[Loadstone] Altitude

Paul Shelton pbs at paulshelton.com
Mon Mar 30 12:48:45 BST 2009


Thanks Shawn.


----- Original Message ----- 
From: "Shawn Kirkpatrick" <shawn at loadstone-gps.com>
To: <loadstone at loadstone-gps.com>
Sent: Sunday, March 29, 2009 6:28 PM
Subject: Re: [Loadstone] Altitude


> 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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