[Loadstone] Altitude

Shawn Kirkpatrick shawn at loadstone-gps.com
Sun Mar 29 23:15:46 BST 2009


You could be right, I'll have a look at this. I'm not exactly sure what 
value the nmea sentence is reporting so there might have been some confusion 
here.

On Sun, 29 Mar 2009, Paul Shelton wrote:

> 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 NMEA
> 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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