# How are altimeter settings estimated?

So, I spotted another weird METAR from the same place that spawned the first one I asked about (it's a USAF base if you're wondering what kind of 'drome it is):

K... 060846Z 00000KT 10SM OVC002 18/18 A2995 RMK AO2A SLPNO ALSTG ESTMD $While I can reckon that the barometric pressure sensor there's on the fritz (again) from the SLPNO and the$, they also have the "ALSTG ESTMD" remark on there. While the estimate looks to be not too far out, or at least more reasonable than the completely bogus figure from last time, it raises a question: how are they estimating an altimeter setting in the absence of the AWOS's barometer? Or does it just mean that their backup barometer isn't as good as the one on the AWOS, so the altimeter setting might be a few ULPs out?

• One of the pre-flight checks is to make sure that your altimeter reads the same as field elevation. If you don’t know the altimeter setting, set your altimeter to field elevation and you get the altimeter setting. – JScarry Oct 6 '17 at 23:48
• @JScarry -- agreed, but I'm wondering how the folks who aren't sitting in front of an altimeter are doing it :) – UnrecognizedFallingObject Oct 6 '17 at 23:52
• The instrument they are using is not certified, hence the ESTMD. When they get a certified one, the ESTMD will drop off the METAR. – mongo Oct 7 '17 at 12:29
• @mongo -- make that an answer and I'll give you an upvote :) – UnrecognizedFallingObject Oct 7 '17 at 15:08
• @UnrecognizedFallingObject a better title might be, why are altimeter settings estimated. The how is rather simple, they just use another instrument. BTW, this is something that AFB tend to run into more than FAA facilities, from my experience. Perhaps the red tape has a different hue. – mongo Oct 7 '17 at 19:58