Accurate Altimeter

10:24 a.m. on January 17, 2009 (EST)
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Does anyone recommend a fairly accurate Altimeter?
Thanks

5:46 a.m. on January 18, 2009 (EST)
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When hillwalking I used to use standard type altimeters based on barometric pressure, these have to be recalibrated in any change in weather pressure, reasonably accurate but always needing to be referenced from a map every now and then. I now use a Garmin Etrex H which gets it's altitude via GPS, never needing calibrated and very accurate. I find when I get to the mountain top, say 1100m, gps is usually less than 5m out

However, if you are in an area when signal strength is not great, this effects the altimeter, though you can usually tell when this happens as a gps unit gives you an accuracy figure(down to 3m).

5:28 p.m. on February 18, 2009 (EST)
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There are basically two types of altitude-measuring instruments for hikers and backpackers, as indicated by mckain's response - barometric altimeters and GPS receivers. As indicated, barometric altimeters depend on the atmospheric pressure, and are based on the fact that the lapse rate is about 1 inch Hg pressure for every 1000 ft altitude. Good altimeters, whether mechanical (Thommens makes by far the best pocket mechanical altimeters, but not cheap) or electronic (many of these available), use the ICAO Standard Atmosphere Tables (you can find this on the web with a Google search, if you want the details). The electronic altimeters found in the Kestrel 4000 and 4500 and Brunton weather instruments, and in the Suunto "wrist computers", Polar heart rate monitors, and a few other electronic devices are quite good, within the limitations of barometric altimeters. There are some strong limitations, however. As mckain noted, since the barometer changes with weather changes (atmospheric pressure and weather are intimately linked), you have to recalibrate barometric altimeters frequently. Plus you have effects like the Venturi effect of mountain passes. And the real atmosphere is never exactly the same as the idealized ICAO Standard Atmosphere. Because of the Earth's rotation, the actual lapse rate within the last 5-10 degrees latitude of the poles differs substantially from the Standard Atmosphere. Hiking up hills on a summer day, when the ground is hot means the temperature change with altitude, and hence the pressure change with altitude will depart far from standard, sometimes by as much as 100 feet per 1000 ft altitude gain.

This is not to say that barometric altimeters are no good. In fact, they are actually quite good for hiking purposes. You just have to understand how they work and take that into account.

A GPS receiver, on the other hand, depends on the signals from the GPS satellites and an assumed shape of the Earth (the geoide). In general, the altitude is within 20-50 feet of the surveyed value (USGS maps are plotted only to a standard of 40 feet accuracy). Sometimes the GPSR can be off by more than that, but if you watch for 10 minutes or so, it will average out to the 20-30 foot accuracy level (unless there is a lot of canopy - tree cover, and/or multipath due to canyon walls). EXCEPT for certain "high-end" models of Garmin receivers which have built-in barometric altimeters which cannot be turned off (you have to recalibrate these frequently, just as with any barometric altimeter - most have an "S" in the model name, such as 60CSx or "HS"). Also, Garmin "S" receivers have a peculiarity of over-riding the GPS-derived reading if the barometric reading is too low pressure compared to the GPS reading.

Either way, count on a minimum of $200, and more like $500+ for an instrument that gives acceptably accurate altitude information. And remember that GPS receivers and electronic altimeters depend on batteries - when the battery runs out, you lose use of the instrument.

August 22, 2014
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