[aprssig] Barometric pressure compensation

Scott Miller scott at opentrac.org
Wed Oct 21 16:56:20 EDT 2009

Thanks Mark.

For now, I went ahead and made it an elevation entry, from which the 
unit itself calculates the offset from sea level - the reason for this 
being that it will eventually have a GPS input option, and will need to 
compute the offset automatically when it's deployed somewhere.

I got frustrated trying to implement the barometric pressure formula 
directly, so I played around in Excel for a while and came up with a set 
of linear models for each of several different altitude ranges.  If the 
R-squared values can be believed, it's pretty close, though I might 
eventually need to make smaller segments for higher elevations.

Anyway, it's now agreeing with the local airport weather station within 
0.01 inHg, so it seems to be working.

I should have the new voice samples for this thing soon, and I'm really 
looking forward to getting them loaded up.  Listening to my own voice 
droning out the weather readings every 20 seconds is getting old.


Mark Williams wrote:
> Hi Scott.
> Maybe it's overly simplistic but I wait for  the pressure gradients to 
> be minimal across the region and adjust the reading to agree with others 
> in the area.  That's essentially what Peet recommends in their setup 
> procedure. Actually, I added the pressure gradient part as my station is 
> remote and not near any NWS sources.  Let's call this the "secondary 
> standard" approach.
> Mark
> ----- Original Message ----- From: "Scott Miller" <scott at opentrac.org>
> To: "TAPR APRS Mailing List" <aprssig at tapr.org>
> Sent: Monday, October 19, 2009 2:38 PM
> Subject: [aprssig] Barometric pressure compensation
>> I'm making good progress on my APRS weather station kit, but I've run 
>> into a minor snag.
>> To be useful, the station barometric pressure reading has to be 
>> reduced to sea level.  This means making an estimate based on a 
>> fictitious layer of air between the station and sea level, and that 
>> estimate makes assumptions about the temperature and lapse rate in 
>> that layer.
>> The NWS has an official algorithm for this, but it'd be rather tricky 
>> to implement in a low-cost station.  The other amateur stations I can 
>> find information on (uWeather, Hobby Boards) seem to have simple 
>> offsets with no temperature compensation, which seems a little too 
>> simplistic.
>> Among professional stations like those by Davis and Peet Bros, is 
>> there any standard for how the sea level reduction is calculated?  Or 
>> do we just assume that we're not going to get any meaningful data 
>> about horizontal pressure gradients from home stations and only look 
>> at one station at a time?
>> Scott
>> N1VG
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