[Loadstone] Altitude

David W Wood g3yxx at guildford-physio.demon.co.uk
Sun Mar 29 17:54:07 BST 2009


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

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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 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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