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?

Henry

On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David
david.vanhorn@backcountryaccess.com wrote:

Per https://www.topoquest.com/map.php?lat=40.02486&lon=-105.24468&datum=nad27&zoom=2&map=auto&coord=d&mode=zoomin&size=m
the address in your signature is close to the 5250 ft (geoidal)
contour.

Henry

On Wed, Jun 8, 2016 at 3:18 PM, Henry Hallam henry@pericynthion.org wrote:

Yo David!

On Wed, 8 Jun 2016 20:33:58 +0000
"Van Horn, David" david.vanhorn@backcountryaccess.com wrote:

Looking at my GPS farm, fair but not great skyview.

A Garmin 18x reports:
Altitude Err:    +/- 264 ft

A u-blox 8:
Altitude Err:    +/- 36 ft

And an adafruit HAT (u-blox 6?:
Altitude Err:    +/- 69 ft

On a Skytraq I have seen +/- 12 feet.

Depends.  :-)

RGDS
GARY

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem@rellim.com  Tel:+1 541 382 8588

A couple of things come to mind:

1. Is this a single measurement or an average over at least 24 hours?
2. Did you get your elevation via the receiver survey mode (recommended)?
3. How close is your "nominal" elevation measurement and what makes you
   think it is truth?
4. The vertical component of the GPS position solution is typically 50%
   worse accuracy and a lot noisier than the horizontal measurement. If you
   have a good horizontal measurement it is unlikely you have a "wrong answer"
   on elevation since your receiver is using the same data, just solving the
   equation for a different variable.
5. What is your satellite mask angle? The geometry (hence accuracy)
   degrades as an increasing function with mask angle. Suggest for the survey
   mode you use as low a mask angle as possible (typically 5 to 10 degrees).

Finally, your 214' error is outrageous. For a surveyed position the answer
should be with +/- 10'.

Michael Perrett

On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David <
david.vanhorn@backcountryaccess.com> wrote:

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces+david.vanhorn=backcountryaccess.com@febo.com] On Behalf Of Michael Perrett
Sent: Wednesday, June 8, 2016 4:33 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS altitude somewhat wrong?

A couple of things come to mind:

1. Is this a single measurement or an average over at least 24 hours?

60+ hours

2. Did you get your elevation via the receiver survey mode (recommended)?

This is what's currently being displayed in LH after a 60 hour survey

3. How close is your "nominal" elevation measurement and what makes you think it is truth?

Damifino.  :)

4. The vertical component of the GPS position solution is typically 50% worse accuracy and a lot noisier than the horizontal measurement. If you have a good horizontal measurement it is unlikely you have a "wrong answer"
   on elevation since your receiver is using the same data, just solving the equation for a different variable.

5. What is your satellite mask angle? The geometry (hence accuracy) degrades as an increasing function with mask angle. Suggest for the survey mode you use as low a mask angle as possible (typically 5 to 10 degrees).

Currently set to 5 degrees, which I know is low, but I wanted to see what the whole sky map looks like.
I will dial it up as we get things settled in.

Finally, your 214' error is outrageous. For a surveyed position the answer should be with +/- 10'.

Ok..

Michael Perrett

On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David < david.vanhorn@backcountryaccess.com> wrote:

time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

I just checked Google Earth and the elevation of your office is 5260', only
about 24' off of your GPS estimate if that is your location.
Michael

On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David <
david.vanhorn@backcountryaccess.com> wrote:

On Wed, Jun 8, 2016 at 4:33 PM, Van Horn, David <
david.vanhorn@backcountryaccess.com> wrote:

Google Earth has the elevation of this address as 5272' which is a good
deal closer to your reading...

--

--Jim Harman

Hi Gary,

Something is terribly wrong with your setup. The Garmin 18x is much, much better than this. I know because the 18x was one of the GPS receivers I brought along on a recent mobile clock experiment.

Here is a plot of 2 hours of GPGGA raw altitude data while stationary in Tucson:
http://leapsecond.com/great2016a/2016a-garmin-18x-2.gif

With clear sky view, the peak to peak is under +/- 8 m, and the (1-sigma) standard deviation is 3 m.
Even at the hotel lobby, with obstructed sky view, the (1-sigma) standard deviation stayed under 7 m.
Your 18x number, +/- 264ft (+/- 80 m), is 10x to 25x worse than this. It doesn't feel right.

Off-list, can you send me a day of NMEA from your 18x? Not gpsd output; but the raw serial ascii data from the receiver. I'd like to get to the bottom of this. We'll all learn something.

Thanks,
/tvb

----- Original Message -----
From: "Gary E. Miller" gem@rellim.com
To: "Van Horn, David" david.vanhorn@backcountryaccess.com
Cc: time-nuts@febo.com
Sent: Wednesday, June 08, 2016 3:24 PM
Subject: Re: [time-nuts] GPS altitude somewhat wrong?

Yo David!

On Wed, 8 Jun 2016 20:33:58 +0000
"Van Horn, David" david.vanhorn@backcountryaccess.com wrote:

Looking at my GPS farm, fair but not great skyview.

A Garmin 18x reports:
Altitude Err:    +/- 264 ft

A u-blox 8:
Altitude Err:    +/- 36 ft

And an adafruit HAT (u-blox 6?:
Altitude Err:    +/- 69 ft

On a Skytraq I have seen +/- 12 feet.

Depends.  :-)

RGDS
GARY

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem@rellim.com  Tel:+1 541 382 8588