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Discussion Starter · #1 ·
I recently picked up a Casio PRW1300 for myself, looking mainly at the compass (as a backup for any critical direction finding, and an easy reference elsewhere) and barometer with a view to predicting something of the weather.

The 'A' in 'ABC' obviously stands for altimeter, which I have been experimenting with, and so far have found rather unreliable and not particularly useful...
I under stand the concept of it, and can see the need to reset it as conditions change, but while it is all very well resetting it when I am at home (80m ASL) before heading out, during a days trip the weather will vary more than enough to have it completely unreliable by the time I get home again, and reading something completely different to when I set out - As I headed out this evening it was reading correctly at 80m, as I got home a few hours later it is reading at 65m... Admittedly this is during the evening as the sun has set, but during the day it can vary by just as much in temperature as the weather changes and the sun appears and disappears, which to me leaves the altimeter function varying in accuracy over one day and what could be one day hiking...

I figure this is just the case for barometer/temperature based altimeters, but is it as useless as it has shown over the last week? I seemed to work pretty well short term, and tracked my altitude journeying across town (referenced against OS maps and google earth), the accuracy a 4-5 hours later leaved something to be desired.

Any other opinions from those more experienced than myself? Is there a particular use it tends to work well towards that I am missing, or has my weather just been exceptionally variable as of late and it will normally settle down more?
 

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I use it to evaluate relative change in altitude. Say you head out in the morning on a mountain hike and the altimeter says 300 feet and an hour later you read 700 feet, you know with considerable accuracy you've climbed 400 feet. To me, it is unimportant what the absolute start and finish readings are, and I almost never calibrate it against a known altitude. It's the difference between start and finish that matters to me. Sure, rapid atmospheric pressure changes throw the altimeter off, but over short time periods, it's not enough to matter.

An opposite application I like is to use the altimeter as a proxy for barometric pressure changes when I'm stationary. My Casio PAG-80 reads out in increments of 20 feet of altitude, which translates to .02 inch Hg (up to about 4000 feet above sea level). If my "altitude" starts rising while I'm sitting still, I know the pressure is dropping and weather change may be imminent. The barometer stays in that mode for only a couple minutes, so I prefer to use the altimeter to monitor pressure change instead, since it holds the altimeter mode for many hours with one button press (and also shows the time).

I'm going to use the watch this weekend as Hurricane Irene approaches the Washington, DC area. I think it will be VERY interesting. Whether I'm watching the barometer reading drop or the altimeter reading rise, I can see the changes as the storm approaches.

So, yes, I think the alti/baro function is quite useful. I just don't worry about the absolute readings, and use the device as a measuring tool for changes.

Roy
 

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Few points:

1. This is a normal way altimeters work. You find the exact same kind of altimeters in aircrafts and they really need to specify the air pressure at sea level to show accurate reading. And the pressure needs to be set periodically because pressure varies over time as you have noticed. If the pressure changes 10 hPa, the shown altitude changes about 85 meters. Use Google to convert units if needed. So you should either listen to weather information or estimate if the pressure changes.

And also think about when to use this feature. You don't need to know whether your home is at 80m or at 65m from sea level. You use altimeter when hiking or climbing a mountain. You need to know if you are 500 meters above the last spot. You don't care if the actual difference is 490 or 510 meters.

2. It really doesn't matter if you want to see the absolute altitude or just difference: you still get wrong results when air pressure at seal level changes. But the greater the actual altitude difference is, more likely it will outweight the possible air pressure difference at sea level. Ie. if you try to measure differences of 50 meters you need to be careful to calibrate for possible air pressure changes. But if you are climbing the Mount Everest, you will get a good ballpark even without calibration.
 

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Any other opinions from those more experienced than myself? Is there a particular use it tends to work well towards that I am missing, or has my weather just been exceptionally variable as of late and it will normally settle down more?
I believe such 'loss of correlation' in devices that measure one thing as a proxy for two different things is always a possibility :-(

The way I implemented it in this ezChronos firmware family was to add the idea of 'altimeter lock' - when you are at a rather fixed altitude you just 'lock' it; as long as you do not forget to unlock before moving to a different one you should be reasonably accurate!
 

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If you don't have a watch with an altimeter lock, can't you just remember your altitude when you stopped moving, then reset your altitude to the same value before heading out again? Not overly difficult.

Alti lock or not, you still have to remember to do something:
--turn the alti lock on/off or
--reset the altitude of the watch without alti lock to the previous value, if it changed

I don't have alti lock and don't think I need it.

Let the alti lock arguments begin! ;-)

Roy
 

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I've heard a lot of claims for alti-lock feature and how it's important etc... should I throw my Riseman to the trash because it doesn't have the alti-lock ?
 

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Discussion Starter · #7 ·
I guess and altimeter lock function is a nice simple one button operation - press to lock at a set altitude, press to unlock it again (resetting it against whatever variable has changed)

Resetting the barometer itself just means making a note of the final value when you stop or comparing against a known value - maps, etc... ad has a tiny amount of extra faff resetting it to a particular value.

As said though, not really a huge problem for most not having the alti-lock, just a handy feature for someone that uses the altimeter a lot.

I will admit to having bought the watch more for the other features, and did pretty much understand the limitations of the altimeter, just interested in double checking really - fine for climbs where any error is within the margin of the climb (10m error in a 50m variable is useless, 10m in 500m not so much), but not really practical as a long running 'what altitude am I at today' reference.
 

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RoyR: Unless you're climbing mountains all day, there shouldn't be too much variation, so that's what I'd do too.

I've been thinking about this a lot recently, and the only way to calibrate the Protrek/Pathfinders is via known altitude calibration, which although might be the most simple way of doing things when hiking up a mountain path (uh, path-finding), it's not useful when ascending/descending mountains without convenient signs to mark altitude (path-making?), especially when spending a day or more at altitude.

As 'Paapaa' mentioned earlier in the thread, aviation altimeters are calibrated differently; instead of calibrating it to a known altitude, they're based on QNH, or 'sea-level barometric pressure'. If you look at the photo of an altimeter from an old F4 Phantom:



You can see the `MB' (millibars aka hPa) window in the bottom right set to QNH. Using this type of altimeter, you should be able to accurately determine altitude knowing only the current QNH for your region.

In regards to `hacking' current Protrek/Pathfinders for accuracy, you could write a (PC, iPhone, Android) program that records local QNH when you calibrate your Protrek (via known altitude), then later in the day, you input your Protrek's Indicated Altitude where it would then calculate the apparent indicated altitude difference between original and current QNH, giving you the correct altitude to adjust to. The downside is that this would require some sort of communications to determine QNH, but METAR is very common in weather reporting and includes QNH.

Catalin: I've thought about programming a BASIC stamp and creating an altimeter, but this `eZ430-Chronos' device looks epic! A programmable watch by Texas Instruments is enough, but with a pressure sensor+thermometer+accelerometer+wireless interface, and retailing for < US$100? What's the catch?
I can already see myself coding an altimeter into this thing; the top line (7-char array) for altitude, the bottom line for QNH (or time). Hold one of the left side buttons to set, and up/down buttons on the right to adjust QNH. Press one of the left-side buttons to toggle QNH/time. What do you think?
 

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I find the barometer actually really useful, and a steady significant drop usually means a storm in most areas of the world I've used it in. Altimeter varies a few hundred feet with variation in air pressure, I've found, but is useful in general and gives me a pretty good idea of altitude if I'm traveling or climbing mountains. In the past, I've used altimeters to find where I was on a river, as rivers flow downhill (!) and every part of a river is at a different altitude - this was useful in the pre-GPS days, now not so much, but I've used my Pathfinder for the same thing - it gets you in the general vicinity, at least, on an average river - probably not too useful in the Amazon, for instance, where it doesn't lose much altitude over great distances.

Mostly, I use relative values - change over time - rather than absolute when dealing with these things -
 

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Catalin: I've thought about programming a BASIC stamp and creating an altimeter, but this `eZ430-Chronos' device looks epic! A programmable watch by Texas Instruments is enough, but with a pressure sensor+thermometer+accelerometer+wireless interface, and retailing for < US$100? What's the catch?
I can already see myself coding an altimeter into this thing; the top line (7-char array) for altitude, the bottom line for QNH (or time). Hold one of the left side buttons to set, and up/down buttons on the right to adjust QNH. Press one of the left-side buttons to toggle QNH/time. What do you think?
There is currently a discount on those and the price is like 25$

Different existing firmware versions might already have most of the features that you are interested in.
 

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The weakness of the Casio altimeters is mainly the missing MSLP (mean sea level pressure). My Suunto X-Lander can either be calibrated using the altitude (if you know your altitude) - it then calculates the MSLP - or you can calibrate it if you know the current MSLP - it then calculates your current altitude - the displayed absolute pressure is completely unaffected by this. A Casio alti/baro watch only displays absolute pressure - so you can't calibrate the altimeter with MSLP. On a Suunto (or any other ABC with MSLP) you need either your altitude or MSLP - on a Casio you need a topo-map or something similar to calibrate the altimeter. But on the X-Lander you'd still have the same changes in altitude when the pressure changes - to partly eliminate that you'd need a watch with alti-lock which determines by the speed of the pressure change if the watch is changing altitude or the weather is changing - this can be faulty of course but it increases the accuracy of the displayed altitude value - and some also have a manual alti-lock which you can engage when you rest for the night for instance - the watch then interprets all pressure changes as barometric and not altitude related.

cheers, Sedi :)
 

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MSLP is definately a convenient way of calibrating the baro/altimeter. Though, it do requires that you are close to the measure point and that there's access to the information. I mainly uses altimeter when I'm on vacation, and here I deliberately choose to not use TV and internet, to me that's part of the vacation.

There's a third way in watches; Some models (not Casio ;-)) have an automatic altimeter lock; It works by constantly taking pressure measurings often - then it's possible to look at the speed of the changes. A fast change is considered as an altitude change, and a slow change is considered as a weather change.

There's a couple of downsides to this: Fast weather changes (=extreme weather, storm/snowstorm/hurricanes/thunderstorms etc) will be considered as, drum solo, an altitude change. But often we know this kind of weather is on it's way, and the alti meter trick works here too ;-) Driving a car can confuse the algorithms. And it will use more battery - I believe this is the reason for Casio not implementing an auto alti lock.

Overall, I find this works very great, much more precise than the Casio way.

That said, I took my Casio 240 with me on a 14 day vacation, where we had 3 bases/stays in a row. When we arrived at one base, I reset the altitude difference meter. Next morning, the difference showed how much drift there was that night. If drift occured, I calibrated the altimeter. Etc. Etc. After those 14 days, my altimeter was less than 10 meters off, when I arrived at home. A key element here, is that I was lucky, and there wasnt much weather changes, the days I was on the move from home to base 1 -> base 2 -> base 3 -> home.

However, an auto altimeter lock is much less hazzle ;-)

And this auto alti lock, also fixes what annoys me the most with Casio AB(C)s: The barometer. Change altitude, and the Casio baro graph will be off.

Practicallity? Well, for me the auto alti lock thingy works great. Both for alti and baro. But it comes at the cost of more battery usage. Casio uses less battery, and is more straigth forward. But requires more user intervention. And the baro graph limitations is next to impossible to work around. Also, local weather probably plays a role here (Tissot T-Touch Expert got a hemisphere setting, but I havent tried the effects of it). But I guess that with some kinds of local weather, one watch works better, and and with other kinds of weather, it's another watch which works better.

But at the end of the day... with any of these watches, if you change altitude and weather related pressure changes simoultanous, both barometer and altimeter will be screwed...
 
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