Join Date: Jul 2000
Thanked 153 Times in 142 Posts
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Re: Vanishing Seamanship
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.
Last edited by RichH; 08-10-2013 at 03:07 PM.