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InvisibleAIRDOG
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GPS and elevation
    #14131690 - 03/16/11 03:18 PM (12 years, 10 months ago)

So im planning on buying a GPS.... a friend sells me the Mio DigiWalker C-520. hes selling it to me in 100 bucks... is it a good deal???

anyone has this device??? does it give elevation?? this is a very important feature i want it to have... soo

thanx i hope someone can tell me...


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Invisiblekoraks
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Re: GPS and elevation [Re: AIRDOG]
    #14136867 - 03/17/11 01:42 PM (12 years, 10 months ago)

I don't have experience with that device; the only thing I noticed with the GPS devices I have used was that the elevation indications they gave were not very useful.


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InvisibleAIRDOG
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Re: GPS and elevation [Re: koraks]
    #14138837 - 03/17/11 07:25 PM (12 years, 10 months ago)

ok thanx... i have bought it already seemen like a good deal to me... if i dont find it useful i will sell it more expensive hehe

:crazy:


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InvisibleRedSnapper
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Re: GPS and elevation [Re: AIRDOG]
    #14140011 - 03/17/11 10:14 PM (12 years, 10 months ago)

Altimeter definitely comes in handy. The one or two times I actually needed it I wasn't carrying it..


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OfflineSeussA
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Re: GPS and elevation [Re: AIRDOG]
    #14141802 - 03/18/11 06:29 AM (12 years, 10 months ago)

> anyone has this device??? does it give elevation?? this is a very important feature i want it to have... soo

I don't know about that device, but in general all GPS devices should provide elevation. That being said, the elevation error can be fairly significant (+/- a few hundred feet).  If I need accurate elevation, I use both a GPS and a (barometric based) altimeter.  Timex makes some nice wrist mounted (analog and digital) altimeters.


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InvisibleDoc_T
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Re: GPS and elevation [Re: Seuss]
    #14141807 - 03/18/11 06:34 AM (12 years, 10 months ago)

^^^ agreed.
I don't use an altimeter, but I've used a topo map and a compass while hiking with geeky friends with GPS.
GPS is great for driving, less useful for walking.


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Invisiblekoraks
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Re: GPS and elevation [Re: Doc_T]
    #14141820 - 03/18/11 06:42 AM (12 years, 10 months ago)

Quote:

Seuss said:
That being said, the elevation error can be fairly significant (+/- a few hundred feet).



My experience exactly. Some of the more upmarket GPS devices have a barometric altimeter to compensate for this, but the entry level ones don't.

Quote:

Doc_T said:
... I've used a topo map and a compass while hiking with geeky friends with GPS.




Same here. It's impossible to beat a good map. I also like to use paper maps, as I think the still beat the crap out of the nifty digital ones with their small screens, limited resolution and hunger for batteries. Just a good map (1:20k; in any case 1:50k or less, with contour lines) and an old fashioned analog compass, thankyouverymuch.


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InvisibleDoc_T
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Re: GPS and elevation [Re: koraks]
    #14141832 - 03/18/11 06:49 AM (12 years, 10 months ago)

Local mountain shop sells USGS topo maps. :nerd: I got the $20 Silva compass, I don't need the fancier features.

I suppose a good GPS would get a better fix over time?
But still not as good as a good map fix.


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OfflineAnnom
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Re: GPS and elevation [Re: koraks]
    #14141945 - 03/18/11 07:35 AM (12 years, 10 months ago)

Vertical GPS errors are usually larger than horizontal due to the geometry of satellites in the field of view (it can't see through Earth).

The actual altitude is with respect to sea level (or is it with respect to the geoid?), so the GPS device needs to know what the sea level/geoid height is and make a correction from the "absolute position" to altitude. Not sure how this is done, but it can cause systematic errors.

A hundred feet error can be caused by vertical errors, but I believe a few hundred feet is generally too much for the GPS position error and has to do with the conversion to altitude. In this case, calibrating the GPS device, like a barometric altimeter, could reduce the errors. Still, a well calibrated barometric altimeter is likley more accurate.

> Same here. It's impossible to beat a good map. I also like to use paper maps, as I think the still beat the crap out of the nifty digital ones with their small screens, limited resolution and hunger for batteries. Just a good map (1:20k; in any case 1:50k or less, with contour lines) and an old fashioned analog compass, thankyouverymuch.

I like maps, but I also like to have a GPS location instead of triangulating mountaintops or trees :wink:, although that is fun too.
I did a few weeks of trekking through New Zealand and used my smartphone/GPS with (offline) topographic maps 1:50k, and 5 spare batteries :grin:. It worked well and I could carry a whole country in 1:50k in my pocket. The only problem was the small screen and limited resolution. A real map gives a better and nicer overview.

What I most enjoy is how GPS can track you, I like that gadget factor. My New Zealand track,


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OfflineSeussA
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Re: GPS and elevation [Re: Annom]
    #14142355 - 03/18/11 09:53 AM (12 years, 10 months ago)

> A hundred feet error can be caused by vertical errors, but I believe a few hundred feet is generally too much for the GPS position error and has to do with the conversion to altitude.

I don't know the cause, but I know it exists.  I do a lot of running with a Garmin GPS based trainer to record my routes.  The horizontal error is usually ~15 feet while the vertical error is usually ~50 or more.  I can be running at sea level and the GPS will show me anywhere from 100' below sea level to 150' above sea level throughout the run.  The error is not instantaneous, but tends to drift over time.


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OfflineAnnom
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Re: GPS and elevation [Re: Seuss]
    #14143125 - 03/18/11 01:07 PM (12 years, 10 months ago)

>  I can be running at sea level and the GPS will show me anywhere from 100' below sea level to 150' above sea level throughout the run.

That is a lot. I can only guess, but your GPS may only use a limited number of satellites (min 4) to save battery/CPU. This can be especially useful for small running devices. In that case, it will favour satellites near the horizon because those will give a smaller error in the horizontal position, which is generally more important. Just a guess though.


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InvisibleDoc_T
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Re: GPS and elevation [Re: Annom]
    #14143567 - 03/18/11 02:43 PM (12 years, 10 months ago)

Sounds plausible. Tune the device to a specific purpose.


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OfflineSeussA
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Re: GPS and elevation [Re: Annom]
    #14143977 - 03/18/11 03:59 PM (12 years, 10 months ago)

> That is a lot. I can only guess, but your GPS may only use a limited number of satellites (min 4) to save battery/CPU.

The receiver can lock onto 12 satellites at a time.  Part of it may be my location in the world.  Our coverage isn't the best.  It often takes a minute or two to acquire a good signal here, while it only takes a few seconds to do the same thing when I visit the states.  Because it is wrist mounted, the antenna is also pretty small to fit inside a "watch". 

In general, for civilian GPS, the vertical (elevation) error is typically 1.5x the horizontal (distance) error.


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OfflineToTheSummit
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Re: GPS and elevation [Re: AIRDOG]
    #14144496 - 03/18/11 05:41 PM (12 years, 10 months ago)

I really only use my GPS for geocaching and I never pay much attention to the altimiter.  The few times I've cross-referenced it with known altitudes (tops of mountains which I knew the actual elevation) it was off by up to 100 feet in either direction.  I still would prefer a map and compass if it were a survival situation and in most of my recreational trips I familiarize myself well with an area before I go making my reliance on maps minimal to begin with.  I nearly memorized topos of areas I was gonna be backpacking through in the past, to the point that I barely even pulled them out during a 1-2 week trip.  Of course a GPS does provide some fun features that a map never could.


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