Altitude Discrepancy

[Guest dbyerman]

I just got the Handy GPS springboard module for my Handspring Visor. I plan on using it for geocacheing this summer, but tested it out a bit around town this past weekend to get a feel for it.

 

I was a bit disappointed, to be honest... especially in the altitude reading. It was inconsistent. My own home was shown, at different times, at 4800', 4700', even once at 4300'. And I didn't *feel* any massive plate tectonic movements at the time...!

 

I'm not looking for anyone to look down their nose at me and tell me I need a "real" GPS receiver. I got this one at a steal of a price and can't really afford to buy a real unit in the near future.

 

I just have two questions: first, is a GPS unit like this one accurate enough to use for geocaching (i.e. is it better at latitude and longitude than at altitude?) and second, what accounts for this discrepancy in altitude? Can I do anything to maximize my chances for an accurate reading?

 

Thanks for your help!

 

--

 

David A. Byerman

david@byerman.com

www.byerman.com

[Guest swangner]

I haven't used the Handy GPS module, but I haven't heard great things about it. I have used the Magellan GPS Companion for the Visor, which works very well.

 

As far as the altitude discrepancy goes, it probably has to do with the # of satellites the unit has a lock on. If this number tends to fluctuate, the accuracy of your position readings (especially the altitude reading) is going to fluctuate as well (in general, the more satellites, the better the accuracy).

 

What I can tell you is that from the manufacturers' specs, the Magellan springboard module seems to be a better unit than the Handy GPS unit:

 

HandyGPS : 25 meter accuracy, 4 hours on a set of 2 AAA batteries

 

Magellan GPS Companion: 15 meter accuracy (in my experience, usually even better than this), up to 10 hours on a set of 2 AAAs

[Guest doc_api]

It is my understanding that most gps units are much more accurate at lat/long than at altitude, esp. with fewer sattelites. If you look around on the GPS units and software board, it is explained a lot better than I ever could.

[Guest Cape Cod Cache]

Altitude is the least accurate of GPS... It wasn't designed for alt, just Lat/Lon. I use GPS sailing other people's boats, and get altitude readings of all sorts of mischeif.

I KNOW I'm at sea-level !!! I have the option of swapping altitude for time functions. Long story short, mountains are a long drive away, but for those of you in hilly areas, get an altimiter.

my $0.02

[Guest ScottJ]

Mainly because of simple geometry, GPS navigation is generally LEAST accurate along the vertical axis.

 

Horizontally, GPS satellites can be spaced along a full 360 degrees of azimuth relative to your position. With satellites on all sides and well spaced, you get a good DOP. Vertically, _ALL_ of the satellites you can see are in one hemisphere, above you. There aren't any in the other hemisphere (below you) because the earth's in the way. So, you get a higher (worse) DOP.