Understanding my Casio barometric altimeter watch

Something the instructions mentions for my Casio ABC is that it's not for applications which require a high level of precision. Since it's a barometric altimeter, it will be thrown off as the barometric pressure changes throughout the day. Best practice with an ABC watch is to recalibrate it every time you hit a known point. On the Wonderland trail which has lots of elevation gains and losses, I would do this a few times per day. Sometimes it was off by a couple hundred feet after a really long climb. Sometimes it was spot on. I still find it to be a valuable tool.

"Best practice with an ABC watch is to recalibrate it every time you hit a known point. On the Wonderland trail which has lots of elevation gains and losses, I would do this a few times per day. Sometimes it was off by a couple hundred feet after a really long climb. Sometimes it was spot on. I still find it to be a valuable tool."

This mirrors my experience, as well. Disregarding the constant recalibrating at every known point, which is irritating but tolerable, the main drawback in my limited experience is what happens if the distance between known elevations is long, both in time and distance. I had this exact situation in October on the Baxter Pass trail, and it caused me to miss my intended campsite. I found out when I hit the pass that I was off by 400', enough to cause me to blow right by the campsite area. The problem had been that there were no known elevations between the last stream crossing and the pass, at least not that I could discern with any precision.

All a barometric watch can sense is pressure. Any changes in the pressure have to be attributed to either changes in elevation or changes in the weather. You tell it what to assume by choosing the operational "mode" (maybe an oversimplified explanation, especially if you have a Suunto with the auto mode that tries to intuit whether you're moving or not to determine how to log the changes–I love mine). If you're climbing or descending, all changes will be attributed to elevation, even if a front is moving in. If my altimeter watch is right within a couple hundred feet after an ascent or descent, I'm happy.

"From very casual observations at home, not in motion while hiking, I noted that the altitude would vary significantly, as much as +/-250 feet per day. Wow, that seems like a lot! Does that mean I will have to set the altitude, referencing a known elevation, five or six times per day while hiking to achieve some reasonable expectation of being able to trust it to read true to +/- 100 feet which is what I was hoping to see? Is the altitude merely reflecting changes in barometric pressure or is the watch somehow defective?"

To get a feel for what to expect, google "Surface Pressure Chart".
e.g. here's Europe from the UK Met Office

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You'll need to wean yourself off "inches of mercury", weather charts worldwide (and anyone in aviation outside the U.S.) use millibars (hPa), sea level 29.92 in Hg = 1013 hPa
You can see that the typical range from cyclone to anticyclone is 980-1045, i.e. about 7%.
So the order of magnitude of ambient pressure variation that you'll see day to day is a few percent.
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Then google "International Standard Atmosphere", for example here
http://code7700.com/isa.html

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If you look at the "pressure ratio" column, you'll see in percentage terms how pressure drops with height. At practical hiking elevations, pressure drops by about 3% per 1000 feet. It's not linear of course – the pressure is determined by the weight of the "column of air" above you, so pressure roughly halves every 17,000 feet (temperature variation makes it a little more complex).
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When your barometric instruments (and the NOAA) convert between measured pressure and implied sea-level pressure they assume ISA conditions. Your "altimeter" watch is, of course, really a barometer; instead of displaying the pressure, it arithmetically converts it to an implied altitude – again, based on assumed ISA conditions. This has a subtle implication that even if you set your watch correctly, and it operates flawlessly, and even if ambient conditions do not change – it will read slightly off as you hike up the mountain if conditions are substantially different from ISA. That's a little arcane, and not something to worry about generally, but worth remembering if you start doing more accurate experiments.

All I know is that when I am standing at one place, and then the apparent barometic altitude appears to have me elevating by 500 feet, it is time to tighten up the guylines on the tent.

–B.G.–

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"If you look at the "pressure ratio" column, you'll see in percentage terms how pressure drops with height. At practical hiking elevations, pressure drops by about 3% per 1000 feet. It's not linear of course – the pressure is determined by the weight of the "column of air" above you, so pressure roughly halves every 17,000 feet (temperature variation makes it a little more complex)."

The Casio manual also makes reference to factoring ISA temperature into its calculation of elevation determination, which introduces another highly variable factor into the equation. Like pressure, it varies all over the place, often considerably from that specified in the ISA chart.

Roger, to save energy, it makes sense to hike uphill when the tide's coming in (sea level rising), and hike downhill when the tide's going out (sea level dropping).

More seriously, you may wish to consult this
http://en.wikipedia.org/wiki/Geodetic_datum

John, is it important to you to have a reasonably accurate altitude with the least amount of fiddling on your part?

If so, the answer is to get a GPS unit with barometric altimeter and GPS-based auto-calibration, such as the Garmin eTrex 30. It operates mainly in barometric mode but recalibrates itself automatically when it calculates a very strong and reliable GPS-altimeter reading that is at a certain variance from the barometric reading, at which point it recalibrates to the GPS-derived altitude and immediately resumes barometric mode.

I used one of the first Casio Pathfinders and a Suunto Vector back in the mid- to late-1990's and they were impressively accurate when the atmospheric conditions were stable, but it was quite annoying to have to constantly recalibrate at known elevations in changing weather conditions.

It is an interesting and educational exercise to mess around with the charts and the math and the topos, but in the field with super-nasty weather conditions when you just need to know the correct freaking elevation – right now! – a zero-fuss system is the way to go.

Bob, that's interesting – I'm not really much of a GPS user, and I didn't know such a thing existed, but it makes perfect sense.

But, in practice – I use a barometric altimeter watch all the time, and it's extremely useful without GPS referencing. Variation with ambient conditions or deviation from ISA is just not a significant issue – as a navigation tool, I'm really not going to care about accuracy +/- 100 feet. For me, it tends to be a "gross error check" device – am I on the correct col, am I traversing at about the right contour to come out above/below known cliffs? Setting it once or twice a day is quite sufficient. And in any case, when it's use becomes most critical, I will automatically be reminded to reset it as I check my elevation at known points during the hike.