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

[Ron Frazier (ALE)]

Hi Randy,

Thanks for the aviation info.  That sounds very cool.  I'm going to take 
a look at those sources and see how they compare to what I get from The 
Weather Channel, Weather Bug, and ForecastFox.

The best way to calibrate the elevation on the GPS is to put in a known 
elevation.  I found the elevation for my house and for one specific 
trail head from some USGS topographic maps.  If I'm starting from one of 
those locations, I'll use that elevation number.  Otherwise, I'll set 
the barometric pressure as you described and the elevation should be close.

Last night, when this storm system came in, the barometric pressure in 
Cumming dropped 2/10" and the elevation reading on the GPS changed over 
200 ft sitting in my house.  I was surprised that it was so much.

After a good bit of reading and experimenting, I've turned off the "auto 
calibration" function which calibrates the altimeter based on GPS.  The 
GPS elevation is just too unreliable.

PS - I was taking private pilot lessons about 20 years ago.  I almost 
got up to solo, but had to quit due to financial difficulties.  It's on 
my things to do again list.

Sincerely,

Ron


On 2/26/2013 11:12 AM, Randy Hagan wrote:
> You could try to use the Altimeter (Barometric pressure) setting for 
> the closest airport.  We use the pressure value on the altimeter to 
> get accurate altitude.  We adjust it as we fly do different areas to 
> ensure we are at the right altitude (Under 18,000 FT anyway).  You 
> could check the Cherokee and Gainesville Airport weather 
> observations.  They would probably be about the same.  If you know how 
> to read METARs you can get the barometric pressure.  If not, look for 
> the four digit number after the first A.  That tells you the altimeter 
> setting.  Example:  261455Z AUTO 10007KT 2 1/2SM RA BKN003 OVC007 
> 03/01 A2963 RMK AO2 TSNO.  The A2963 is the Altimeter (or barometric 
> pressure) setting.  IT's 29.63 in. 29.92 is standard pressure (which 
> doesn't happen much).  You can check the weather at airnav.com.  Just 
> click the airport tab, enter the 3 letter identifier (CNI is Cherokee, 
> GVL is Gainseville).  Scroll down and look over to the right side and 
> you will see METAR.  They take them every hour (The example above was 
> taken on the 26th at 1455Z - or 9:55AM Local).  I'm not sure how 
> accurate you GPS will be, but it should get you close.  I think for 
> every inch of mercury you are off, you increase/decrease altitude by 
> 1000 feet.
>
> Regards,
>
> Randy
>
>
>
> On 2013-02-25 20:07, Ron Frazier (ALE) wrote:
>> I don't think Cumming has an airport, but I'll keep it in mind.
>>
>> I took the GPS out with me to supper.  When I left, my house was at
>> 1210 feet above sea level.  By the time I got back, it had sunk (on
>> the display) to 1170 feet.  In the last 20 minutes of me sitting here,
>> it has raised back up to 1185 feet.
>>
>> So, I guess I cannot expect too much from the elevation function of 
>> this device.
>>
>> Sincerely,
>>
>> Ron
>>
>
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-- 

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Ron Frazier
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