Magellan GPS 315 averages when I am walking!?

[Guest dmdeitz]

I just started using a GPS 315. My first cache hunt with it turned up zilch. On the way back to the car, I was running late and was at a brisk walk (or so I think). I looked down at the GPS (I waypointed the car's location). It was saying averaging. it gets to about 4 seconds then shows a speed of 2.6 - 3 mph (is that brisk?). then averaging for about 4 seconds then back to the speed... is that a defect, feature, etc?

[Guest arffer]

Remember that the majority of models used for geocaching have an accuracy of 50 feet. Less than 50' and the unit isn't sure you have moved. Anytime the Maggy thinks you are standing still, it starts to average. Add to that any partial signal loss under very heavy tree cover, and I can see your GPSR switching between averaging and mphs. An example of this is the fact that the compass doesn't start showing true readings at speeds under 2mph, there just isn't enough movement to determine speed and direction.

[Guest dmdeitz]

thanks.. that's what I figured - how does it know it moved vs. just averaging?!

 

thanks!

[Guest arffer]

Based on the number of sattelites locked in, and the quality of the signals, the GPSR knows what its Estimated Position Error is. As I understand it, and movement under that EPE it assumes is an error and that you are not moving, so it goes into averaging mode. Any movement greater than the EPE it figures is good, and that you have therefore moved, so it comes out of averaging mode and tries to figure out your speed and direction.

[Guest Kerry]

How would EPE affect or trigger averaging? Averaging begins when the unit is ?stationary? (or thinks it is) so to do this it must continue to take fixes to create that average, while trying to determine if it?s still ?stationary?. When the unit commences averaging the EPE doesn?t mysteriously change at all as the EPE is supposedly a figure to represent the accuracy based on satellite geometry etc and some other data. The averaging is commenced due to the interpreted distance (being less than some value) between each pair of successive fixes (the current fix and the previous one), which is a different thing to what EPE is representing.

 

The other thing with new and old versions of the same make/model GPS is how the software parameters are invoked to trigger a ?stationary? situation. Pre and Post SA would be a totally different requirement.

 

Cheers, Kerry.

[Guest arffer]

If the amount you move is less than the EPE, the GPSR will always assume that the movement it saw was due to error.

 

You have to move a distance greater than EPE for the GPSR to recognize it as true movement.

 

That's all my point was. If I'm wrong, well okay. It just seems to be contrary to the definition of 'error' if the GPSR will recognize a movement that is smaller than EPE.

[Guest AZMark]

Slow walkers will always be about 15-20 feet in front of there GPS. Makes it kind of a pain once u get close to a cache, cuz you have the tendency to slow down anyhow. But, I have learned to just stand still for awhile and let the darn thing catch up.

 

Anyone else vote for the ability to turn averaging off in the next rev of the flash?

 

Maybe if we all ask Magellan nicely?

[Guest arffer]

AZMark,

 

Afraid I can't sign that petition. I love the auto-averaging feature.

 

I head towards the cache untill the GPSR either gets to 125 feet, or until the distance starts heading back up meaning I missed a turn.

 

Then I stand still for say 3 minutes for the auto-averaging to work. Then I take a compass bearing off the GPSR and pace off the distance it says to the cache.

 

Then let it auto-average again for a minute or two, and see how close I am. Then I find the cache!

 

I would however vote to have the EPE still displayed along with the averaging time, rather than how it is now where the EPE display is replaced with the averaging time. It would be very handy to have the Maggy take the averaging into account when displaying the EPE so you could watch the EPE drop the longer you allowed the averaging to continue.

[Guest bwware]

Hmmm...not sure about turning it off. But I do have a problem with it. Why does it seem to take forever(!!) for it to average? I mean, I can sit down on the ground, no tree cover overhead with 8 full sats getting to my GPS but yet it sits there, 30mins, averaging! What am I missing?

 

Bill

[Guest Blab]

From what I understand, it will never stop averaging. It will continue to average, making more and more caculations. The longer it averages, the more accurate the measurement. I would say that after 20 minutes, you will not get much more accurate. Each reading taken during those 20 minutes has the potential to be just as inaccurate as any other reading, but it all averages out.

[Guest arffer]

That's correct. Averaging will never stop. The longer the better...

 

Here is a good article on how time makes a difference.

 

If you just look at the pretty pictures, you will see that the most significant improvement occures in 5 minutes. After that it slopes off.

 

I personally stop around 100-125 feet from the cache, give it 2 minutes to average, take a compass bearing and pace off the distance, then give it 4 minutes to average and see where I'm at.

[Guest AZMark]

All I'm asking for is a ******* toggle in the setup! Averaging on-off.

 

You want it you got it, you don't bing it's gone.

 

Throw me bone here fellows.

 

AZMark

[Guest ClayJar]

Consider this a bone. (It won't hurt to have one more toggle, and I have seen where it could be useful.)

[Guest Kerry]

As I see it there is really not much of a purpose for averaging (today) and especially auto averaging. The averaging should be a user selectable function.

 

I've quickly compared some data based on the same GPS both with and without Selective Availability with regards averaging.

 

http://www.cqnet.com.au/~user/aitken/gps/gps_avg.htm

 

Only preliminary at this stage but one should get the general idea.

 

Any comments/thoughts/ideas I'd like to hear them.

 

Cheers, Kerry.

 

[This message has been edited by Kerry (edited 31 July 2001).]