Etrex... Normal?

[+tomcatuk]

Bought myself a little yellow Etrex and a question for those experienced in geocaching....

 

Went out today and found a cache, (My eight!) it was right by the side of an underground line...

 

Now the reading said I had 60 odd feet togo and I spotted the stump where it was, the gps then suddendly started counting up in feet while I was standing still and finally stopped at 160' in the opposite direction....

 

Is there a set procedure that I need to do when starting off or is it just me expecting to much... had a good few sats locked in 6 or 7 I think it was... but it just seemed to go a bit mad...

 

This also happened the other day at a different location, but reading the logs it seemed to happen to quite a few other's at this other place as well..

 

I bought the Etrex as my first GPS so was getting a little worried....

 

Steve

[davester]

Perfectly normal. Add a few trees into the equation and your experience will be even worse.

[+leecee]

Nothing to worry about.. the little moving arrow only points the way as long as you are moving or walking above a certain speed. As the GPS doesn't contain a magnetic compass, it doesn't know which way it is "facing" when you are standing still. Once I get to about 30 feet or so, I often use a magnetic compass and the bearing from the GPS display to home in on the cache.

[+John Stead]

I think the idea is for those who use a GPSr under variable cover. If it loses the signals it assumes for a while that you are continuing in the same direction and speed as before - and if you stop it thinks for a while you are still moving. If you wait a while it will (if it has a satellite fix) recalculate its position and give you a correct distance, but unless it has an electronic compass as in the Vista the direction will not be accurate until you are moving again. It still is often better to watch if the distance is decreasing to be sure you are moving in the right direction until it sorts itself out.

Now under trees all of that may or may not apply - you may have to find a clearing and take a bearing and distance from there having checked the satellite display to be sure you have a good fix.

Patience is often more than a virtue and becomes a necessity. This is why sometimes coordinates for new caches are wrong - hiders have not waited long enough for the GPSr to settle. Sometimes it is advisable to take more than one reading and average. And to check it is right move off for a hundred yards or so and then see if it brings you back to the right spot. But I digress!

Have fun!

[+Boneychest & Catsuey]

..... suddendly started counting up in feet while I was standing still and finally stopped at 160' in the opposite direction....

 

If the distance is getting further out when you are at the cache, check that you have the GPSr set at the WGS84 system and not British OS grid. On the yellow Etrex setup>units page, check that you have the position format set to hddd* mm.mmm' and the map datum at WGS84 (I had accidently set mine set to British OS grid once and that put me anything up 400ft out).

[+Lance Ambu]

Yep pure normality.

 

I think the magellans do more of the averaging thing, see "boomerang effect" in GPS forum, than the etrex series.

 

My venture can suddenly jump around as you slow down, it is good advice to lift your eyes of the little yellow fella when you get to about 3o feet and look for the cache rather than expect the GPS to be inch perfect.

 

It all adds to lifes rich tapestry, Enjoy!

[+kbootb]

Perfectly normal.

 

Try this, go to a place, mark the waypoint. Come back every now and then over the next few days, stand in exactly the same spot and see how far it says to the waypoint you recently captured.

 

Even standing still at the place you will see the distance vary over a period of about 10 minutes. My etrex has a magnetic compass so even when stationary it will point towards the waypoint. Interesting to see it dance around.

 

Any buildings, hills in the area will reflect some of the signal and add to the 'noise' no matter how many sats the unit is reporting as seeing.

 

How gps works

 

The above link will take you to a very readable tutorial on how gps works and will give a clue to its limitations. I am in awe of the people that worked all this out.

[+Brenin Tegeingl]

I have a yellow Etrex, and can not get within 11m of the first micro of a multi that I placed with it, yet all visitors to the cache have had no problems with the co-ords. To top it all I've tried over a 100 times, as it's 4PawDrives usual walkies area. Shows you how funny etrexes can be.

Dave

[+tomcatuk]

Many thanks to all for your replies... glad to know it's normal...

 

Regards Steve (Tomcatuk)

[+The Forester]

It's called "multipath" in the jargon.

 

It is caused by the raypath of the received signals having bounced off various surfaces before being received. Your receiver has no way of knowing which signals hve come in a straight line of sight and which have bounced off something on the way, so it just incorporates duff pseudo-range data in the computed fix.

 

Because the satellites are constatntly moving at many thousands of knots, the multi-path effect is constantly changing. That's why the indicated position appears to hop around even if you are standing perfectly still.

 

They are fact of life in many urban locations and places where there is a lot of nearby steelwork.

 

Cheers, The Forester