[Loadstone] Altitude

Paul Shelton pbs at paulshelton.com
Sun Mar 29 23:56:07 BST 2009


Well it's possible that when this code was first put in it might have been 
correct.  I understand that earlier versions of the SIRF chipset, i.e. 
before III did report the geoidal altitude in the NMEA sentence.  The 
standard is apparently to report the MSL value in the sentence and all newer 
receivers including SIRF III do this.  This is all based on internet 
research but I've seen it corroborated on several sites so I'm inclined to 
believe it.

Paul

----- Original Message ----- 
From: "Shawn Kirkpatrick" <shawn at loadstone-gps.com>
To: <loadstone at loadstone-gps.com>
Sent: Sunday, March 29, 2009 5:15 PM
Subject: Re: [Loadstone] Altitude


> 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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