GPS: planing and tacking

Moderator: GreenLake

Re: GPS: planing and tacking

Postby GreenLake » 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.
~ green ~ lake ~ ~
GreenLake
 
Posts: 7136
Joined: Mon Jun 18, 2007 3:54 am

Re: GPS: planing and tacking

Postby jeadstx » Thu Oct 08, 2015 5:23 pm

As someone who's work has involved dealing with electronic map data for many years, printed and electronic maps are not always up to date. Best case with most maps and charts is a minimum of 6 months, commonly over a year. NOAA charts have a site that list daily changes to their charts so that notations can be made, but even their printed and downloadable electronic maps are not necessarily current. I have seen people list an aerial at work as being current (2015) in references (usually pulled from Google), but visually checking the aerial I could tell it was older. The more remote, often the older. Many map and chart services will list the current date of their data, but it is only made up of the data that was currently available to them. There is no way to keep data on an iPhone or GPS truly current. There is always a lag time with releases. For aerials it depends on the last time the satalite photographed it. Once photographed the data has to be geo referenced and indexed to line up with everything else. The topo maps follow the aerials in being updated. A lot of work goes into developing maps and charts, it takes time.

The downloadable charts I make for the Tx200, I reference the date of the data used. Even tho I make the charts as accurate as I can, I include a "Not to be used for Navigation" note just like NOAA does on their charts. Yes, if you look long enough you will find such a note on at least the publically available NOAA charts.

Things that can affect a GPS or iPhone are tree cover (on land), cloud cover, and the position of the satalites themselves on any particular day. Not sure about electrical lines, but interference from them wouldn't surprise me. With phone based GPS, signal strength as previously mentioned my is a factor as well. When I'm sailing with GPS, I often check what my accuracy is, especially if I'm in an area where navigation might be tight.

John
1976 Day Sailer II, #8075 - Completed the 2011, 2012, and 2013 Texas 200
1952 Beetle Boat Swan Catboat
Early Rhodes 19
1973 Mariner 2+2, #2607 - Completed 2014, 2015 and 2016 Texas 200
1969 Day Sailer I, #3229
Fleet 135; Canyon Lake, Texas
jeadstx
 
Posts: 1216
Joined: Tue Jul 08, 2008 11:10 am
Location: Dripping Springs, Tx

Previous

Return to Racing/Performance

Who is online

Users browsing this forum: No registered users and 9 guests