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Elevation Data accuracy?


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I own a Vista & 176c, I use both for driving, The

vista has a calibratable altimeter, my 176c doesn't. While driving, I have a data field box

set to elevation on my 176c, It seems to be quite

inaccurate, while driving over the Golden Gate Bridge in San Francisco, it displays negative

elevation, then close to 1000 ft above sea level.

The bridge is 225ft above sea level according to

my blue chart data card. Any answers? or is it just a part of civilian gps inaccuracy? I hate

uploading my track to mapsource, just to see a

inaccurate elevation graph!!!!!

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Is your 176c mounted where it gets a good skyview of the satellites? I record tracklogs of my bicycle rides and just checked a few that cross the GG bridge. The elevations were all in the 240' - 260' range at the peak (the 225' shown in BlueChart is probably clearance under the bridge).

I use an eMap and I'd expect similar altitude performance from the 176c, so I'm surprised you saw so much variation.

 

I find the elevation profiles to be quite accurate and useful even without a barometric altimeter like in the Vista. An example of a ride that I did last week and also in the spring is shown at:

http://prathman.home.attbi.com/Image33.gif

Note that the profiles are very consistent (recent ride is in blue, the spring ride in red). I also checked the elevations at the peaks and other key points against USGS topo maps and found them to agree within 30'.

 

The ability to set the altitude manually in 2D mode is useful if you know that your altitude has changed significantly since the last lock. For example, when first getting a 2D lock when leaving the airport I got a position that was way off since the eMap still had my altitude as 36000' from the previous lock on the plane. Resetting it manually to 50' resulted in a good position fix. Once the unit has a good 3D fix it will adjust the altitude automatically.

 

[This message was edited by peter on November 28, 2002 at 11:17 AM.]

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Manually inputting elevation or even the ability to manually enter elevation with most comsumer grade receivers will only exist with a 2D fix. A 2D fix is something that is not commonly encountered these days.

 

Generally it was only a viable option in some marine applications but today it's not something that's really worth considering.

 

Elevation in general will always be less accurate (relative) than horizontal (for all users) and in cases where multipath is a problem then there certainly will be spikes (big spikes).

 

Simply accept 3D mode and accept the elevation for what it's worth and sometimes that's not worth a lot.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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