Navigation charts, the blue line...

[G.I.B.]

Here is an example of a standard NOAA navigation chart. The blue color changes at the 18 foot mark. I know the draft of the old galleon's was around 18 feet. Is this how this contour color change came about, or is there a different reason?

 

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[stevemc]

You never know where it really started, but in the old charts, they didnt have that, just soundings in various areas. I am sure they didnt have the zoning like today until hydrography(bottom mapping) was done fully. Most of the US charts today were done from hydrography from the late 1800s, including the Bahamas. In the old days-1600, 1700s early 1800s, they could only use a sounding lead to see how deep it was. Until they invented a ping sonar they had to rely on the lead to tell them how deep it was. So I am sure it didnt start way back in the day of galleons. Around 1940s they started doing cable drags-which is 2 big boats with a heavy cable between them, dragging the bottom. They would drag until it hit something. Then they would mark it. That is how many wrecks were found. Do you live on St Thomas? My brother used to live there. Nice place! Nice diving! Great history! Steve.

 

[ivan salis]

by following the draft line and seeking obstuctions you may just find a wreck . but of course you know that already.

 

[wreckdiver1715]

Does the legand say that it is in Feet or fathoms?

 

[mad4wrecks]

I think Guyin BacK may be onto something. It is interesting to speculate that the 18-20 foot range was picked as the shallow water contour line because it harks back to the general draft of the galleons.

 

[G.I.B.]

Does the legand say that it is in Feet or fathoms?

All in Feet, and now I really don't get the rhyme or reason to the color contours.

Along the west coast of Florida, it's at the 18 foot line. I looked at your area, and, the chart is in feet, but with a different contour line depth.

Scanning around, I'm unable to see what the determining factor is as to where/why the contour color is placed.

Hummm.... maybe is has to do with the chart scale?

The 18 foot line does make it easy to visualize search areas/patterns.

 

[Dinkydick]

It's the three fathom line usually it ment safe water.

Dinkydick

 

[ivan salis]

the (3) fathom line or "shallow water line " as its known in the naval world --- 3 fathoms (or 18 feet) was the minimum safe water depth for most old sailing vessels --a fathom being 6 feet -- so 18 feet of water was the safe minimum depth -- in water under that depth a deeply loaded vessel would strike bottom and break up -- by following the 18 foot depth line contour and looking for "obstructions " you might just find a wreck or two ---- Ivan

 

[mnm]

If the guys are discovering history within a certain vicinity , couldn't you assume everything else would be within the area less a coquina ridge or manifestation of a boulder or two dumped in place as a cover , sheesh , how far aspread could it really be , they are right on it , Blow !!!!!!! Some holes dag nabbit ........