pbs at paulshelton.com
Sun Mar 29 16:45:48 BST 2009
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:
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.
More information about the Loadstone