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post #24 of Old 02-22-2010
wwilson
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Quote:
Originally Posted by Omatako View Post
AFAIK WAAS is only available in the US and there's a heck of a big world out there.
Omatako you make a valid point though, I thought the European & Japanese systems were up and running and compatible.

MSAS for aviation use was commissioned on September 27, 2007
The official start of EGNOS/SBAS operations was announced by the European Commission on 1 October 2009

My real point however was less a tribute to GPS, than a reminder that there is a whole lot of error in taking compass bearings then transferring them to a paper chart even if the coastal pilot ignores the inherent error that map/chart makers estimate and provide.

A + or - 2˚ compass sighting of an object 1nm away is pretty good on flat water and unobtainable on a pitching deck. That alone can produce 47 meters of error!

A 0.5mm wide pencil line on a 1:10K scale chart adds another 5 meters of error, @ 1:20K 10 meters of error, @ 1:40k=20meters of error, @ 1:80k=40 meters of error.

NOAA has specified the accuracy for its nautical charts in terms of the accuracy with which features are plotted on the chart from their original surveyed position. The plotting positional accuracy of most features is approximately 1mm at chart scale. To put this into perspective, at 1:20,000 scale 1mm is 20 meters of error and at 1:80,000 scale 1mm is 80 meters of error.

You are an advocate of coastal piloting techniques - me too, but it doesn't produce "ground truth" only another estimate with a pretty good error chain of its own!
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