Gps Elevation And Altimeter (garmin)

[+JohnAengus]

On the Garmin units there is the GPS Elevation and then you have the altimeter on some units. What is the difference between these measurements?

 

Thanks,

JohnAengus

[Neo_Geo]

The GPS elevation is derived from the unit's calculation of data gathered from the satellites. If you're getting a good lock on the satellites, the reading is pretty accurate - within about 20 feet or so. The altimeter's reading is based on ambient air pressure. The altimeter needs to be calibrated at least once for each use. If a strong, fast-moving high or low pressure weather system is moving through, it'll need to be calibrated several times. The difference between GPS readings and barometric readings can vary greatly depending on whether or not the altimeter is calibrated. And if you turn your GPSr on during a flight on a commercial airliner, the plane's pressurized cabin will cause a barometric altimeter to read between 5,000 and 8,000 feet - even though the plane is really at 41,000 feet! That's one reason I don't like the models with the sensors - that and the fact that they need to be calibrated. I just wanna turn on my GPSr and go (without having to mess with pushing a lot of buttons, etc.).

Edited April 14, 2006 by Neo_Geo

[+S&G.Davison]

And the thing about GPS elevation it is dependent upon the co-ordinate system that is used .. The systems attempt to put a perfecr sphere over the world.. but the world is not perfect .. so "sea level" in the co-ordinate system may be more accurate in one part of the world than it is in another part of the world.. there is a great article on GPS and elevation readings here

[peter]

And the thing about GPS elevation it is dependent upon the co-ordinate system that is used .. The systems attempt to put a perfecr sphere over the world.

Not exactly. GPS receivers start out with coordinates based on an 'earth-centered, earth-fixed' reference frame and then convert that to equivalent coordinates using a perfect ellipsoidal model of the earth as defined by WGS-84. Most consumer receivers (incl. all Garmins and Magellans) then do an additional conversion to the altitude based on an internal table giving the estimated difference between the geoid and the ellipsoid at that location.

 

So our receivers already take care of most of the coordinate transformations mentioned in that article and should give a good approximation to elevation above MSL

[+JohnAengus]

So, why do we need the barometric altimeter in addition to the GPS elevation?

[peter]

So, why do we need the barometric altimeter in addition to the GPS elevation?

In most cases we don't; and most GPS receivers don't include a separate pressure sensor. As NeoGeo indicated, *if* you have good, unobstructed GPS reception then the altitude reading is pretty good (I've found about 95% to be within 35'). It's not quite as accurate as the horizontal position due to the inherent geometry (best altitude determination would require some satellites to be below you), but good enough for most applications. But when you're in mountanous terrain the reception is frequently degraded by the hills and foliage and you can get multipath reflections that further impair accuracy. When the receiver can only get good signals from 3 satellites it can no longer determine altitude and then assumes that your elevation isn't changing. I've sometimes had tracklogs where I was steadily climbing but the profile instead shows an almost level line followed by a jump of hundreds of feet because the receiver had only 2D reception for an extended period.

 

Adding the pressure sensor can give you good altitude data even when the GPS reception isn't good. And even under good reception conditions it gives a more stable reading, with fluctuations of under 10' compared to the 30' fluctuations frequently seen using GPS alone. Much of the recalibration mentioned by NeoGeo can be done automatically by the Garmin units using the GPS altitude averaged over a period of time when reception is good to compensate for weather-related pressure changes. When set to this 'auto-calibration' mode you don't need to do the periodic manual calibration normally required for pressure-based altimeters.