ETA - calculation of estimated arrival time on Oregon. Distance accuracy?

[Duvivis]

Prepared a walking route on basecamp and put onto Oregon GPS. Then when navigating the route one can see on the Trip computer page, ETA at destination. How is this calculated? Does it just take average speed to that point, assume the same speed to the destination and calculates ETA based on distance remaining? Is there any allowance for uphill sections - eg using Naismiths rule which would mean slower speed uphill? (Route has contour heights on it so the information is available.)
How accurate is the recorded Track distance? Recording intervals can be changed to a shorter time or shorter distance. Standard interval seems to be 30 secs or 100 metres. What error would occur using these intervals? - assuming wild walking route with many twists and turns and ascents.-
I have a Suunto watch which also records distance by GPS and it is frequently 10% or more different to the Oregon recorded distances.

 

[Nuvi-Nebie]

Does it just take average speed to that point, assume the same speed to the destination and calculates ETA based on distance remaining?

I would say the answer is yes, I think it is very unlikely that the gradient is taken into consideration because you would also need information on the walker's ability to handle the gradient, e.g. age, sex, weight, fitness etc.

Reducing the time / distance between Track points will theoretically increase distance accuracy, however there is always error in each co-ordinate measurement, so there is no point in taking a measurement every 1 metre if that measurement has a 10 metre positional inaccuracy