Attached is a new model I am evaluating for the section of wood on trees below high side ground level and the ground interface. This section of tree is a frustum with a slant base. For some of the big redwoods this piece of wood is substantial, 5,000 cubic feet or more sometimes. It should be accurately modeled in order to estimate the total volume of the tree. Up until now I was using a 1/2 wedge model, which does not really reflect the shape of the trunk form. The formula used is that for a "slanted cone". The formula and derivation can be found at:

https://math.stackexchange.com/question ... -a-frustum

see attachment for further details. I had to rework the formula a little to work for a slanted frustum.