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Altitude Help For Gps 60cs


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I seem to be getting inaccurate altitude readings on my 60cs. I have not calibrated the altimeter, but my understanding was that in the default state that the GPS altitude readings would correct the altimeter.

 

Can anyone point me to a web resource that lays out all the interplay between the altimeter altitude, the GPS altitude and barometric pressure for Garmin units?

 

Thanks

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I don't know of a website but I can try to briefly explain.

 

I'll begin with the fact that my experience with GPS altitude is that it can be very accurate. A lot of points along the Blueridge Parkway are survey marked for elevation and I was always surprised how close the altitude readings on my eTrex Venture and Legends were. It always bugged me that when I went to a Vista, I had to calibrate the altimeter before I could get accurate readings. The advantage of the barometric altimeter over the GPS altitude, however, is that it works even when you don't get a good GPS signal and even if you have the GPS features turned off to save batteries.

 

Having said this, I think the obvious solution if for Garmin to offer GPS elevation readings as well as elevation readings based on the barometric altimeter.

 

My understanding of Ambient Pressure, Barometric Pressure and elevation is that to solve for any one of these parameters, you have to know the value of the other two. Well, ambient pressure is easy because it is directly measurable. Giving the GPS unit the value of the other 'known' is the process of calibration. You can calibrate the altimeter by giving the unit a known value of elevation or a known barometric pressure.

 

When the GPS is in the Auto Calibration mode it occasionally uses the GPS elevation to solve for barometric pressure. Knowing this, the best way to keep the readings for all three parameters very accurate is to have an accurate GPS reading often enough for the unit to auto calibrate.

 

Keeping in mind that barometric pressure is a measure of ambient pressure normalized to mean sea level (msl), the value of the barometric pressure readings is that it gives a reading of the pressure associated with a weather front that is independent of the elevation of the weather station’s elevation.

 

The GPS units with barometric altimeters allow manual calibration by entering the unit’s known elevation or the unit’s known barometric pressure. When the unit has a GPS satellite lock it can also use the GPS elevation as the input for the known elevation. When using a known value of barometric pressure for manual calibration, the closer you are to the weather station reporting the barometric pressure and the more stable the weather conditions, the more accurate the calibration will be.

 

I hope this helps and I know how you feel. I dealt with the same issues when I first got my Vista. The good news is that the autocalibration of the 60CS and 76©(S) are much more advanced than the Vista.

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Thanks a lot - very clear explanation of a complicated subject.

 

Since my posting I got bits and pieces from the usegroups. A couple of interesting tidbits: almost all of the altitude measurements displayed in the 60CS's fields are readings from the altimeter. The only GPS altimeter field seems to be the one accessible from the satellite page (which I had not discovered before).

 

The logic of the altimeter/GPS combination is as follows. GPS elevation data is less accurate than your long/lat position. It is also much more subject to jumpiness because of the nature of GPS reception, changes in satellite geometry, etc.

 

The elevation data from the altimeter is generally more accurate than the GPS measurement: 1) if properly calibrated; and 2) assuming there is not much weather-related change (i.e. non-altitude connected change) in the barometeric pressure. It also gives much steadier readings than the GPS-derived figures.

 

My understanding of the interplay is that the primary driver of the altitude measurement is the barometer, and this is what is reported in the tracklog and I presume the datastream. However, the GPS-derived reading will be used to correct the barometric reading if there is a large disparity. This will happen if a weather front comes in quickly.

 

Before doing any calibration, I was consistently getting altitude readings of -100 feet at my home (real elevation +17 feet). I will experiment and see how it goes. It's fun learning about this stuff.

 

I wish the user could choose whether the altimeter is on or not. Sometimes rough numbers are enough, or you don't have a source for calibration.

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Ok, so its not really relavant, but I think its fun to roll the windows up and down in the car while driving and watch the altimeter page jump up and down by 50 ft! Kind of gives you a visual of the pressure changes that occur in a moving car when the windows go up and down. Now I understand the reason for the blowout of windows in airplanes and the havoc that goes on when it does.. At least in the movies anyway.

 

:mad:

Darren

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Taken from a local forum:

 

Evidently, the normal operating mode of most GPS units gives NOT the elevation above Mean Sea Level- which presumably is the elevation given on USGS topological maps: http://www.topozone.com - but instead gives the elevation above the WGS84 ellipsoid; though many units supposedly provide a MSL option.

 

MSL presumably correlates closely with the Geoid (surface of constant

gravitational potential) that it also serves to define. The difference between the Geoid and the WGS84 ellipsoid is given by the Earth Gravity Model, EGM96; typically, the Geoid is about 30 meters below the WGS84 ellipsoid in North America:

 

Also see:

http://users.erols.com/dlwilson/gps.htm

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Taken from a local forum:

 

Evidently, the normal operating mode of most GPS units gives NOT the elevation above Mean Sea Level- which presumably is the elevation given on USGS topological maps: http://www.topozone.com - but instead gives the elevation above the WGS84 ellipsoid; though many units supposedly provide a MSL option.

I don't know what your source is for this statement, but it is not true for current Magellan or Garmin GPS receivers. While a GPS internally determines the elevation above the WGS84 ellipsoid, both companies use tables in their firmware to correct for the geoid vs. ellipsoid height. Therefore the reported heights are estimates of elevation above MSL, not above the ellipsoid. The tables appear to be rather coarse and therefore have significant errors in some places (about 15' for Garmin's table where I am), but it's still much better than not correcting at all. IIRC, the old Lowrance GM100 did not correct for geoid vs. ellipsoid, but I think their more recent models include a correction as well - don't have access to an iFinder to check.

 

The NMEA GGA sentence includes the value for the geoid vs. ellipsoid correction being used in the receiver and you can compare this with values from the NOAA site:

http://www.ngs.noaa.gov/cgi-bin/GEOID_STUF...003_prompt1.prl

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I've actually been pretty impressed with the accuracy of GPS units for altitude - even the ones without a barometric sensor. But they can easily be fooled and give bad readings so you have to be aware of the possible pitfalls.

 

For the GPS portion to give good altitude readings by itself requires much better reception conditions than for decent horizontal readings. Although 4 satellites are the theoretical minimum for a 3D lock, I don't trust the altitude unless I have at least 5 satellites locked and check the geometry to see that they're dispersed around the sky. But with 5+ satellites I've found my GPS to be within 40' well over 95% of the time.

 

The main advantage of the units with barometric sensors is that they can continue to give good altitude readings even if the GPS reception is marginal for awhile. But they are subject to atmospheric changes so either recalibrate manually fairly frequently or use the automatic GPS-based recalibration and try to ensure that you give the unit the best possible satellite reception so the recalibration algorithms have good data to work with.

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Hi I tried to autocalibrate my GPSmap60CS and it works much more better than I expect ! I compare my altitude reported by GPS with altitude reported by map on some summits and there was 1-5m difference only without any manual calibration.

 

On other hands I adrressed another problem to garmin support:

 

<i>Hello, I experienced problem with a Altimeter. On Altimeter page data fields "total ascent", "elevation maximum" and "elevation minimum" doesn't work. On other hands Altimeter works fine, curent elevations seems to be ok, graphs are ploted correctly. What I'm doing wrong, or it shoudld be an HW or FW problem ? In setup page is correctly set-up variable elevation. </i>

 

Did you have the same experience ?

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