djlewis Posted October 28, 2003 Share Posted October 28, 2003 I'm moving this to a separate thread, since I took it pretty far off topic -- my apologies for that.-----------------------------------------Dan Lacey wrote:> I recently got a Timex Helix from Campmor. The battery quit >in 2 months,the altimeter is reliable within a couple of >hundred feet,if you can ever get it to recalibrate! The >temperature function is useless as long as you wear it. But >it did keep accurate time(till it quit) OK, thanks for the warning on this item.>I carry a carabiner watch on my PFD now,and a small clip on >compass with thermometer. Gee,I really haven't missed the >altimeter while kayaking! As I understand things, an altimeter is, perforce, a barometer. It depends on which variable you assume is varying and which is constant. For an altimeter function, you need to assume constant barometric pressure, and vice versa.For us sea kayakers, we are usually pretty close to seal level, right, so altitude is obviously constant. Therefore, an altimeter actually makes a great barometer.What confuses me is how you can use a barometer/altimeter as a reliable altimeter when the weather is changing. At low altitude, the pressure changes with altitude by roughly 1.0" mercury for every 1000 feet. That's a perfectly likely change in pressure with a storm approaching. So, if your altimeter tells you you just climbed 1000 feet higher, how do you know a significant portion of that wasn't actually a meterological drop in pressure, and you only climbed, say, 750 feet.For example, on Peaks Island on Sunday, the atmospheric pressure changed from 29.9 to 29.5 between 11:00 AM and 8:00 PM, when we know from RickS's report, that weather in Casco Bay was deteriorating. (http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KMEPEAKS1&day=27&year=2003&month=10)If I'd been climbing a mountain under those conditions, my altimeter would have been off by 500 feet over that 9 hour period. Hmmm... maybe that's within tolerance for a hiker who's interested in the altitude... they are probably climbing and descending a lot faster than that. Besides, they can improve their accuracy by ~looking~ at the weather and factoring in likely baromteric changes.OK, maybe I answered my own question. Does this make sense?--David. Quote Link to comment Share on other sites More sharing options...
Guest guest Posted October 28, 2003 Share Posted October 28, 2003 David,I use a wrist Altimeter for climbing purposes at some very high altitudes. I was glad when they came out with the Sunto about 5 years ago, they go to Andean elevations of over 20,000 feet, many models before this stopped at 14,000 feet which made them useless for climbing big mountains.For the Altimeter to be useful, you must constantly recalibrate it to known elevations at places you can identify from a topo or what ever. I reset my altimeter often, sometimes twice a day. They are nothing more than a wrist barometer. My Sunto can be viewed as "elevation" good for climbers or "inches of mercury" good for sea kayakers and sailors and weather prediction.I believe there are now gps based altimeters that measure the distance to the satellite orbit from some elevation you input as a base line. Quote Link to comment Share on other sites More sharing options...
christopherG Posted October 28, 2003 Share Posted October 28, 2003 i believe sunday was the 26th, professor! Quote Link to comment Share on other sites More sharing options...
cmfos Posted October 28, 2003 Share Posted October 28, 2003 Yes, I went through these mental gymnastics on altimeters/barometers about a year ago when I got a fancy watch that did everything except wash the dishes: it had a wind gauge indicator, compass, altimeter, barometer, thermometer. It was also a piece of junk but that's another story. Anyway, I'd calibrate the altimeter for what I thought was the elevation for my house, only to find that my house had dipped about 300 feet in altitude in the subsequent hour as a low pressure airmass was moving in. So when I went to my college textbook and looked up the relationship to altitude and pressure, I found something probably similar to what David found: that small changes in pressure can lead to large changes in the altitude as measured by the watch. For a fella like myself who changes altitude very slowly, such an altimeter really is not of much use. For someone who changes altitude quickly like Ken Cooper scrambling up mountains or a pilot in a Piper Cub, then an altimeter is a useful device to have even though it does need to be recalibrated from time to time.Certain GPS receivers do measure altitude like the Garmin Summit. That model is nice because it'll even plot the elevation change vs. the distance you've hiked so you can get a nifty look at all the hard work you did getting to the top of Cannon Mt for example. But for sea kayaking folks like us, an altimeter is not too handy.I would also mention that Ken Cooper pointed out to me long ago that GPS receivers measure movement in all three dimensions. So if you hike a mile, then climb straight up a mountain for a mile, then the GPS will say you traveled two miles. This is kinda simple I know but this does affect your speed and distance readings when you're paddling. On a trip out to Jewel Island in Maine, the conditions were quite benign on the way to the island and our group averaged probably 3.5 kts out there. Coming back, we had lots more waves and so, as if by magic, I found that we'd paddled a signficantly longer distance on the way back and we'd gone FASTER (about 4 kts). This difference was probably attributed to the extra 'distance' we were traveling over the waves that were moving us up and down.Jim Quote Link to comment Share on other sites More sharing options...
christopherG Posted October 28, 2003 Share Posted October 28, 2003 Dear Jim, I like the colour of your writing. There is a nice flavour to it ("it was also a piece of junk; but that's another story..." and so on)Regarding altimetry: of course pilots adjust their altimeters all the time: that's one vital function of air-traffic controllers -- they constantly advise local barometric pressure (usually in millibars -- only in the USA, as far as I know, is it still given in inches, by the way). When flying from one flight information region to another it is quite conceivable that the general, average pressure of the one may be quite different to that of the other: hence, on changing setting, a significant flight adjustment might be necessary, too. For exactly this reason, all aircraft above a certain altitude (which lower limit is locally-ruled) change to the "standard setting" of 1013,2mb and one's altitude is then no longer described as being in feet, but as a "flight level".Regarding how high one has climbed on any given day, in theory one needs only consult a decent contour map to calculate this? I seem to have Boy Scout memories of counting those contour lines on the wonderful Ordnance Survey maps (one inch to the mile and splendid detail)...halcyon days!Enough? It really is quite an interesting subject...for your own personal and mobile barometers, then, you need to be able to adjust local pressure before leaving the warmth of your home to set out for the peaks (Ken the Cooper et al). Quote Link to comment Share on other sites More sharing options...
Guest guest Posted October 28, 2003 Share Posted October 28, 2003 Mr. Chris,It is an interesting subject, your post is a bit cryptic, so I'm not sure of your intent. You're of course an expert with altimeters and flight.I don't think you understand, my friend, the use of an altimeter and its part in route finding on a big glaciated mountain in a remote part of the world where there are no topos available and where you've left the warmth of the nearest village a week before. Even on mountains where topo is available, its your altimeter that tells you which one of those topo lines you're on, how else do you know? They don't paint topo lines across the ice and snow with little elevation signs on them.Maybe I'll see you someday and we can talk about it and I can share some of my experiences. Quote Link to comment Share on other sites More sharing options...
djlewis Posted October 28, 2003 Author Share Posted October 28, 2003 Luckily for us sea kayakers, we do the vast majority of our paddling at sea level. I guess we could be on a mountain lake, or a descending river, or going through locks, but in those cases, predicting storms is not as vital as when on the bounding main.So, if I understand this business correctly, an altimiter/barometer makes a terrific barometer for us, because altitude is held constant, so changes in barometric pressure can only be meterological. And that's what I'm looking for.Gee, that reminds me... when I started this *!@$#$!*&%$##@!! so-called sport, I figured to simultaneously continue hiking the Whites and other places. Guess what...--David. Quote Link to comment Share on other sites More sharing options...
jeffcasey Posted October 28, 2003 Share Posted October 28, 2003 GPS devices give your coordinates in 3 dimensions, so you always have an altitude. The effects of triangulation mean that the altitude often has the largest uncertainty, especially with only a few satellites in view. I doubt, however, that the waves on that return trip were significant enough to add to the effective distance travelled (in other words, I doubt that the GPS had enough resolution to pick up your altitude going up and down, and integrate all that up and down motion to give you greater distance). More likely, your course made good over the sea bottom was wavering around as conditions changed or your heading changed. If you downloaded your route from the GPS to your topo map software, you probably would have seen a jiggly course. It is often very revealing to look at how straight your course was after the fact, and compare upwind, downwind, crosswind, crosscurrent headings, etc. /* extra geek warning on: Pressure is one of the most notoriously mangled measures, with many barbaric (pun?..never mind) units still in popular use. I've had occasion to take readings on three instruments, all in different units, for the same measurements...inches water, torr (mm mercury), millibar, Pascals, psi....aaarrrrggghhhh!!). warning off */ I picked up a Brunton Sherpa from REI a year or two ago...it is an altimeter/barometer, wind gauge, thermometer. Cute, but only a novelty...I would never count on it, thus it has never been worth the bother to carry it along. Not waterproof. http://a1072.g.akamai.net/f/1072/2062/30d/...645068_1521.JPG Regarding the ongoing battle between mssrs. Godfrey & Cooper, I'll agree a bit more with the former. Gadgets have always found ways to fail, especially in cold and altitude. I've even had GPS devices tell me I was a full quadrant (7.5') away from the mountain I was sitting on (albeit an early model GPS). I trust just about ANYTHING before I trust a gadget...paces counted, contours, local topography clues. I've never failed to find a topo for any mountain that needed climbing (even in the Caucasus, where we had to use old German WW2 topos... hmmmm...). Quote Link to comment Share on other sites More sharing options...
Jed Posted October 28, 2003 Share Posted October 28, 2003 >Luckily for us sea kayakers, we do the vast majority of our >paddling at sea level. I guess we could be on a mountain >lake, or a descending river, or going through locks, but in >those cases, predicting storms is not as vital as when on >the bounding main. This is a matter of degrees (no pun intended). Weather can be very dangerous on a large lake with violent weather looming just beyond the steep mountains to the west. If fact the weather is easier to see on the ocean so on lakes is where you may want to hone your sky reading skills.>So, if I understand this business correctly, an >altimiter/barometer makes a terrific barometer for us, >because altitude is held constant, so changes in barometric >pressure can only be meterological. And that's what I'm >looking for. Please note that a Altimeter / Barometer is a barometer first and foremost. Any altitude reading is a calculated figure while the barometric pressure is the primary read of the sensors.Also note with regard to barometric pressure and weather: It is not the absolute value of the pressure that is of importance but rather the direction and rate of change.Barometers can tell you what is happening when the cloud cover obstructs your view but the pressure doesn't come near to being the whole story so accuracy is relative depending on your expectations.>Gee, that reminds me... when I started this *!@$#$!*&%$##@!! >so-called sport, I figured to simultaneously continue hiking >the Whites and other places. Guess what... Jedjluby@teamnorthatlantic.comLife is too short to own an ugly boat. Quote Link to comment Share on other sites More sharing options...
djlewis Posted October 29, 2003 Author Share Posted October 29, 2003 > This is a matter of degrees (no pun intended). Weather can >be very dangerous on a large lake with violent weather >looming just beyond the steep mountains to the west. If fact >the weather is easier to see on the ocean so on lakes is >where you may want to hone your sky reading skills. Good point.>Also note with regard to barometric pressure and weather: It >is not the absolute value of the pressure that is of >importance but rather the direction and rate of change. Which is why it still works fine for kayakers anywhere. No matter what altitude they start at, paddlers, unlike hikers, will simply not be ~changing~ altitude during a trip anywhere near enough to complicate relative barometer reading of the weather.--David. Quote Link to comment Share on other sites More sharing options...
mskulick Posted October 29, 2003 Share Posted October 29, 2003 Let go of those millibars. 29.92" HgYour altimiter is graduated in units of you-know-what!Propeller-Butt. Quote Link to comment Share on other sites More sharing options...
Jed Posted October 29, 2003 Share Posted October 29, 2003 >>Also note with regard to barometric pressure and weather: It >>is not the absolute value of the pressure that is of >>importance but rather the direction and rate of change. >>Which is why it still works fine for kayakers anywhere. No >matter what altitude they start at, paddlers, unlike hikers, >will simply not be ~changing~ altitude during a trip >anywhere near enough to complicate relative barometer >reading of the weather. This is of course true, with the one possible exception or marathon WW paddling. But your theme seems to assume the use of an altimeter adapted for use (via human processing) as a barometer. The point I mentioned in the last post that an (every) "Altimeter / Barometer is a barometer first and foremost. Any altitude reading is a calculated figure while the barometric pressure is the primary read of the sensors."There is no altimiter that works via any other mechanism than by reading the changes in barometric pressure (save for the GPS variety). So the overwhelming majority of wrist computers w/ an altimeter feature will have as their data source a virtual barometer. Few manufacturers are short-sighted enough to not include the barometer function even if there marketing guru's do tell them that altimeter's are sexier.Jedjluby@teamnorthatlantic.comLife is too short to own an ugly boat. Quote Link to comment Share on other sites More sharing options...
cmfos Posted October 29, 2003 Share Posted October 29, 2003 >GPS devices give your coordinates in 3 dimensions, so you >always have an altitude. The effects of triangulation mean >that the altitude often has the largest uncertainty, >especially with only a few satellites in view. Yes that's true although not all GPS receivers display the elevation profile. My Legend does not whereas my sister's Summit (designed with the hiker in mind) does.>>I doubt, however, that the waves on that return trip were >significant enough to add to the effective distance >travelled (in other words, I doubt that the GPS had enough >resolution to pick up your altitude going up and down, and >integrate all that up and down motion to give you greater >distance). Yes, this sounds right. I think that the variability on these things can only get as precise as 16 feet (and that's imprecise writing BTW). Still, I think that small variations can be detected. For example, I was hiking with my sister and her husband down to the Rio Grande in New Mexico last Christmas. I had the Garmin Summit in my hand on the way down and I saw a nice little elevation profile of our intrepid journey down the canyon. On the way back, I hung the Summit from my neck and let it swing back and forth across my chest like a pendulum. When I looked at the elevation profile of our return trip, it was NOT a perfect mirror image of the first leg of the trip like I'd expected but instead, it had a more flattened profile. After puzzling about this for awhile with my brother-in-law, we concluded that the Summit was detecting the pendulum-like motion of swinging wildly back and forth as extra distance traveled which thus made it look like we had hiked farther than we had on the way down. So even though this swinging motion of about 3-5 feet each way was lower than the 16 foot variability, the unit could still pick it up. Jim Quote Link to comment Share on other sites More sharing options...
oldsalty Posted October 29, 2003 Share Posted October 29, 2003 You took longer to hike back out than to hike down in. Those things usually plot the elevation profile against a time axis instead of a distance travelled axis. Quote Link to comment Share on other sites More sharing options...
djlewis Posted October 29, 2003 Author Share Posted October 29, 2003 >So even though this swinging >motion of about 3-5 feet each way was lower than the 16 foot >variability, the unit could still pick it up. You'd think (even hope) that they'd use a smoothing function for calculating distance. Even better (or worse, depending on how geeky you are) the smoothing parameters could be set by the user, something like: how small a back-and-forth variation should be ignored when figuring distance?There's an easy way to test this. Stand still and swing the gps unit back and forth on a tether, or around your head, or toss it up and catch it for a few minutes, and see if it registers any distance. If not, then increase the arc, or walk back and forth a couple of feet, climb up and down stairs, etc etc, until it starts to register. That will tell you its sensitivity.--David. Quote Link to comment Share on other sites More sharing options...
donperry Posted October 29, 2003 Share Posted October 29, 2003 >You'd think (even hope) that they'd use a smoothing function for calculating distance.That's what the 'Battery Save' function on a Garmin Summit does, reducing the number of tracklog points saved in memory. Quote Link to comment Share on other sites More sharing options...
donperry Posted October 29, 2003 Share Posted October 29, 2003 I believe the Etrex Summit and Vista are the only Garmin handheld models that incorporate a baritron. The baritron is very sensitive and numbers reported by the altimeter are more resolute than elevations computed by the GPS receiver. Garmin says the ‘Auto Calibration’ function will adjust the altimeter reading to within a hundred feet. Manual calibration at a known elevation is the most accurate way to correct the altimeter.Elevations generated by the altimeter, right or wrong, are the values recorded in the GPS memory, along with latitude, longitude, and speed. Garmin claims MapSource software will extract all of this from the GPS unit and save it in a tracklog file. A Garmin representative just verified that for me by opening a recorded flight profile showing location, altitude and speed. Maptech packages (and Topo I suppose) don’t use the elevation recorded in the GPS because it would be in conflict with their data. The altimeter on a Summit or Vista detects changes while bobbing up & down in the waves. A higher sampling and display rate could probably give a good report of average and maximum wave heights. A barometer page for a stationary unit would be nice to have too. What the baritron is used to display is up to the marketing and sales departments.They are not so concerned with the needs of nerds. Quote Link to comment Share on other sites More sharing options...
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