[pct-l] Altimeters

["David B. Stockton" <davstock@tiac.net>]

Unfortunately, Rich Calliger's statement shows misunderstanding about how 
altimeters work, whether electronic or mechanical:

>>
These watches try to do to much. Another example, altitude determination 
requires thermal compensation of the mechanism..this is "easy" in an 
hand-held unit..there is a bimetallic strip.... but electronically it is a 
lot more complex and hence prone to larger errors and requires daily (if 
not 2-3 x) recalibration for reliable accuracy. (Read the fine print of the 
manual!!)
>>

(1) All altimeters require constant recalibration because they base their 
altitude estimation on ambient air pressure (that is, the barometric 
pressure).  Even if you do not move vertically, your altitude will appear 
to change because the ambient air pressure is changing.  If a storm is 
coming through, your apparent altitude may move even faster.  Since the 
barometric pressure is constantly changing, the altimeter's estimation of 
your altitude is constantly drifting.  It is the altimeter user's 
responsibility to keep ahead of the barometer by resetting the altimeter as 
often as he/she knows his/her altitude fairly precisely based on other data 
(triangulation, landmarks, benchmarks).  This could be MANY times in a day 
and has nothing to do with the altimeter's accuracy.

(2) A device's (or algorithm's, or microprocessor program's) accuracy is 
not necessarily a function of its complexity, and has almost nothing to do 
with its "electronic" complexity.  A simple, but poorly implemented device 
could be much less accurate than a complex device.

For example:  I write "1000 feet" on a piece of paper and carry it around 
as my altimeter.  Quite simple, but accurate only once in a while (and 
when?).

Second example: I store "1000 feet" in a file in my computer and carry IT 
around as my altimeter.  Very complex electronically, but has nothing to do 
with the program (just a file access) that "calculates" the altitude 
(inadequately simple).

(3) As partly stated in (1), the constant recalibration required by a 
mountaineer has nothing to do with temperature compensation of an 
altimeter.  In reality, the advent of temperature compensation for 
altimeters vastly improved their accuracy, especially in airplanes.  The 
actual amount of active compensation on a wristwatch altimeter is likely to 
be minimal because one's wrist's microclimate when hiking is likely to be 
very stable (say, within fifteen degrees F) for days, whereas the altimeter 
in the jetliner that can land on a runway under computer control may suffer 
temperature changes of over a hundred-fifty degrees F in, say, half-an-hour 
(-40 to 110 is easy to imagine).

(4) There are actually two types of calibration supported by the Suunto, 
one of the kind discussed in (1) and another that helps recalibrate the 
temperature compensation, which would only be done occasionally (I imagine 
1x/month would be too often) at a well-calibrated site such as an airport 
or USGS benchmark.  Since author did not specify which aspect he was 
hammering, we cannot be sure that he was referring to the proper section of 
the manual when he made his comments.

>>
I rate myself an expert on these watches now, as well as being a graduate 
practicing engineer I loath the day I got suckered" into laying down my 
hard-earned...
>>

The author may have a lot of experience returning the devices, but 
"expertise" would involve conducting carefully controlled tests in a 
chamber where one could control temperature and pressure and collect data 
over time, then analyze that data.  Perhaps "partial expert" would be a 
little bit more accurate, but in practice it's the most rewarding to let 
others call one the expert.  One rewarding aspect of engineering is to get 
a new device, put it through its paces, and see how it fares.  The author 
did that, found them wanting, and got his money back too.  I'm not sure 
where the "suckered" part comes in.

I have a Suunto, but don't have enough experience with it yet to come to 
any conclusions except to say that, in my opinion, the user interface is a 
little bit complex for easy field operation.  I will take me a while longer 
to absorb the button-pushing logic to feel comfortable with the entire 
range of functions that it provides.  The manual is easy to read and 
follow, though, and there is a one-page summary that I expect to carry with 
me this summer.  The display is large and easy to read.  The one-button 
compass fix is super.  It is a bit of a power hog: about 18 months per 
battery (but you can change it yourself), depending on how often you use 
the compass and altimeter.  I carried mine on an 8-day trip in the 
Adirondacks with no problems in physical durability.

If you buy an Avocet, don't use it to move furniture -- I did a few years 
ago and tore up the way-too-soft buttons.  The Suunto seems to be much more 
rugged.

-- Dave

David B. Stockton
davstock@tiac.net


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