered the depths are NOT the same.

Obviously an oil tanker wouldn't go anywhere near the place but a small boat may think from the chart that the area is all the same when the pics show it's clearly not!!!

Compare my local harbour chart with the Bembridge one - they both show similar conditions but the reality is VERY different. Once my area is covered there are no nasty pointy bits to spoil your fun!!!
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Old 19 July 2007, 19:18   #2
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Those Bembridge rocks are certainly very nasty and cover a large area. Mapsource doesn't show the area as rocky but a chart gives you a better impression.

It calls them Sharpus Rocks whatever they are !
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Old 19 July 2007, 19:32   #3
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Quote:
Originally Posted by Biggles View Post
Those Bembridge rocks are certainly very nasty and cover a large area. Mapsource doesn't show the area as rocky but a chart gives you a better impression.

It calls them Sharpus Rocks whatever they are !
Hell yes - and there's me thinking electronic charts were just as good - shows when I last looked at a paper chart!!!

I will have to check on my Raymarine to see what that shows.

OS maps show brilliant coastal detail - I use memory map on a pda with GPS which is great for close in work. Look how much detail this shows - the difference between this and the chartplotter is really frightening!!!
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Old 19 July 2007, 19:45   #4
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So the OS Map actually depicts the danger area better than the Admiralty chart although it doesn't give depths.

But it really shouts 'Danger' for that area.
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Old 19 July 2007, 20:00   #5
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Quote:
Originally Posted by Biggles View Post
So the OS Map actually depicts the danger area better than the Admiralty chart although it doesn't give depths.

But it really shouts 'Danger' for that area.

Tell me about it.

In fact maybe we should give up on nautical charts - after all boats only tend to have problems when they hit the land..............
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Old 20 July 2007, 05:12   #6
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Quote:
Originally Posted by Biggles View Post
So the OS Map actually depicts the danger area better than the Admiralty chart although it doesn't give depths.
Woohhhaaa!!
Lets be VERY CAREFUL HERE!!!
If we have newbies read this it looks like we are saying you are better off navigating by OS that Admiralty.
No WAY!!!
Look closely at the two clips of the charts. Do you think the OS surveryor guys went out on the Foreland, Black Rock and Long Ledges? This is very misleadning because it appears the run is 'open.' In fact as the Admiralty chart shows there is a reef right across, it just rarely is seen.
We must encourage people to learn to read a CHART not a map and even do a basic bit of tidal height prediction. Many of these places are safe, IF you do your homework.
The Admiralty chart IS the source of truth. If it is inaccurate I am sure the Hydrographer dept would love to hear from anyone who has specific info.

On another note, when looking at Admiralty charts, if they are a large scale, they will group a larger area together and show the shallowest depth, without the detail, so not every rock, or every channel many be seen, but the depth is still shown as a hazard my being the minimum (ie that over the highest rock). As the scale decreases more detail is shown and POTENTIAL channels between the hazards open up. It is never the other way round. Even on the chart for the whole E Channel Sharpus rocks and the bembridge ledge depth is shown as 2.7m drying, almost to the ledge bouy which in reality is in 17+m of water! If you look at the chart for East wight you get the channels and the nasty looking art work.

Please guys, lets encourage good seamanship and navigation.
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Old 20 July 2007, 06:42   #7
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Quote:
Originally Posted by genoa View Post
Woohhhaaa!!
Lets be VERY CAREFUL HERE!!!
If we have newbies read this it looks like we are saying you are better off navigating by OS that Admiralty.
No WAY!!!
Look closely at the two clips of the charts. Do you think the OS surveryor guys went out on the Foreland, Black Rock and Long Ledges? This is very misleadning because it appears the run is 'open.' In fact as the Admiralty chart shows there is a reef right across, it just rarely is seen.
We must encourage people to learn to read a CHART not a map and even do a basic bit of tidal height prediction. Many of these places are safe, IF you do your homework.
The Admiralty chart IS the source of truth. If it is inaccurate I am sure the Hydrographer dept would love to hear from anyone who has specific info.

On another note, when looking at Admiralty charts, if they are a large scale, they will group a larger area together and show the shallowest depth, without the detail, so not every rock, or every channel many be seen, but the depth is still shown as a hazard my being the minimum (ie that over the highest rock). As the scale decreases more detail is shown and POTENTIAL channels between the hazards open up. It is never the other way round. Even on the chart for the whole E Channel Sharpus rocks and the bembridge ledge depth is shown as 2.7m drying, almost to the ledge bouy which in reality is in 17+m of water! If you look at the chart for East wight you get the channels and the nasty looking art work.

Please guys, lets encourage good seamanship and navigation.
WELL SAID Genoa.
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Old 20 July 2007, 08:05   #8
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Hell yes - and there's me thinking electronic charts were just as good - shows when I last looked at a paper chart!!!
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
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Old 20 July 2007, 09:11   #9
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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.
K
I have not used the Garmin kit, but with Navionics and CMap the data is there, and like the paper version, it depends what you have bought. If you have paid for the detailed charts you get the detail, if not you don't. How many people on the forum have bought the detail and how many rely on the preinstalled summary stuff designed for planning not navigation?

Quote:
Originally Posted by Erin View Post
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
A plotter, whether it be a digital screen or a mark on a bit of paper is NOT synonomous with navigation. The later requires you to do more including understanding tidal ranges and curves. It is to easy to think that cos you can read a TomTom in the car, that you can navigate. I know since I bought the Rib I have become more lazy than when I sail, but I still check the charted depth against reality and expected for the time to see wots wot (occassionally). These skills are vital and we must encourage people to understand them so that they have a feel for what they are getting into.
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Old 20 July 2007, 09:14   #10
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Quote:
Originally Posted by genoa View Post
Woohhhaaa!!
Lets be VERY CAREFUL HERE!!!
If we have newbies read this it looks like we are saying you are better off navigating by OS that Admiralty.
No WAY!!!
Look closely at the two clips of the charts. Do you think the OS surveryor guys went out on the Foreland, Black Rock and Long Ledges? This is very misleadning because it appears the run is 'open.' In fact as the Admiralty chart shows there is a reef right across, it just rarely is seen.
We must encourage people to learn to read a CHART not a map and even do a basic bit of tidal height prediction. Many of these places are safe, IF you do your homework.
The Admiralty chart IS the source of truth. If it is inaccurate I am sure the Hydrographer dept would love to hear from anyone who has specific info.

On another note, when looking at Admiralty charts, if they are a large scale, they will group a larger area together and show the shallowest depth, without the detail, so not every rock, or every channel many be seen, but the depth is still shown as a hazard my being the minimum (ie that over the highest rock). As the scale decreases more detail is shown and POTENTIAL channels between the hazards open up. It is never the other way round. Even on the chart for the whole E Channel Sharpus rocks and the bembridge ledge depth is shown as 2.7m drying, almost to the ledge bouy which in reality is in 17+m of water! If you look at the chart for East wight you get the channels and the nasty looking art work.

Please guys, lets encourage good seamanship and navigation.
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.
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