Projection differences...

[+Spaceman Spiff]

What does the GPS have to do to do a projection from a point.

We have one cache here were a projection of 20 miles gives you 250 feet difference between an e-trek and a magellan.

 

How far can you project in a multi-cache before the device differences become an issue?

 

As an example I did a projection 100 Miles 0 true degrees.

And I get as much as 360 feet difference Street Atlas, Magellan Meridan and e-trek.

[+gnbrotz]

The difference will depend on how far apart your starting coords are and how far away you are projecting. With the range of error of a GPS, (small that it is, relatively speaking), two units will rarely, if ever, give the exact same position while at the same location.

 

As an exercise, I used some software to run some sample projections. I started with two waypoint just 25' from each other. I then projected off of each one at the same bearing for a distance of twenty miles. The two new projected points were 593 feet from each other. When I moved the original waypoints within 5' of each other, the two projected points were still 95' apart.

 

If you are planning a cache that will require a long distance projection, perhaps it should lead to an easily distinguishable landmark rather than an actual cache. From that new spot you can give more specific directions, such as "approx 40' east of the statue" or something similar.

 

Greg

N 39 54.705'

W 77 33.137'

[The 2 Dogs]

I haven't had much luck with projections on the Magellan 330. Seems to be way off every time I use it. Maybe I am doing something wrong. Don't forget the Magnetic vs True issues.

[+Logscaler and Red]

The way I do the projection trick when hunting a location, or placing a projection waypoint in a multistage cache, is to mark where I am at as the goto point.

 

Then SUBTRACT OR ADD 180 to the heading I needed to go. This gives you a reverse course to steer by.

 

Then all you have to do is head at that reverse heading for the distance you needed to go to.

 

And I use a Mag 330 and have had zero problems doing projections forwards or backwards.

 

Projection only.

 

Later. logscaler.

 

"It is not fair to have a battle of wits with unarmed people."

[+Spaceman Spiff]

I am talking about is setting up a projection that someone ELSE is to follow...

 

In the above example.. I set each unit with the same starting point...

 

Had each unit do a projection 100 miles 0 degrees true...

They each come up with a diffent answer...

On the same bearing line, they different positions.

 

All magellans get the same answer...

All of the e-treks will get the same answer..

But they could be 300 feet difference between the e-treks answer and the magellan..

 

So I am asking.. what kind of calculations does the DEVICE have to do, to calculate the projection.

[bnolan]

My guess would be that the internal algorithm that calculates the magnetic declination is different between the units. A fraction of a degree of difference in declination would account for such an error.

 

I remember there was a firmware upgrade for my garmin III+ that updated the way declination was calculated. My local declination changed from 16 degrees E to 15. So if you had two of the same GPS with different firmware versions you could also get different results doing projections.

 

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I may be lost, but at least I am making good time.