Certainly should be do-able and I’m almost surprised it’s not out there already.? ?The Garmin stand-alone GPS systems have had this since at least 2011 when the Etrex-30 came out.? Maybe before, but that’s where I entered the GPS world.? Since pressure also varies with temperature (and thus inversions), weather, and large-scale air motion, the usual approach is to slowly reset to the GPS-based elevation when the no motion is detected for a long time.? That approach essentially lets the GPS re-calibrate barometer’s pressure-to-elevation curve.? I don’t know, but I’d guess there’s some sort of Kalman filter or other simple Bayesian model with a long time constant buried in those Garmin boxes.? ?

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Those of us who are sufficiently old to have had simple digital altimeters -- or aneroid ones for those of us even more sufficiently old! -- remember recalibrating them regularly.? I recall researching at the time why I had to do that.? I noted some of reasons above, but at least as a decade or so ago, the simple altimeters marketed to hikers were worse than that.? Most appeared to use simple exponential models of the atmosphere based the weight of the stack of air above in a one-dimensional column, assuming air was an incompressible fluid.? Those models didn’t account for adiabatic heating/cooling.? That approach works for small variations down near sea level but fails spectacularly in high mountains.? At least that’s what my decade old review suggested to me.? Clearly that problem can be worked.? There are equations out there that include temperature lapse rates.? But GPS or manual resets work too. ?Just don’t trust the first pressure-to-altitude equation you find on Wikipedia.? ????

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As an aside, I remember sitting in Wheelbarrow Camp, the big camp area a few miles north of Forester Pass on the JMT near treeline, several years back when all I had was a digital altimeter.? I watched the elevation rise 1000 feet and drop back in course of a few minutes.? I thought my expensive altimeter was failing but all was fine thereafter.? All I can figure is that a big slug of air slid down the Bubbs Creek drainage, leaving some lower-than-normal pressure behind.? Then all re-equilibrated.? The atmosphere is a dynamic thing and sometimes all you can do is just sit back a watch what it’s doing with joyful wonder.?

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And, as a second aside, here’s a calibration data point to play with.? When I was up on Whitney in 2009 – last time I opted to join the big parade up -- my digital altimeter/barometer recorded 602 mb.? ???

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