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Old 24-11-2011, 09:15   #1
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Reading a Chart in Metres

Need help with understanding charts that say
"Soundings and heights in Metres".
The numbers have a little number dropped to the right that I think means.
21 = 2.1 meters or 6.8 feet.

Is my thinking correct?

Mark
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Old 24-11-2011, 09:23   #2
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Quote:
Originally Posted by Cotemar
Need help with understanding charts that say
"Soundings and heights in Metres".
The numbers have a little number dropped to the right that I think means.
21 = 2.1 meters or 6.8 feet.

Is my thinking correct?

Mark
Err yeah.

Dave
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Old 24-11-2011, 09:32   #3
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Re: Reading a Chart in Metres

Quote:
Soundings

The depth of water indicated by figures all across the seaward area of the chart may be in fathoms or metres (according to the chart title). Close inshore these depths may be represented in smaller denominations, either as fathoms and feet or as decimals of a metre. This is probably the most confusing and dangerous aspect in the conversion to metric charts and the navigator must be aware of it.
The depths are indicated by a normal-sized sounding figure with a smaller figure below and to the right of it. They are read off thus:

Fathoms: 52 (5 fathoms 2 feet = 32 feet) [1 fathom = 6 feet]
Metres: 52 (5.2 metres = 17 feet approx.)
The quote is not 100% true copy, see the link below.

Navis.gr - The Nautical Chart
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Old 24-11-2011, 10:26   #4
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Re: Reading a Chart in Metres

On French charts (and many others), depth are stated in
- meters and decimeters (1/10 m) up to 20.9 m
- meters and half meter (0.5 m) from 21 m up to 30.5 m
- in meters above (i.e. 31 m and deeper)

A rough conversion: multiply by 3 to convert meters to feet.
Useful : know your draft in meters (usually around 2)
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Old 24-11-2011, 10:54   #5
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Re: Reading a Chart in Metres

Quote:
Originally Posted by sailorF54 View Post
A rough conversion: multiply by 3 to convert meters to feet.
Useful : know your draft in meters (usually around 2)
3' 3" in a metre.

When calculating depth that extra 3" can add up.
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Old 24-11-2011, 11:35   #6
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Re: Reading a Chart in Metres

Quote:
Originally Posted by David_Old_Jersey View Post
3' 3" in a metre.

When calculating depth that extra 3" can add up.
By using 3, you're on the safe side
e.g. 6 meters is 19.7 feet.
Now 3 x 6 is 18 feet.

I agree it is useless if you want to find out whether you can safely navigate a shallow channel or clear some rock
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Old 24-11-2011, 11:55   #7
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Re: Reading a Chart in Metres

Quote:
Originally Posted by Cotemar View Post
Need help with understanding charts that say
"Soundings and heights in Metres".
The numbers have a little number dropped to the right that I think means.
21 = 2.1 meters or 6.8 feet.

Is my thinking correct?

Mark
Your correct Mark.

Heres a link to download US Chart No.1 which will explain all (I think)
U.S. Chart No. 1
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Old 24-11-2011, 12:26   #8
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Re: Reading a Chart in Metres

Quote:
Originally Posted by David_Old_Jersey View Post
3' 3" in a metre.

When calculating depth that extra 3" can add up.
Beyond knowing one's draft, which is generally around 2 metres, the extra 3" a meter can add up, but does it really matter? Two metres being 6 feet 6 inches, if my draft is 6 feet I would not go where the depth is shown as being 6 feet 6 inches as I feel it is too close for comfort. Same as I would never try to go under Julia Tuttle Causeway in Miami, which has a clearance of 56 feet even though my max height is 54 feet 6 inches.

So time the metres by 3 to get to feet. The extra 3" does not make a difference, as long as we are still talking about boats and related issues.
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Old 24-11-2011, 13:09   #9
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Re: Reading a Chart in Metres

So look folks:

There are 3.28 feet in one metre. Thus, if one multiplies the depth in metres by three and then add (mentally) 10% to that number one comes damn close to reality.

Example: to convert 10 metres to feet, multiply 10x3=30, add 10% of 30 which is by inspection 3 and you get 33 feet... a reasonably accurate approximation.

Not that hard, is it?

Cheers,

Jim
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Old 24-11-2011, 18:26   #10
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Re: Reading a Chart in Metres

Thank you all for clearing this up. Will be in the BVI reading these charts in Feb. and want to make sure my metric navigation is upto par.

Mark
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Old 24-11-2011, 20:42   #11
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Just in case: if you are using vector charts, you can just switch the units to feet and all depth indications will change to feet and inches.

ciao!
Nick.
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Old 25-11-2011, 05:01   #12
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Re: Reading a Chart in Metres

Nick,

Unfortunately the charts I have for the BVI are raster (Scanned paper charts made electronic)

Mark
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Old 25-11-2011, 06:39   #13
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Re: Reading a Chart in Metres

Being unable to think in Metric (which is really one of the worst inventions of mankind), I have copied a page from Bowditch that converts meters to Sensible, laminated it, and posted it above the nav station. It takes an extra step to glance at the chart and convert, but it's better than trying to navigate with unusable information.
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Old 25-11-2011, 06:58   #14
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Re: Reading a Chart in Metres

Quote:
Originally Posted by Benz View Post
Being unable to think in Metric (which is really one of the worst inventions of mankind)...............
Eh? Maybe deserves a 'rethink'.

Thinking in metric is easy enough - remember metric is the measurement unit that makes up US Dollars and Cents so it does not get much simpler.

It's the converting from one unit to another that's usually the trouble.........

Cheers
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Old 25-11-2011, 07:14   #15
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pirate Re: Reading a Chart in Metres

Hell.... its not that hard guys...
1 metre is 1 yard + 3 inches... or 3' 3"s
So 2 metres is 6'6".... 3 metres = 9'9"
Whats so hard about that FC's.....
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