My understanding is that for a large pile (like 0.5 acre base) that even if the base plane is pretty flat like say 5-feet slope over 200 ft (long side of pile) and maybe 2 or less feet across 100 feet perpendicular to that, it’s most accurate to use triangulation with lots of baseplane boundary points versus linear fit. Linear fit to me seems only appropriate if the pile is on extremely flat ground (asphault or ground previously graded to be perfectly flat), or for a way to do a cut/fill analysis where the baseplane boundary point elevations represent a very flat surface (or very flat surface with linear slope in one direction) and the terrain is “rough” within the boundary and you want to see what you need in total to add/remove in total without calculating volumes of every pile/hill within the baseplane?
Is that about right?
If it is “pretty” flat it’s not going to make a whole lot of difference either way but if it is a constant slope linear is just fine. Triangulated is for scenarios where there are larger piles with a large grade break or multiples. You would want to place a point at the top and bottom of these features. This is why I always have the elevation layer on when digitizing. It make it very evident where those breaks are, especially if you dial in the elevation range slider to isolate to the elevations of the specific stockpile.
On a rather large pile of about 8000 cubic yards (25 feet tall at the peak) with 5 feet of slope the long way and 2 feet of slope along the shorter width, triangulation produces about 2.5% less volume than linear fit in a job I’m working on. There isn’t any significant cut volume on either (triangulation is < 1 cubic yard and linear fit is < 6 cubic yards of cut) so I’ve got the base plane boundary nice and tight with the boundary points not sitting on any small holes/dips/mounds.
My thinking is triangulation still has to be more accurate as the change in slope is rarely perfectly uniform, it can be slightly concave or convex or have slight undulations. It seems to me the linear fit is less accurate in most cases except a perfectly flat base plane. But as you say the flatter it is the less difference. 2.5% is not insignificant though. I dropped pins all around the base plane boundary to make sure no boundary points were sitting in a hole or on top of a small mound that wasn’t easily distinguishable in the surface model map.
It seems to me the linear fit is best for a cut/fill analysis where all the boundary points are sitting at the target grade elevation (you could use as few as 4 boundary points for a square/rectangular area.) Where triangulation seems it should always be best for measuring a stockpile volume since it’s essentially interpolating the base plane using more data (triangular slices). So triangulation can accommodate slight undulations in the base plane instead of potentially overestimating the uniformity of the slope like the linear fit would do.
It’s kind of funny I just thought of it like trying to measure the volume of cheese on a pizza. If you measured the amount of cheese on 12 slices that’s going to be more accurate than assuming the cheese is the exact same thickness across the entire pizza.