Charts & navigation

codprawn · 19 July 2007, 23:09

< Edit: split from this thread JK >

Quote:

Originally Posted by MikeCC

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!!!

Biggles · 19 July 2007, 23:18

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 !

codprawn · 19 July 2007, 23:32

Quote:

Originally Posted by Biggles

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!!!

Biggles · 19 July 2007, 23:45

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.

codprawn · 20 July 2007, 00:00

Quote:

Originally Posted by Biggles

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..............

genoa · 20 July 2007, 09:12

Quote:

Originally Posted by Biggles

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.

alystra · 20 July 2007, 10:42

Quote:

Originally Posted by genoa

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.

Erin · 20 July 2007, 12:05

Quote:

Originally Posted by codprawn

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

genoa · 20 July 2007, 13:11

Quote:

Originally Posted by Erin

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

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.

codprawn · 20 July 2007, 13:14

Quote:

Originally Posted by genoa

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.

codprawn · 20 July 2007, 13:17

Quote:

Originally Posted by Erin

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.

codprawn · 20 July 2007, 13:21

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.

genoa · 20 July 2007, 13:59

Quote:

Originally Posted by codprawn

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.

genoa · 20 July 2007, 14:02

Quote:

Originally Posted by codprawn

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!

codprawn · 20 July 2007, 14:06

Quote:

Originally Posted by genoa

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!!!

genoa · 20 July 2007, 14:16

Quote:

Originally Posted by codprawn

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?

simonmcnamara · 20 July 2007, 14:17

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.

Biggles · 20 July 2007, 14:20

Quote:

Originally Posted by genoa

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.

genoa · 20 July 2007, 14:29

Quote:

Originally Posted by Biggles

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.

codprawn · 20 July 2007, 14:41

Quote:

Originally Posted by genoa

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!!!