Last weekend, I did a mountain hike with Mrs. F and others.
I recorded it on my Suunto watch, and Mrs F. on both a Garmin watch and a Garmin bike computer.
The watch died early, but the bike computer had been set to a power-saving mode which reduces the number of points recorded.
I did the full route which came out at 17k, which is close to the advertised 16k considering there was a bit of bumbling about going on.
Mrs F's group stopped a bit early on account of running out of time.
But her tracklog only showed 11k, which seemed too low, considering the final bit they skipped was only about 3k.
I chopped my tracklog to the same finish point, and compared the distances.
Mine was 14.3k, hers was 11k.
Plotting them both together, looking at them zoomed out, they look identical.
But mine had 15000 points, and hers only 5000.
The route was very ziggly-zaggly, but nonetheless I'm surprised by the magnitude of error caused by the downsampling.
Both seemed to capture the ziggles and zaggles to a reasonable extent at least visually.
Basically, about 3k difference over a 14k route.