First a wooden frame, measuring 70.7 x 70.7 cm on the
inside (0.5 m2), is
placed on the ground and pinned into place with aluminum pins to keep it
from shifting. A grid is marked on the frame and the stick that lies
across it in this photo. At each of the 25 points on the grid, the
distance from the top of the frame to the ground surface is measured and
recorded. This serves as our reference for the ground surface as
the hole is dug. As each layer is removed, the depth from the top
of the frame is measured again so that we can calculate a precise volume
for that layer. Care is taken to make the sides of the hole very
straight and square, and to avoid leaning on the frame, possibly shifting
our depth reference.
Because we
need to calculate soil volume separate from rock volume, all large rocks
in the hole must be accounted for. Rocks from a lower layer that
protrude into the excavated layer are left in place and their presence
noted. Rocks protruding from the side of the hole are also measured
and noted. Rocks within a layer can simply be weighed (and volume
calculated from mass and their known density), or measured in 3 dimensions
if they are too large to weigh. The depth measurements can get rather
complicated because of all the special notations for rocks, so all the
data is analyzed in a computer program to calculate the volumes for soil
and rock in each layer.