Elevation Readings

Did you calibrate the Vista's barometric altimeter before-hand at a known, correct point? If so, changes in barometric pressure/ changes in weather over the time of ascent/descent will throw the calibration off.

The Vista also has the means to calibrate the altimeter using GPS elevation but normally, GPS elevation readings are off by quite a bit...and the calibration takes place slowly over time.

I'd say the unit did admirably (600 ft off), especially if you didn't calibrate.

I'd agree; sounds pretty close.

Altitude is an interesting concept in GPS -- altitude as a difference in height from WHAT? What's the reference?

For most of the units we use, altitude means the difference in height from the WGS-84 geoid. It's an approximation.

That's one reason the eTrex Vista has a barometric altimeter (and the ability to calibrate it; visit a local airport, as pilots are picky about current barometric pressure).

As a pilot, we always listen to ATIS which tells us the barometric altimeter pressure. The standard pressure is 29.92 but rarely have I seen that. Once a pilot get's that information, we set our altimeter to the pressure to get the exact altitude before takeoff and before landing. GPS's will be off unless they can be calibrated.

On the water, with our Garmin GPS 76, the GPS elevation is never near sea level, which the boat always is at, since it is on a bay connected to the Atlantic Ocean. The tide varies about 8 feet. The conclusion must be that the chart datum is not near sea level.

dobs108

Funny about this topic and a Garmin GPS coming up.

My wife and I drove our car up to the summit of Pikes Peak on June 15 of this year. My Garmin 1490t unit was on all the way up to the summit of 14,110 feet. My GPS sitting idle for 5 minutes showed an altitude of 14,103 feet -- a screen shot was taken.

The key must be keeping the unit still for a short period of time and the altitude will then be more accurate.

Unit specific. When I had my first Magellan GPS on the boat, I would always have it on 3D (this was back in the dark ages when GPS first started) and so I would get an indication of what the error was for that day (also in the dark ages of GPS the Military introduced errors into the civilian GPS receivers). Since the boat was on the ocean anything beyond elevation 0 feet was an indication of error.

These days when using the Nuvi 500 the altitude readings for summits are within the 30 foot error expected on the GPS receivers.

Remember that the eTrex Vista has a barometric altimeter and electronic compass in addition to the normal GPS altitude and "moving GPS" compass features we're all used to. Some of the replies here are correct for most GPS units but may miss the point of the eTrex vista model.

"How accurate is the GPS elevation reading?

Answer:

GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (or what is commonly called mean sea level). Basically, they are two different systems, although they have a relationship that has been modeled.

The main source of error has to do with the arrangement of the satellite configurations during fix determinations. The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions. It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating."

I'd always thought that would explain the continued updating of elevation at stops using my Garmin MobileXT, continued refinement from a [now] stable position.

We were on the cog wheel powered train and were moving all the time until we got to the top. Maybe that caused the differences.

Lots of "Good-to-know" stuff in this thread

we drove to the top of Pikes Peak on Jun 18 and my Nuvi 750 said I was at 14,130 feet, 20 feet above the advertised 14,110 feet. We took a picture of the reading also. I just attributed it to the 20' or so distance accuracy variation that GPSs seem to have. Didn't know about the barometric pressure affecting that.

Barometric pressure doesn't affect GPS elevation readings. But in addition to the GPS-elevation and GPS-directional info of all GPS units, the eTrex Vista has a standard barometer that, when calibrated for the day, will give elevation readings--it also has an electronic compass that is not part of most GPS units.