Re: FAA regs & GPS barometers

Message posted by JB737 on May 27, 2007 at 18:19:06 PST:

You're probably right, except that if Groom were following FAA regulations, then I think they'd have more than 1 flight attendant on each packed JANET 737 flight.

It's 20 years since I was a pilot, so I am not sure if each VOR and/or airport gets its own weather forecast now, or if sometimes they just use a generic one for the area. The latter might make sense in a flat area with many airports.

My hiking GPSes are indeed both Garmin Vistas, with barometer. I generally disable it, though, partly to save battery, and partly due to the mysterious and apparently rather horrible algorithim they use in auto-calibration mode.

I suspect that their recalibration decision algorithm assumed lousy GPS altitude accuracy back in the old SA days. And even back then, auto-calibrate seemed to be far worse than turning auto-calibrate off.

I try to manually calibrate the barometric elevation to equal the known elevation. Failing that, I calibrate it to equal the GPS elevation, somewhere that I can reasonably expect to be able to check the true elevation of later. Often this means either on a large, flat area where I mark a GPS waypoint, or a peak or other marked point on a topo map.

It is best to do this at the beginning of a hike, and again at the same point, at the end. If done at intermediate points, I try naming my waypoint to remind me of what I did there, or actually make a note of it.

Unfortunately I did only 1 calibration on my Easter hike, and for the most part left the barometer off. Hence, I relied primarily on GPS elevations. The good news is that it was a reasonable match to my preprogrammed route waypoints, the names of which I always put the Google Earth or USGS elevation into (e.g., A5450, B5675, whatever). It seemed to chug along just fine, generating the elevation profile of my hike as I went, apparently using GPS elevation.

I do recommend hiking with no fewer than 2 GPS receivers. Before my Easter hike, one of my 2 Vistas was dead due to leaving dead batteries in it for 5 years unused. Its replacement didn't arrive until after the hike, so I carried my no-elevation-data Garmin i2 cheapie car nav as my backup.

It was pretty funny, actually. On the way back to the car, which I had marked as a destination waypoint on the i2, it would give an ETA based on some type of car speed, despite being offroad and actually moving under 1mph at times. Yeah, right, I'll be at the car in 5 minutes.

But it was extremely cool to program intermediate waypoints into it, zip it back into my little bellypack, and forget it. Then I'd navigate using the Vista. But when I'd get to a waypoint, the i2 would suddenly speak up "arriving at destination" followed by "on right", "on left", or silence (if less than 20ft or so off course). Sometimes it would keep me company by randomly spouting "recalculating...proceed 0.2 miles" or something like that.

They're all very light, and use only 2 AA batteries each, so I might indeed take all 3 next time. The spare Vista is solely in case I smash the first one during a fall or whatever, which I nearly did several times, due to having it in my hand almost continuously.

Next time, I'll put velcro or something its belt clip fits onto, on a few likely places to carry it more easily.


In Reply to: Re: The NWS approximation explains it posted by lone wolf on May 27, 2007 at 0:05:14 PST:


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