Nevada State Parks have the most numerically precise characterisation of hiking trails I have yet seen. For example, see this map and list for Valley of Fire State Park or this one for Beaver Dam State Park. For each trail, they report numbers on length, typical grade, maximum grade, typical cross slope, maximum cross slope, typical tread width, minimum clearance width, surface firmness (typical and worst), and surface stability (typical and worst). If it wasn't for omitting measurement uncertainty I'd even describe it as a scientific measurement.
Source: Valley of Fire trail map
Most of those properties are reasonably intuitive to understand (although it was the first time I saw a table like this). The legend gives some attempt at visualising that all but two trails are "not firm" and "not stable", but doesn't actually define what it means. The private company Beneficial Designs, which has made those signs, sells the Rotational Penetrometer for $4895, and claims that numerical estimates of surface firmness and stability are "required in trail signage", and states that more information is available on request, but doesn't actually define what the numbers on surface firmness and stability mean.