@LGoSo I think there would be three points to consider here:
As I understand it, the model is a Rolson 50799. The manufacturer doesn’t specify its accuracy, but in that price range, I believe it will be between 0.1% and 0.5% (meaning the margin of error would be between 0.5 m and 2.5 m over a distance of 500 m).
For such uneven terrain, a wheel with a circumference of around 1 m might not be the most suitable option; for these cases, measuring wheels with a circumference of around 2 m are used.
If the terrain has a noticeable slope, as seems to be the case, we wouldn’t just be assessing the accuracy of the watch’s GNSS, but also that of its altimeter. (In reality, the distance should be compared against the result of using the ‘3D Distance’ option on the watch, which I assume Suunto uses by default since it isn’t available as a toggle.)