[ale] WAY [OT] but geeky - how do I calibrate GPS barometric altimeter

[Scott Plante]

Derek, 
I'm pretty sure Ron realized that--that's why he mentioned getting it from online sources for his locality. The problem was they all reported different values. 


Separately, 


A family member gave me a fancy weather station / atomic clock (well, it supposedly reads the atomic clock radio signal) with an outdoor remote unit this Christmas. It tells the indoor/outdoor temperature, pressure, and other stuff. Testing it out, I place the remote unit right next to the indoor unit and they consistently reported values 10 points off! It also never read the clock signal. Oh well, so much for technology. 

Scott 

----- Original Message -----

From: "Derek Atkins" <warlord at MIT.EDU> 
To: "Atlanta Linux Enthusiasts" <ale at ale.org> 
Sent: Tuesday, February 26, 2013 11:17:53 AM 
Subject: Re: [ale] WAY [OT] but geeky - how do I calibrate GPS barometric altimeter 

Ron, 

... 

The issue is that the barometric pressure is constantly changing. 
Pilots use barometric altimeters, and the rule is that they need to 
reset them every 100 miles or every hour. An airport will report the 
current altimeter setting every hour, but some automated systems will 
report it every minute, and yes, it can change rapidly if you have 
strong storm systems. 

Basically, this is a long-winded way of saying that you need to be 
constantly resetting altimeters. When they are set properly they are 
very accurate. However they need to be reset frequently to remain 
accurate. 

One tip: the barometric pressure doesn't usually change significantly 
over short distances. Most of the local airports will have the same 
setting, so you could theoretically just dial the weather at, say, 
Kennesaw (770-425-3406), to get a local altimeter setting that should be 
"close enough" for you. Worst case you'll be maybe +/-50 feet off, but 
is that really too much error for your use-case? 

-derek 

-- 
Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory 
Member, MIT Student Information Processing Board (SIPB) 
URL: http://web.mit.edu/warlord/ PP-ASEL-IA N1NWH 
warlord at MIT.EDU PGP key available 
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