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Old 27-01-2013, 14:23   #1
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Are All Charts Wrong?

I recently bought some binoculars, and set out to learn how to use the mildot reticule in them.

Having accomplished that, I thought that I would therefore be able to use the height of towers and lights to estimate range.

Well, the problem is this: all the NOAA charts for Puget Sound carefully list the heights of all the lights and towers in both feet and meters.

The problem is that none of the figures match.

The height listed in feet is never the same as the height listed in meters.

So, since all the NOAA charts for Puget Sound give at least one set of incorrect listings, is there anything else that these "charts" list incorrectly?

And oh, does anyone know which, if either, of the height listings are correct?
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Old 27-01-2013, 14:39   #2
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Re: Are All Charts Wrong?

Jammer,

Welcome to the world of old charts. A lot of the data is questionable, and a lot of it is just wrong. Plus you have the issue of the charts being made before GPS, so everything may be shifted a little bit. In one anchorage the chart was off by more than a mile.
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Old 27-01-2013, 14:45   #3
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Re: Are All Charts Wrong?

So the coefficient (conversion) between feet and meters is wrong? I wonder how NOAA screws that up?
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Old 27-01-2013, 14:53   #4
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Re: Are All Charts Wrong?

Quote:
Originally Posted by David M View Post
So the coefficient (conversion) between feet and meters is wrong? I wonder how NOAA screws that up?
I'm assuming this is just a matter of rounding right? We are talking "slight" difference correct? Show us some numbers?
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Old 27-01-2013, 14:53   #5
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Re: Are All Charts Wrong?

Quote:
Originally Posted by Jammer Six View Post
I recently bought some binoculars, and set out to learn how to use the mildot reticule in them.

Having accomplished that, I thought that I would therefore be able to use the height of towers and lights to estimate range.

Well, the problem is this: all the NOAA charts for Puget Sound carefully list the heights of all the lights and towers in both feet and meters.

The problem is that none of the figures match.

The height listed in feet is never the same as the height listed in meters.


So, since all the NOAA charts for Puget Sound give at least one set of incorrect listings, is there anything else that these "charts" list incorrectly?

And oh, does anyone know which, if either, of the height listings are correct?

The heights are listed in ft, the values you are seeing with the height is the visible range in miles, so 15M means visible for 15nm. I can't remember if it is visible or geograghic range that is listed.

Where on the sound are you? I used to be, and wish to be again, in Seattle.
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Old 27-01-2013, 14:55   #6
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Re: Are All Charts Wrong?

Quote:
Originally Posted by Adelie View Post
The heights are listed in ft, the values you are seeing with the height is the visible range in miles, so 15M means visible for 15nm. I can't remember if it is visible or geograghic range that is listed.

Where on the sound are you? I used to be, and wish to be again, in Seattle.
Ahhhh that actually makes sense
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Old 27-01-2013, 15:12   #7
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Re: Are All Charts Wrong?

Are you sure you are reading the chart properly--NOAA charts usually have the height of a light in feet. For instance, the Pt Partridge light is listed as FL 5s 105ft 13M, which means it is a flashing light at 5 sec intervals, 105ft high and visible to 13 miles.
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Old 27-01-2013, 15:14   #8
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Re: Are All Charts Wrong?

The heights in feet were probably calculated using a sextant with degrees, minutes and seconds. The metric heights were probably measured using a metric sextant with degrees, minutes and tenths, hence the minor differences.

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Old 27-01-2013, 15:17   #9
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Re: Are All Charts Wrong?

The interntional standard symbol for a metre is m - lower case.
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Old 27-01-2013, 15:38   #10
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Re: Are All Charts Wrong?

Uh, oh.

Well, at least I get to learn something.

The Alki Point Light, in Puget Sound, is listed as

Fl 5s 39ft 15M

I read that to mean "a white light, flashing every five seconds, at a height of 39 feet (15 meters)".

But I can see how it means "a white light, flashing every five seconds, at a height of 39 feet, visible for 15 miles."

Thanks, gang. Me learning is the best possible result.

Now.

Is that statute miles or (I assume) nautical miles?
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Old 27-01-2013, 15:50   #11
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Re: Are All Charts Wrong?

Quote:
Originally Posted by Jammer Six View Post

Now.

Is that statute miles or (I assume) nautical miles?
G'Day Jammer,

Well, it really does not matter, for it is only an approximation.

Cheers,

Jim
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Old 27-01-2013, 15:58   #12
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Re: Are All Charts Wrong?

U.S. Chart No. 1

charted ranges are nominal ranges given in nautical miles.

Page 66 - that leaves another 109 pages full of things you think you know better than you do
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Old 27-01-2013, 16:04   #13
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Re: Are All Charts Wrong?

If I remember right (and it was a long time ago) the visibility distance is based on a height of eye of 15 feet.
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Old 27-01-2013, 16:28   #14
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Re: Are All Charts Wrong?

Quote:
Originally Posted by Vasco View Post
If I remember right (and it was a long time ago) the visibility distance is based on a height of eye of 15 feet.
IALA seem to go into some considerable detail..

http://www.iala-aism.org/iala/public...oc_228_eng.pdf

Quote:
In the case of a light that appears as a point source, the luminous range D is defined as the
maximum distance at which a light can be seen, as determined by the luminous intensity I of the
light, the meteorological visibility V and the ‘required illuminance’ (formerly known as threshold)
Et at the eye of the observer. At this distance, the illuminance E at the observer’s eye is
reduced to the value Et


4.1 Definition of the nominal range of lights intended for the guidance of shipping
IALA recommends that the nominal range of maritime signal lights intended for the guidance of
shipping should be defined as follows:
The nominal range of a maritime signal light is the distance in nautical miles at which this
light produces an illuminance at the eye of the observer:
• of 2 × 10
-7
• of 1 × 10
lx for night time range
-3
It should be assumed that meteorological visibility V equals 10 nautical miles (T
lx for day time range
M
Note: Please see A1.2.3 for further considerations of required illuminance values.
=
0.7411) and that the atmosphere is homogenous
Just as I thought..,.

Always stuff the learn with boats
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Old 27-01-2013, 16:37   #15
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Re: Are All Charts Wrong?

Read all that have no idea what it means or says. You mean it isn't 15 feet?
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