Altimeter in Garmin Summit GPS

2:55 p.m. on February 15, 2002 (EST)
(Guest)

Does anyone have any experience with the altimeter function in the Garmin Summit GPS? I know Bill S. used to do extensive reviews of these types of things in the good old days.

Thanks in advance.

6:36 p.m. on February 15, 2002 (EST)
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Garmin Summit GPSr

A good place to look is on Joe and Jack's website:

joe.mehaffey.com

Their review of the Summit is at:

http://joe.mehaffey.com/etrexsum.htm

You might also look at their reviews of other eTrex models, notably the Vista, which also has the altimeter. My general impression is that, for a barometric altimeter, you are better off with something like the Avocet or even the Suunto "wrist computers" (even with the strange "sea level" anomalies that the Suuntos have, which may have been fixed recently). The purely GPS-signal-derived altitude is usually more accurate than the barometric altitude, and the barometer add-on is a pretty costly step up across models in Garmin's GPSR series.

I don't have a lot of personal direct experience with the Summit/Vista Garmin barometric altimeter, but what little I do have was very unimpressive.

10:35 p.m. on February 15, 2002 (EST)
(Guest)

Re: Garmin Summit GPSr

Quote:

A good place to look is on Joe and Jack's website:

joe.mehaffey.com

Their review of the Summit is at:

http://joe.mehaffey.com/etrexsum.htm

The purely GPS-signal-derived altitude is usually more accurate than the barometric altitude, and the barometer add-on is a pretty costly step up across models in Garmin's GPSR series.

Quote:

I don't have a lot of personal direct experience with the Summit/Vista Garmin barometric altimeter, but what little I do have was very unimpressive.

I have used the "plain vanilla" eTrex, with the GPS altimeter and it works great. I have been within 2 feet of the known altitude of USGS benchmarks on several occasions. In running cross country courses, it has always been within twenty feet and very useful in navigating. Cheaper, lighter, more accurate - what's not to like?

1:12 p.m. on February 19, 2002 (EST)
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Re: Garmin Summit GPSr

Quote:

I have used the "plain vanilla" eTrex, with the GPS altimeter and it works great. I have been within 2 feet of the known altitude of USGS benchmarks on several occasions. In running cross country courses, it has always been within twenty feet and very useful in navigating. Cheaper, lighter, more accurate - what's not to like?

Depends on what you want to do. A problem with GPS receivers arises when you have poor geometry or are only able to lock onto 3 SVs (usually poor geometry, since this results from canyon/canopy problems and can only see 3). When you have only 3 SVs, you only get a 2D fix. If your assumed altitude is off, your horizontal position is displaced by an amount dependent on the altitude error. I have seen situations in which the horizontal displacement was over a kilometer. Several years ago, Magellan introduced a pressure sensor in several of their models to aid getting fixes and to stabilize the variations produced by Selective Availability (the intentional dithering of the signal introduced by DoD to reduce accuracy for non-authorized users). This helped tremendously in TTFF, as well as aiding 2D fixes. It would still help in situations having bad geometry or the minimum number of SVs.

One other place where the 2D fix pops up that I have seen way too often is when someone whips out their GPSr and reads off a position as soon as it shows a fix, without checking to see whether it is a solid 3D fix. If your altitude at you last fix was, say, 2000 or more feet different from the new location, you can get an position error of a couple hundred feet, and worse, the EPE display will not show how bad the error circle is (especially on Garmins, because of the strange algorithm they use for figuring EPE).

Competition in the GPSr consumer market being what it is, Magellan appears to have dropped their pressure aiding and followed Garmin in adding barometric altimeters in their most expensive models instead. This is not totally bad, since receiver and antenna design have gotten much better, and TTFF and time to 3D fix are much shorter. But the user should be very sure to check that there is a valid 3D fix and that the geometry (indicated by the DOP and EPE) is reasonable. Garmin's EPE is actually more of a relative number than a statistically valid one. So look for Garmin EPE values under 10 feet (real error circle in the CEP sense is more like 15 meters when Garmin is indicating 10 feet). In that case, you will be within 20 or 30 feet over half the time, and fairly often be within 5 or 10 feet. But keep in mind that USGS map accuracy is 40 feet for 7.5 minute quads (the diameter of an 0.5 mm pencil lead on the map), and altitudes are designated as +/- half a contour interval. If you are reading the altitude off a benchmark that is designated for altitude measurement, be sure to check the USGS and NGS websites for the accuracy level assigned to that particular monument. Claiming to be within a couple feet of the altitude brings up several caveats - first and most obvious is where were you holding the GPSr - on the BM, at waist level, at eye level??? Second is how does your altitude read compare with the official recorded listing of accuracy, not just what is stamped into the brass plaque. Those 3 to 5 decimal numbers are the calculated number and not rounded to reflect the stated accuracy. The intention is to allow retroactive checking of the field measurement reductions. And, how close is the reading if you wait 15 minutes, or come back a day later, or a week later? And, here in California, of course, when was the monument placed - before or after the latest major earthquake. We do get some pretty significant displacements of the ground, horizontally and vertically.

July 25, 2014
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