For an eTrex Vista HCx, you can set it up to either run in "Auto Calibration" mode or "Barometer" mode. In auto calibration, it uses GPS measurements to periodically recalibrate the elevation that is read from the barometer. According to Garmin's website, this setting gives you the greatest degree of accuracy since it helps to correct for the errors associated with barometers (see below). In barometer mode, you are basically just using it like a bike computer with a barometric altimiter. You can specify "variable elevation" for when you are moving or "fixed elevation" for when you are stationary.
An article on errors of barometers follows, from: http://www.tpub.com/content/meteorology/TM-11-427/TM-11-4270042.htm ERRORS OF ANEROID BAROMETER. Factors which introduce errors into the readings of the aneroid barometer are hysteresis, temperature, scale errors, and drift. a. Hysteresis. (1) One of the chief causes of error in an aneroid barometer is hysteresis. All elastic materials tend to change shape whe placed under stress. As soon as the stress is removed, they return almost to their original shape. The retardation or failure of the mate- rial to assume completely its original form is known as hysteresis. All aneroid barometers are subject to the effects of hysteresis although an attempt is made to keep these effects to a minimum by the proper selection and treatment of materials. Over a long period of time hys- teresis effects gradually tend to disappear. (2) Hysteresis occurs in the pressure sensitive cell of the aneroid barometer and for that reason appreciable errors may be noticeable immediately after a large and rapid pressure change. These errors can be greatly reduced by waiting a sufficient length of time before taking a reading. It is difficult to make a specific recommendation as to the interval of time necessary to reduce hysteresis errors to a minimum since hysteresis varies with the amount of change in pressure, with the rate of change, and with the time involved that is, the length of time at which the barometer has been subjected to a pressure differing from current atmospheric values. (3) For pressure changes occurring at any fixed station, hysteresis errors are small enough to be neglected. When the barometer is sub- jected to a- comparatively large pressure change for a short time only, and then restored to its previous values, hysteresis effects are small and quickly disappear. Hysteresis errors are probably most pronounced when a barometer is transported by airplane, especially if the flight is of several hours duration and the plane flies at high altitude. There- fore, to be reasonably sure of reducing hysteresis errors to a minimum under such circumstances, it is advisable to wait at least 24 hours before taking a reading. b. Temperature. A second and exceedingly important error which affects the aneroid is temperature changes. Without compensation for Changes in temperature the errors of an aneroid would be quite large. Several methods of temperature compensation are used in the barome- ters covered by this manual. (1) Aneroid cell. If a barometer is maintained at a constant pres- sure, a change in temperature, if not compensated, will cause a move- ment of the pointer on the dial. This indicated change in pressure is due to a physical change in the dimensions of the instrument and also to variations in the stiffness of the pressure sensitive cell. This latter condition may be compensated, for the most part, by leaving a small amount of air in the cell. Thus when a change in temperature occurs, the change in pressure of the air inside the cell partially bal- ances the change in strength of the metal. This does not entirely com- pensate for changes in temperature. Therefore, temperature correction curves are provided for some models of Barometer ML-102-(*) and M L - 316/TM. These corrections, however, are small, indicating that the greater part of the error has been compensated. (2) Bimetal temperature compensation shaft. Barometers ML- 102-B, ML-102-E, and ML-102-F use an additional temperature compensation in the form of a bimetal pivot shaft made of a strip of invar and a strip of brass welded together lengthwise. Since invar and brass have different coefficients of expansion, changes in temperature cause the bimetal shaft to bow slightly, thus changing the indication of the pointer (3) Temperature correction chart. ... c. Scale errors.[in analog readouts or in the accuracy with which a digital readout is presented] ... d. Drift. (1) Another error to which the barometer is subject is drift (often called creep), which is due to changes in the aneroid cell occur- ring slowly over a long period of time. Drift is caused by molecular changes in the metals of which the cell is made, and by alterations in the shape of the cell, due to the tendency of all materials to assume a new permanent shape when placed under steady stress. In meteoro- logical textbooks, drift is sometimes referred to as "secular change," for it is an error taking place over a long period of time. (2) Drift is manifest by a gradual increase in the difference between the indications of the aneroid barometer and a mercurial barometer with which it may be compared. Errors of drift can be largely elimi- nated by setting the barometer to indicate the pressure shown by an accurate mercury barometer. (3) Some barometers drift because of very minute leaks in the evacuated cell. Usually a leaking cell is detected before the instru- ment leaves the manufacturer's plant, but occasionally a leak can develop in a barometer that has been in use some time. The indica- tions of a leak are progressively lower readings than are normal for the station. There is no way to remedy the defect. Return the barome- ter to the depot and requisition a new one. (4) For the first few months after the aneroid is built, the drift effects are more pronounced, but after this time they usually become negligible. In general, the longer the barometer is in use, the more consistent the performance is likely to be. (5) Occasionally a barometer will show a persistent drift. TO detect this condition, keep a record of the amount of resetting necessary to eliminate drift. If, over a period of time, this total exceeds 12 millibars, requisition a new barometer and return the defective instrument to the depot On Feb 16, 4:02 pm, Roy Yates <roydya...@gmail.com> wrote: > For the reasons Leslie noted, the Garmin units like the 205 or the older > etrex Legend that just use GPS to measure altitude changes work quite > terribly; I have seen errors by as much as a factor of 2. On the other hand, > the units (like the 705) that have the barometric altimeter seem to be > roughly about as accurate as other bike computers that just have a > barometric altimeter but no GPS capability. > > In principle, Garmin could be using a proprietary algorithm that does data > fusion on GPS measurements, topo databases, and the barometric altimeter. > What they actually do, AFAIK, is not disclosed. However, my guess based on > observation is the models with barometric altimeter are mostly using that. > > ...Roy > > On Tue, Feb 16, 2010 at 10:20 AM, Leslie <leslie.bri...@gmail.com> wrote: > > On Feb 15, 4:32 pm, Jim Thill - Hiawatha Cyclery <thill....@gmail.com> > > wrote: > > > GPS should record elevation changes more accurately, and both GPS and > > > cycle computers should record distance traveled more accurately than > > > mapping software does. In the old days (10-15 years ago), GPS > > > elevation measurements were suspect because there weren't always > > > enough satellites to triangulate accurately in the 3rd dimension. I > > > think this has improved lately. > > > I've never used a bicycle computer, so I can't speak to comparing its > > data to web-based map service, but I can touch upon the GPS elevation > > issue. > > > While it's true that the satellite constellation has filled out (and > > is currently being rearranged from a 21+3 to a 24+3 configuration over > > the next year or so), and GPS receivers are now running at least 12 > > channels or more, that accuracy has improved. Unfortunately, the > > vertical accuracy will always lag the horizontal, because the earth > > isn't transparent. > > > If you were to stand at a window with a GPS unit and look at the > > 'skyplot' view, you would notice that the receiver is picking up > > signals from satellites out in front of you that you can see from the > > window (if you could actually 'see' them), but that the ones behind > > you are blocked by the building, which thus skews your calculated > > location. If you then went outside and were standing in the middle of > > a field, you could see satellites all around, then the calculated > > location can be more precise. > > > The same thing is going on with elevations, but in that case, it's the > > earth that's obscuring signals from satellites on the other side of > > the earth. Because all of the satellites within view are being used > > are all overhead, and not 'behind' you, there's a bit of a skew; the > > smallest bit of error makes for a much larger change in elevation, > > than you would get for horizontal calculations. If the satellites > > that you're viewing/using are spread out around the horizon, then > > you'll be better off than if they were all clustered directly overhead > > - that would help minimize that error - but, that error will still be > > greater than your horizontal. > > > FWIW, the maps that I usually make, I will pull my horizontal location > > off the GPS, plot it on a topo, then use the elevations off the map, > > in order to compare to other locations (that aren't GPS'd). [Unless, I > > can actually GPS the other locations myself, too, in which case I'll > > then emphasize their relative elevations over absolute elevations.] > > > With that said, yes, it's improving. I've got a few Trimble units > > here in my office that are sub-meter, and I can pull off sub-foot with > > them, given enough time (and even cm-level accuracy w/ a LOT of time > > and post-processing); but, vertically, I wouldn't guarantee anything > > more than sub-foot, and that's w/ a long residence time, not an > > instantaneous value, as you would have if you're biking along. > > > (Hope that's not totally confusing....) > > > -L > > > -- > > You received this message because you are subscribed to the Google Groups > > "RBW Owners Bunch" group. > > To post to this group, send email to rbw-owners-bu...@googlegroups.com. > > To unsubscribe from this group, send email to > > rbw-owners-bunch+unsubscr...@googlegroups.com<rbw-owners-bunch%2bunsubscr...@googlegroups.com> > > . > > For more options, visit this group at > >http://groups.google.com/group/rbw-owners-bunch?hl=en. -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. 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