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Old 08-11-2013
JonEisberg JonEisberg is offline
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Re: Vanishing Seamanship

Quote:
Originally Posted by RichH View Post
My personal belief is that with modern 'electronic' navigation there are many 'errors of resolution' ... or improper 'scale up'/magnification of the base charting from which the e-charts are made, especially charting that is derived from old lead-line soundings, etc.

In the paper chart days, skippers would pass by most 'danger zones' at considerable margin of safety distances because they knew that the old charts simply were not that accurate. With todays e-Nav much of the base data is still derived from lead-line data, especially in areas not frequented by the 'large commercial boats'; and still may include the relative inaccuracy of lead line soundings, etc., which when originally surveyed the 'expected' inaccuracies ('tolerance ranges' of measurement) defined the 'scale' of the charting. With e-Nav its really easy to 'magnify' at a scale that is much much different (and entirely inappropriate) than the original scale and the 'resolution enhancements' that the modern e-navigator uses can be 'waaaaay off'.
The message here is if youre navigating in areas that were charted/defined/derived by the pre-WWII (and later) survey methods, dont depend on extreme MAGNIFICATION of an e-chart to insure your safety. Another way to put this if the base data was properly accurate for 1:50,000 or 1:75,000 scale charts (NAD1927, etc.), the 'stack up' of intrinsic error due to the eNavigation magnification scale up of the 'intrinsic margin of error' in the 'old data' will leave you very vulnerable when reading the exact same chart MAGNIFIED to 1:5,000 scale (WGS1984). The resolution error comes from scaling up inappropriately to a a higher value of the intrinsic 'margin of error'.

This is the same as 'believing everything that you read in print to be true' ... and with no consideration a 'grains of salt' taken with such readings.
Several times before, here and elsewhere, I have cited Nigel Calder's HOW TO READ A NAUTICAL CHART, which offers his usual lucid, analytical take on precisely these issues... May as well do it again, towards the end of this piece he did for OCEAN NAVIGATOR, he gets to the heart of what you are talking about... With so many folks today relying on e-charting exclusively, it's very important for 'navigators' to understand this stuff...

How accurate are our charts? - Ocean Navigator - January/February 2003

In my observation, what amazes me the most today, is the lack of appreciation for the distinction between your position on a chart, and your position on the earth's surface... The faith that many sailors today have that those are always identical is simply astonishing...

The last time I was in Allan's Cay, I met another cruiser who was headed back across the Yellow Bank towards Nassau. Only problem was, he was gonna be doing so in extremely poor light conditions... No problem, however - in speaking with him, it became apparent that he truly believed that Monty & Sarah Lewis had charted every single coral head that existed along the track between Allan's and Porgee Rock, and as long as he steered around the little asterisks on his EXPLORER CHARTS, he'd be fine...

Now, to be fair, he probably would be, the odds of hitting a head with 6' draft along that route probably ARE somewhat slim... But still, it's the attitude and faith placed in the accuracy and completeness of cartography even as brilliant as Monty & Sarah's that often leaves me simply shaking my head at some of the 'seamanship' I see being applied today...

UFB...


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