[time-nuts] time-nuts Digest, Vol 130, Issue 27

Bob Camp kb8tq at n1k.org
Wed May 20 19:42:38 EDT 2015


Hi

One of our standing jokes when we lived by the shore was that we actually 
lived underwater. The GPS routinely put is 30 to 50’ below sea level ….

Bob

> On May 20, 2015, at 3:36 AM, Björn Gabrielsson <bg at lysator.liu.se> wrote:
> 
> Hi Bob,
> 
> You are confusing the readers here.
> 
>  1) zero altitude relative the WGS84/GPS ellipsoid are often tens of
> meters over or below sea level, depending on your location.
> 
>  2) zero altitude relative to a geoid (EGM96 or something else) is very
> close to sea level.
> 
> Then if you think of the ellipsoid as a second order model of the geoid...
> we can start comfusing people again.
> 
> --
> 
>    Björn
> 
>> Hi
>> 
>> The gotcha is that the GPS numbers are related to a geoid model and not to
>> sea level.
>> You can indeed find points that are “underwater” based on the geoid,
>> but quite dry in
>> real life (and vice-versa).
>> 
>> Bob
>>> On May 19, 2015, at 12:26 PM, Demian Martin <demianm.pds at gmail.com>
>>> wrote:
>>> 
>>> I would buy that (Google Maps being off) except that I'm less than 2
>>> miles from the SF bay and -5M would have me underwater. That may well
>>> happen but not for a few years at least. Also the Arbiter does match
>>> Google maps pretty closely. It doesn't really matter a lot, just a
>>> curiosity.
>>>    Demian
>>> 
>>>> Date: Mon, 18 May 2015 18:12:58 -0400
>>>> From: Bob Camp <kb8tq at n1k.org>
>>>> To: Discussion of precise time and frequency measurement
>>> 	<time-nuts at febo.com>
>>>> Subject: Re: [time-nuts] Thunderbolt Height Error
>>>> Message-ID: <1AE37B6B-35FD-4C5F-95D6-01316D6C4AE1 at n1k.org>
>>>> Content-Type: text/plain; charset=us-ascii
>>> 
>>>> Hi
>>> 
>>>> The simple answer is that Google maps may or may not be correct. There
>>>> are a lot
>>>> of examples of them being off by 10M or more. That said, my *guess*
>>>> would be that the
>>>> Thunderbolt is closer to the truth.
>>> 
>>>> Bob
>>> 
>>>> On May 18, 2015, at 2:34 AM, Demian Martin <demianm.pds at gmail.com>
>>>> wrote:
>>>> 
>>>> I have 2 GPSDO's. A Thunderbolt and an Arbiter 1083A. The Arbiter is
>>>> old but
>>>> it works fine (and has a Wenzel 5 MHz streamline oscillator in it). It
>>>> has
>>>> the 1995 firmware issue, and I could get new firmware for it ($$) but
>>>> I'm
>>>> not using it as a clock, just a frequency source.
>>>> 
>>>> 
>>>> 
>>>> I just moved and have re-setup both. They share an antenna. I got both
>>>> to do
>>>> a self survey. The Arbiter was really close to what Google maps
>>>> indicate is
>>>> my location. The Thunderbolt was about the same except it has me
>>>> underground. The arbiter has the height as +30M. The Thunderbolt as
>>>> -6M.
>>>> What setting do I have wrong in the Thunderbolt? Would it affect the
>>>> operation as a frequency standard in any way?
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> Demian Martin
>>>> 
>>>> San Leandro, CA 94577
>>> 
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> 
> 
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