On this hike, I took both a Garmin GPSMap 64S and an older model GPSMap 62S. I had the 62 on in the car to record my mileage driving to the trailhead, but did not turn the 64 on until I arrived and parked.
Below is a slice of the hike shown on Garmin Basecamp, with the 64 in blue and the 62 in red. Both units are set to record data at 3 second intervals.
This map shows a portion of my hike from 45 minutes to 60 minutes into the hike, and on the return from 1 hour, 52 minutes to 2 hours, 5 minutes into the hike. There is an amazing amount of blue line variation on the trip out (this is data from the 64, turned on at the trailhead), despite the fact that the GPS has been on for 45 minutes. The 62 is showing very similar tracks on each leg, and the 64 return data is right with the 64. But that 64 outbound is clearly an outlier.
This is not something most users would ever notice when taking a single GPS receiver on a loop hike, but is much clearer on an out-and-back hike with multiple units. The lesson here is to turn on the GPS receiver when leaving the house. The longer it has been on, the more accurate the data it collects.
