I recently did some field testing in order to compare my two GPS units, a Garmin Edge 305 and a Garmin Etrex Vista. Note that neither I or MTBGuru are affiliated with Garmin in some way, these just happen to be the units I own and generally like: the Edge is a good cycle computer, and with its SiRFstar III chip an excellent track logger. My Vista is a bit older, has a less sensitive GPS receiver chip and some other annoyances (such as dropping time info when saving tracks) but you’re able to upload topo maps which you can view on its big display, so for navigation the device is pretty good.What did I want to find out? I already knew the Edge had much better reception under canopy, in wooded areas or narrow canyons. In these circumstances, the Vista would sometimes lose signal, with interrupted tracks as a result. The noise on measured latitude and longitude on the Vista is higher - how would the measured distances compare?
I was also interested in how the two altimeters would compare. Both units have a barometric altimeter. The Vista’s altimeter can be used in two modes: ‘auto-calibration’ on or off. When on, this auto-calibration uses elevation data based on the GPS signal to periodically recalibrate the sensor (about every 15 minutes). When off, it’s the barometer only that produces the elevation measurement.
In addition to this, you can also manually calibrate the altimeter (useful if you know the altitude at which you’re at). The Edge on the other hand features a barometric altimeter that is quasi-continuously being recalibrated based on its GPS-signal.

So I went for a ride with the two devices, up in the Santa Cruz mountains. From the Russian Ridge parking lot on I started logging, and headed up to Borel Hill:

I hadn’t used or calibrated the Vista’s altimeter in a while and it was initially quite out of whack (about 400 feet).
At the summit of Borel Hill, 2572 feet high, I manually recalibrated the Vista and set ‘auto-calibration’ to off. In the meantime, the Edge was giving me the right altitude within 10 feet.

The graph above shows the raw elevation data from both units over the whole ride; the interesting part is that after being recalibrated on Borel Hill, the Vista’s barometric altimeter (without GPS ‘auto-calibration’) tracks the altimeter from the Edge almost perfectly. This is not too surprising: though they are in absolute terms not very accurate and require frequent recalibration, altimeters have very good relative accuracy and over a time frame of a few hours won’t drift much. A weather front that suddenly moves in is about the only thing I could think of that would knock it off quickly - otherwise baseline drift is a very slow effect. And I have actually still to experience any such effects attributed to fast-moving weather fronts; but then I live in an area where we don’t see much of these.

On the ride, I repeated the same section of trail multiple times - I did the same descent and climb three times, as an out-and-back. The graph below (a blow-up of the first graph) illustrates this:

The red dotted line connects points that represent one single spot: the top of the climb. The extra bump on the second peak is to be ignored, as I climbed there a bit higher to a ridge to take a break. At the third peak I took another quick break and played a bit with the controls of the Vista - that may explain the weird little dip in its signal. But as you can see, drift over the course of about an hour is minimal (less than 10 feet), and consistency of the data is quite good. Only at the third repetition of the climb, a slight deviation between the two units can be observed.

So concerning elevation, the two units aren’t very different. The Edge’s altimeter is just much more convenient because you don’t need to recalibrate - the ‘quasi-continuous’ automatic recalibration seems to work well due to its very accurate GPS signal. I used the Vista here with ‘auto-calibration’ off, because I wanted to rule out sudden shifts that would occur every 15 minutes. On another occasion, I’ll look at the effect of the latter. I will also keep the discussion on cumulative elevation gain and loss numbers for later - these are a function of the noise of the sensor and the filtering applied.
Other interesting data to look at would be the pure GPS elevation data but neither of these two devices allow extraction of such data (I’d need an Edge 205 or Etrex Legend which don’t feature a barometer to obtain this).

The difference in accuracy of the GPS-signal between the two units can be easily observed by plotting out latitude and longitude against each other (figure below) - this is the same descent+climb repeated three times as in the previous figure:

The graphs plotted here consist of an overlay of six times the same track segment (three times down and three times up). The spread on the Vista’s signal is clearly larger than the Edge’s, in particular around the area on the right, indicated by the red dotted circle, which was a section of trail under fairly dense canopy. Note that the Vista did not lose signal here - in other conditions where it would do that, the signal would of course be messed up even more as a number of points would be missing.

How does this translate into distance? In a way, distance is ‘cumulative location’; you’d expect that things will average out a bit, and indeed, if you plot distance versus time for the same 3x repeated track segment, you’ll see the Vista and Edge returning roughly the same graphs:

The slope of this curve is of course the speed, and you can quickly identify the descents, climbs and breaks. Distance is calculated from latitude and longitude using the great circle distance formula as mentioned in another post.

In conclusion, both devices are about equally good in generating elevation and distance data (with the Vista having a calibration issue), but for logging tracks the Edge’s SiRFstar III chip is quite superior. Another unit, the GPSMAP 60Cx seems to combine the best of both worlds, but it comes at a higher price, and in a bulkier package.