[time-nuts] GPS altitude somewhat wrong?

Henry Hallam henry at pericynthion.org
Wed Jun 8 18:18:46 EDT 2016

Some of that is probably the difference between the geoid (what your
surveyed maps report height relative to) and the WGS84 ellipsoid (what
your GPS reports heights relative to).  At Boulder that difference is
only about 15 meters, though.

Generally with VDOP < 2 and a reasonably modern receiver the accuracy
of a GPS altitude measurement should be better than about 20 meters.
I'm not sure if the Thunderbolt counts as such.

How flat is Boulder?  Do you have a proper surveyed elevation of your location?


On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David
<david.vanhorn at backcountryaccess.com> wrote:
> I have just installed a Thunderbolt here to get our time and frequency equipment all on the same page.
> As I was looking at the display on Lady Heather, I was noticing that the GPS altitude seems rather wrong.
> We are in Boulder CO, which is nominally 5430' and the antenna is about 20' off the ground.
> The display (near overdetermined position) reads 1589.72991 meters or 5216 and change in feet.
> Altitude is a big deal around here. :)
> I suppose 214' isn't that outrageous, but it does bring me to a question:
> How accurate is the altitude number really?
> Thanks.
> --
> David VanHorn
> Lead Hardware Engineer
> Backcountry Access, Inc.
> 2820 Wilderness Pl, Unit H
> Boulder, CO  80301 USA
> phone: 303-417-1345 x110
> email: david.vanhorn at backcountryaccess.com<mailto:david.vanhorn at backcountryaccess.com>
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