GPS tracks

[flyguy]

Interesting thing I have seen on the cross-country tracks laid down by two different GPS units. (see how smoothly I avoided saying GPSes.) My cheapo Magellan will lay down a completely different track/course for a long cross country as compared to the Garmin I checked, between the same airports. Example: Central Calif (Los Banos) to Atlanta. The Garmin will give a great circle path that goes up to Kansas and then back down. The Magellan will draw a track to follow down across Oklahoma or Texas, much further south that is, and definately not a great circle. When you check it out over a shorter range the Magellan exactly tracks a straight line drawn on a chart. I have flown right down the track and checked visual checkpoints along the way. Interesting. It gets better though.
If you set a waypoint on a sharply defined peak that is say 100 miles away, like Mt. Shasta going up the central valley, the Magellan will take you off in a curve and then curve you back in to the waypoint. That is compared to the direction that you can see goes directly to the peak. The times I have checked, it was off to the right.
I think I know what is going on but it is just a little more odd than I would care to put in print without someone who knows about this confirming.
Anyone know anything about this or can refer me to an expert site or reference book. There is nothing in any manuals that I have seen.
Darryl

[Pearson]

Re: GPS tracks

Darryl,

I use a Trimble GPS to fly all over the country in the middle of the night. I have noticed that the nearer you get to the zero degree magnetic variation line, the less curved the great circle route is. I have not done any scientific study or even paid it a lot of attention, but it is enough of a difference for a half asleep freight pilot to notice.

Why not pick two points exactly on a certain line of variation, as taken from a sectional, note the amount of curvature of the great circle course. Then plot a course ninety degrees perpendicular, and the same distance, to one of those original two points and see if the curvature is greater. I think you will find that the farther west you are, greater variation, the more pronounced the curve will be.

You might try checking your GPS at various points along a track to verify that it has the right variation. Maybe you have to tell it.

I can tell you from experience that if you take off from LAX going to FtWayne, Indiana, your original mag course will be around fifty degrees. By the time you get southeast of Chicago your track needs to be around 110 degrees.

I can also tell you that if you don't have a GPS and you want to take a great circle route, you can get radial and dme points from VORs along the way by using DUATS. You are basically dead rekoning from fix to fix, but only for 100 to 150 miles at a time. As you cross over or abeam a fix, you adjust acordingly just like you did on your first solo cross country. If you had an old RNAV unit you could program each fix in and have a great circle route. Of course dead rekoning 100 to 150 miles at a time is a lot different when you are in a Taylorcraft as opposed to a kerosene burner at the higher flight levels. Also, it probably wouldn't be feasable during the daytime with the higher traffic level. But believe me, there are freight dogs in the night following vectors from ATC for thousands of miles. It is a whole nuther world out there on the back side of the clock.

[Mark Julicher]

Re: GPS tracks

Hmmm. Interesting. Now I need to go out and compare my cheapo Magellan to my Lowrance Airnav. (of course I always worry that the Lowrance will revert to being a fish finder, but that is a different discussion )

I used to fly Omega on cross country flights but I could not file as an RNAV equipped aircraft. The reason was that Omega flew Rhumb Line (straight line on a chart) and RNAV computed great circle. The difference is not significant at a two hundred miles but it gets way off on long hauls.

[flyguy]

Re: GPS tracks

Rich,
Color me ignorant. I didn't even think about the magnetic part of the heading indicator/track. Except in the grosser long distance part I am going to have to rethink the entire concept. Cheesh, talk about missing the boat.
Thanks for the major pointer. Funny it seems so simple on the surface, LOL.
DC
Mark, thanks for the word "rhumb" line. I know the word, but sure had forgotten what if meant.