Re: GPS: planing and tacking
Posted: Thu Oct 08, 2015 7:33 am
kokko wrote:Most purpose-built gps devices will be accurate +/- 6 ft at a 95% confidence level.
Theory and practice. As I wrote, I've seen land-based tracks that were shifted compared to the actual street grid (and some had me bike through water). This happened at that particular time, but not at other times, using the same dedicated GPS device in the same area. All I can conclude is that GPS reception may have been compromised that day. Now, most of the time we don't double check any self-reported accuracy data on the GPS (whether once, let alone in real time). So, it's useful to assume larger error bars on principle.
When navigating relative to charts, you pick up charting errors of unknown magnitude. I already mentioned that some navigational aids are only plotted approximately. Floating marks will move with the current, and certain marks are repositioned to track shifting channels, not all of those get translated faithfully.
Nigel Calder has written a nice book "How to read a nautical chart" which goes into that subject to some depth. Makes nice winter reading.