ABC – ups and downs

It’s a funny old business – height and the measurement thereof.   I mention this in passing only as the ABC model watches that are around at the moment all feature an Altimeter.   Though in actual fact they are to all intents and purposes Barometers which are used to indicate height.   The question of course has to be – are they any good?  Are they as good for example as my car Garmin GPS?   Which begs another question – is the car GPS any good ?

Well I played around with both items this weekend and found some odd little quirks.

First I decided to calibrate my Casio ABC watch here at home.  I duly checked the Barometer which was 995mb this morning as indicated by the Casio.   I then had to find out the height above sea level of my house and I checked Google maps and found it was 116m, so I used this as my datum point.   I then duly set the watch to read 116m at 995mb – so far so good.

Casio showing 995mb Barometric pressure.  Note the graph showed better weather yesterday but over the last 20hrs or so no change history.  Also note the trend icon @3 which also shows no change.

Casio showing 995mb Barometric pressure. Note the graph showed better weather yesterday but over the last 20hrs or so no change history. Also note the trend icon @3 which also shows no change.  Each dot indicates 2hr intervals and 1mb up/down values

But when I checked my car GPS it told me the altitude here at my house was 108m – now that’s 8m difference or around 26ft and yet just 35m down the road (not up, you notice) it read 114m.

It seems that GPS altitude is maybe not as accurate as I thought for all sorts of reasons and boy are they complicated!   Now I’ll not go in to Ellipsoid Earth calculations or altitude measurements that actually indicate height above WGS84 as opposed to barometric above sea level indications, or quality vertical measurement, because frankly I don’t understand it all either!  BUT suffice to say that in general a GPS car unit requires at least 3 preferably 4 satellites and a clear sky above to get a meaningful altitude reading and even more preferably including connection to a satellite that’s “under” you – which you won’t get as the earth is blocking it from you.

Anyway playing around further I drove a route that swung by a particular crossroads and it was interesting to check the reading at the crossroads, but driving in from two different directions.   On the first run the crossroads altitude was indicated as 70m by the car GPS, but on the second run when I approached it from another road it now indicated 96m – quite a discrepancy.  Since reading up a bit I now know that the GPS was not managing to get the required signal reception, as there was a little tree cover on the second road.   Hence the difference.

In contrast my Casio ABC watch on both runs indicated the crossing was 72m above sea level.   My Casio also indicated the barometric pressure was now 998mb, so both figures had changed from my house location.  This was fine as the crossroads are definitely lower than my home, so as expected, lower altitude meant higher pressure and vice versa.   The weather hadn’t changed in the 10 minute drive.   Indeed when I returned to my home location the pressure was 995mb again and the altitude was as before 116m.

Note – In town today I stopped at local supermarket and both car GPS and Casio indicated 28m.  It was wall to wall clear sky so the GPS managed it’s maximum efficiency and nice to see they both agreed with one another.

But practically the Casio barometric Altitude versus the car GPS altitude is the clear winner, which I have to say did surprise and please me no end!    I also note that the better hand held GPS units also use Barometric Altimeters, so GPS direct read is not the favored system and remember the aviation industry still use pressure Atmospheric/Barometric systems and for quite a few good reasons.

The fascination of the Barometric system of course (as far as Altitude is concerned) is that also changes with the weather.  For example had I gone off in the car and a nasty weather front blown in, the atmospheric pressure will have dropped.   When I got to those crossroads my Casio Altitude reading could easily be incorrect by 40m and indicating an Altitude of something like 115m or higher!   So basically as long as the barometric (atmospheric) pressure remains fairly constant, then your Altitude readings will be reasonably accurate.   If not then you really need a decent map with height indications on it, so you can recalibrate your ABC watch with the correct height at that pressure.   You see?

There are lots of things you can do with the ABC watch if you are a hiker or climber of course, with recording of ascent/descent altitudes and so on and it can get pretty involved, but always remember weather change means pressure change so calibration is the key – so if doing these pursuits my advice for what it’s worth – TAKE A MAP (with co-ordinates/height indications and all the rest of the data).

As far as the Casio PRW3000T is concerned it is proving to be quite brilliant, Altimeter, Compass and Barometric Pressure being uncommonly accurate.   It is VERY clear to read (makes Suunto, Garmin and some others look poor in comparison – the display is streets ahead of them all, it’s light to wear, not too big at all and I’ve yet to take it off.   However I do at night so it can receive it’s Radio signal (though I forgot one night and it received without any problem at all).   Power save is interesting when it’s dark – after a time the display goes off and even when receiving a signal the dial is blank.   Any light at all comes near and it’s instantly on again – fascinating!

So my older compass watches are relegated to the drawer, though the odd one is a reminder of technological progress so will be in my display cabinet – the rest will be on their way to auction no doubt.

Ah the ups and downs of the watch collector!  😉

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