NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: GPS & Magnetic Bearings
From: Brooke Clarke
Date: 2003 Feb 9, 15:11 -0800
From: Brooke Clarke
Date: 2003 Feb 9, 15:11 -0800
Hi Robert: You're very close. The World Magnetic Model is based on empirical observation, not a theory. The model uses a 12th order polynomial based on past observations. My Motorola GPS uses flash memory and can be updated for a fee. The model is much more complex than just the variation (I misused deviation in the prior post), it has magnitudes for all three vectors North-South, East-West and the largest one Up-Down. Have Fun, Brooke Robert Eno wrote: > Just a quick note on the magnetic variation: I was told by an authority on > the subject that after 5 years, the accuracy of the magnetic variation > information contained within your GPS software starts to drop off. The > reason being, is that the calculations for determining the variation is > based on a set rate for a particular epoch. As we all know, the rate of > change in variation is generally steady over a short period of time, > however, over the long haul, that rate starts to change. So for example, if > in 1995, the rate of change for a certain area was 12 minutes eastward per > year, that rate of change might be, say 20 minutes eastward per year in > 2003. > > That is my understanding but I could have misinterpreted the message. > > Comments? > > Robert Eno > > ----- Original Message ----- > From: Brooke Clarke> To: > Sent: Sunday, February 09, 2003 2:39 PM > Subject: GPS & Magnetic Bearings > > > Hi: > > > > I know my old Motorola GPS uses the World Magnetic Model that is updated > every 5 years, mine has the 1995 version. This model will tell you the > magnetic deviation if you know the date and your location, both of which a > GPS receiver knows. > > > > GPS has no provision to determine direction. For example on a trip to > Japan I had a way point for my hotel in a big city. Many blocks away I knew > I needed to head in a certain direction to get back, but the GPS has no > compass functionality. The answer was to walk very quickly for a block and > then see which direction the GPS said I was going based on the change in my > position. Some GPS receivers now incorporate a fluxgate or other > > electronic compass into the receiver for this reason, but the GPS system > derives bearing from change in position. For more on sensors and the WMM > see my web page: http://www.pacificsites.com/~brooke/Sensors.shtml#Earth's > Magnetic > > > > Have Fun, > > > > Brooke > > > > Jared Sherman wrote: > > > > > Fred- > > > > > > > > > GPS knows nothing about steering directions, it only knows the actual > course taken by the GPS antenna. If you are steering 340 but making 350 due > to leeway, the GPS will only tell you that you are making 350. Or, that the > GPS antenna is making 350 so it presumes the rest of the boat is moving that > way too. GPS literally "can't" be swung like a compass can. > > > > > > GPS knows nothing about true/magnetic, unless someone tells it to > display one versus the other and gives it deviation information. I'm not > sure if that is available from the GPS system (I don't think so) but the > deviation data for the world can be stored in ROM and then "generated" by > the GPS unit running the current date against that to extrapolate the > correct information for the time and position that the GPS system has given > it. > > > > > > There was some discussion...here?...some time ago about one brand of GPS > that indeed had a firmware error and was apparently calculating > magnetic/true incorrectly. French or English I think--a brand that doesn't > appear here on the US market. > >