[Loadstone] Altitude

Shawn Kirkpatrick shawn at loadstone-gps.com
Mon Mar 30 00:28:24 BST 2009


Oh, that makes things fun. The code was originally put in when I had a sirf 
ii receiver so it looked right to me. I've changed it so it'll be correct 
now. I guess people with older receivers will just have to play with the 
settings until it works.

On Sun, 29 Mar 2009, Paul Shelton wrote:

> 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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>>>
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