[Loadstone] Altitude

Paul Shelton pbs at paulshelton.com
Mon Mar 30 00:01:13 BST 2009


Altitude does seem to vary quite a bit but I don't think the problem I am 
reporting will change this if it is corrected.  The variation will still 
occur but the "window" will be shifted by the offset for your location.
----- Original Message ----- 
From: "David W Wood" <g3yxx at guildford-physio.demon.co.uk>
To: <loadstone at loadstone-gps.com>
Sent: Sunday, March 29, 2009 11:54 AM
Subject: Re: [Loadstone] Altitude

> Paul:
> Very interesting.
> I posted to the list some while ago that the altitude function seemed to 
> be
> at error as my house which is 161 yards from the steps to the foreshore in
> Hythe Kent, is apparently anything up to 150 feet below sea level, and 
> this
> figure can vary quite rapidly.
> Assuming that your theory is correct, then these variations would be 
> double
> the figure which would be expected from satellite drift.
> David W Wood
> T: 01483 575876 (APPOINTMENTS)
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> E: reception at guildfordphysio.net
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> -----Original Message-----
> From: loadstone-bounces at loadstone-gps.com
> [mailto:loadstone-bounces at loadstone-gps.com] On Behalf Of Paul Shelton
> Sent: Sunday, March 29, 2009 4:46 PM
> To: loadstone at loadstone-gps.com
> Subject: [Loadstone] Altitude
> I've been looking at the altitude reading reported by Loadstone vs. the
> value in the NMEA sentence and I believe there is a problem.  I've done 
> some
> research on this and as I understand it, the value reported in NMEA, at
> least for most modern receivers, is the MSL value and that the geoidal
> (WGS84) value can be calculated by adding the correction in the NMEA
> sentence to the MSL value.
> It appears to me that instead of taking the MSL value and adding the 
> offset
> to arrive at the geoidal value, that perhaps the reported value in the 
> is being used as the geoidal value and that the offset is being subtracted
> from this number to arrive at the MSL value.  I'm just speculating about 
> the
> subtraction because the offset at my location is -27 meters and I show a
> higher value for MSL than I do for geoidal.
> The following is taken from a log file:
> $GPGGA,151117.000,3601.0738,N,09554.3711,W,2,10,1.00,230.7,M,-27.0,M,000
> This shows that my MSL altitude is 230.7 meters and I have Loadstone set
> for MSL and I'm using imperial units.  If I convert 230.7 meters to feet, 
> I
> get 756.9 feet.  At the same time that this log file was being captured,
> Loadstone was reporting an MSL of 849 feet.  This is approximately what 
> you
> would get if you subtracted -27 from 230.7 meters.  Actually, the
> calculation seems a bit off but it's in the ballpark.
> So to recap, I think the value from the NMEA sentence should be reported 
> in
> Loadstone as the MSL altitude without any calculations (other than unit
> conversions if necessary and that the offset value should be added to the
> MSL value to arrive at the geoidal value.
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