Hi,

I've experimented with both a Garmin GPSMAP 60CS and a Garmin eTrex Vista. The details here pertain to the Vista, which I prefer for cycling. I don't know which of the readings is more accurate, but that won't stop me from rambling a bit:

In my experience, the total ascent/descent figures I calculate from the track log are a bit higher than those indicated directly by the unit. I suspect that the unit is performing some sort of data smoothing on the barometric altimeter readings before adding them to the ascent or descent totals, and as a side-effect they miss some shorter/slower sections of vertical travel. I think the track log altitudes recorded are read from the baro-altimeter, versus the satellites, so it comes down to how those are post-processed. (If track log altitudes came from the satellite "altimeter", I'd consider that the larger ascent/descent numbers could be caused by the unit interpolating positions between satellite calculations and over-shooting at slope changes.)

After playing with the units for a bit, I've settled on turning the barometric altimeter auto-calibration off, and calibrating the units manually with the GPS altitude at the start of each trip (with a good satellite lock). If I'm at a place I've seen many altitude reading of (e.g., my driveway), I'll just manually enter the altitude. I ditched auto-calibration upon noticing that, after climbing the same steep hill repeatedly along the same course (in the name of science!), the altitude profiles of the same road in the track log drifted with each successive pass. Eliminating auto-calibration got rid of this, as far as I could tell. So, in my experience, auto-calibration introduced more systematic error than arose from barometric drift (which is AFAIK the whole reason for auto-calibration to exist). My test rides were over periods of 1-2hr, in elevations of about 0-800ft as reported by the GPS, with no significant storm systems moving through (this being San Diego and all...).

When collecting cycling track logs for analysis, I set the the track log mode to "auto" and "more often", and I always download the "ACTIVE LOG" directly to the computer instead of having the unit "save" it first. Saving the track log within the unit seems to drastically reduce the resolution of the data, and also discards timing info. I run the GPS receiver in "normal" mode, without WAAS and without "battery saver".

Hope this helps,

-JAB