All volume calculations in the TIN program are based on two simple mathematical formulas for a triangular prism. Tin versus level volume calculation Every triangle is divided in two parts above and below horizontal plane.
Every triangle is trimmed by face plane.
The part inside is divided in two parts above and below face plane. Every triangle from TIN1 is projected to TIN2. Newly created TIN1' matches TIN1 x, y coordinates. Z values are calculated by interpolation using TIN2. The same procedure is applied to the class TIN1' above TIN1.