Understanding my Casio barometric altimeter watch

[John Klinepeter]

There is no cute warm and fuzzy stuff here. Numbers and boring high school level analysis. Move on if you want to read about staying warm and dry in an EE quilt inside a Cuben shelter!

I tend to check all my equipment before taking it into the field. My new altimeter watch deserves the same attention. Besides, it's cold outside and I am looking for something to do.

In a recent article Ryan Jordan mentioned that he uses a Casio "ABC" watch. Altimeter/barometer/compass. I needed a watch and was interested in the altimeter feature so I purchased the less-expensive sibling of the aforementioned watch. Mine, the model PRG-270-1, does not have the automatic time referencing but all other features seem to be the same.

After becoming acquainted with the watch I set the altimeter to my home's known altitude of about 3150 feet. This number was determined from consulting a topographic map. 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? I would be tempted to deem the watch to be too finicky if I need to adjust it more than two or three times per day. Maybe my expectations are too high vs. the physical realities of the world?

There is nothing like a good set of measurements and a little studying to bring clarity. I set up a table to enter values of barometic pressure and altitude over time from several sources for comparison. Here are the results of my measurements:

Weather stations:
NOAA is located at the Glacier Park International Airport site which is about 7 miles from my house and sits at 2972 feet ASL. Close enough to be relevant? Their barometric readings are given in a table as related to sea level and as true pressures.
Wunderground is a private site whose data can be accessed from the Web. This site is about 1/2 mile from my house and sits at 3133 feet ASL, virtually the same elevation as my house. Its barometric readings are adjusted to sea level.

My equipment, all located indoor:
Skyscan Weather Station reads barometric pressure NOT adjusted to sea level.
Gischard hiker's altimeter, analog model, perhaps 35 years old. Barometric readings are NOT adjusted to sea level. The last few readings were made after tapping the case with my finger; prior readings may be off due to a sticking needle.
Casio watch barometer readings are NOT adjusted to sea level. The barometer can be adjusted but I left it at the factory setting.

Initial setup: I set the Casio and the Gischard to 3150 feet.

Initial fudge factor: I adjusted the Wunderground barometric pressures by subtracting 3.24 to simulate the actual station pressures (as opposed to the sea level pressures) by using the table below. This makes the values appear to be more of an "apples to apples" comparison for easier reading, i.e., there are no head-scratching 30.0 in. Hg vs. 26.8 in. Hg readings. The values in the table reflect this fudge factor. They did not turn out as close to the other column readings as I hoped; maybe there is a factor I do not understand. In any case I am looking for trends and not absolute figures.

One thing that stands out immediately by observing the table below is that it does not take much of a pressure change to cause a 100 foot change in apparent altitude. A change of 0.10 inch of mercury equals a 100 foot altitude change! *

Elevation vs. Atmospheric Pressure (stolen from the Web):

Elevation, ft.= Inches of Mercury (Hg)
Sea Level = 29.92
…..
…..
2500 = 27.31
2600 = 27.21
2700 = 27.11
2800 = 27.01
2900 = 26.91
3000 = 26.81
3100 = 26.71
3200 = 26.61
3300 = 26.52
3400 = 26.42
3500 = 26.32

Data vs. Expected Results: I am extremely pleased by how well the alitmeters responded to changes in pressure. For instance, look at the difference between the first and last NOAA pressures, 0.41 inches of Hg. If 0.10 inches of Hg equals a 100 foot altitude change then 0.41 should equal 410 feet. In fact it does if you look at the first and last values in the CASIO ALTITUDE column! The other pressure readings also do an admirable job of yielding about 400 feet.

Final Comments: To answer my questions above, yes, it is necessary to frequently check and adjust the watch altitude for best results. Yes, the watch is working as it should even though the altitude reading wanders dramatically while remaining at rest.

Oh, a bonus! I don't have to finger-tap the Casio to get the correct reading.

*The 0.1 inch of mercury per 100 feet altitude change does not hold throughout the entire spectrum of normal hiking elevations; more complete tables are easily found with a web search.

Edited to make table more legible. Thanks to Bob Gross for the idea.

[Ian]

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.

[Anonymous]

"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.

[Todd T]

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.

[Ralph Burgess]

"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

.
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.
.
Then google "International Standard Atmosphere", for example here
http://code7700.com/isa.html

.
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).
.
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.

[Bob Gross]

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.–

[Mobile Calculator]

…

[Anonymous]

"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.

[Ralph Burgess]

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

[Bob Moulder]

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.

[Ralph Burgess]

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.

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