[nugzboltz]

I'm in the middle of the desert riding on pretty much flat ground. However, my 305 edge reports +/- 10 ft elevation changes on my rides; am I missing something or can I not trust the elevation readings from my 305?

 

[3034]

+- 30 ft vertical at best

 

[wmodavis]

By virtue of the way elevation in a GPS is calculated it is the least accurate of the positional information presented. I do not believe the GPS accuracy estimates apply to the elevation. Someone correct me if I'm wrong on that.

 

[nugzboltz]

Yeah, elevation calculations by GPS are not something I would rely on, but one of the touted features on the 305 is its barometric altimeter for precise elevation data. When I see my data on Garmin Connect saying that I had 200+ ft of gained elevation on a 3 mi ride on flat terrain, I just have to shake my head.

 

[slocaus]

I'm in the middle of the desert riding on pretty much flat ground. However, my 305 edge reports +/- 10 ft elevation changes on my rides; am I missing something or can I not trust the elevation readings from my 305?

Take a look at this thread http://forums.mtbr.com/showthread.php?t=498435&highlight=elevation

As stated, GPS elevation accuracy is tough to measure, barometric altimeters help, but a good software program using DEM data can correct tracks to give quite accurate information. I use TopoFusion for that information, and have read the link in the above thread, and tried all the different data sources that Krein has built into the program. I feel my reading are accurate in the program, but not the raw GPS data.

 

[Harold]

Yeah, elevation measurements using either the barometric altimeter or GPS (say, the 205) are estimates at best. On a short, flat ride like you had, you could have been experiencing changes in barometric pressure due to weather. That will make the GPS THINK you're changing elevation.

Calibrating often when you are at a known elevation, like a USGS benchmark, or a peak marker will help, but not eliminate the problem. Even DEM data has an effective resolution. For most, a 30m square area (pixel of DEM data) is given the same elevation. So short, minor changes can be missed if you rely too heavily on that data. If you're lucky, you might be able to find better than 30m resolution DEM data, but not likely.

 

[PCC]

If I'm not mistaken, the Edge 305 uses a barometric altimeter unless you turn it off in the settings. The variances in elevation we all have experienced is due to the barometric pressure changing as the temperature of the day changes.

GPS elevation can be accurate but it is a slow process as the GPS unit calculates and recalculates your elevation form the information that it receives from at least four satellites. If it doesn't see four satellites then it is not able to calculate elevation.