Put in gutter spikes at the four corners of the test unit, which will serve as reference points for the four sections of the test unit: northeast, northwest, southeast and southwest.
Tie a string to the spike that is the highest elevation of the four corners. In cases where the entire unit is at a consistent elevation, use the southwest corner as the primary spike. Tie the string to the base of the gutter spike, right at ground level. On any floor plans or profile maps, designate the depth of this primary spike as zero.
Extend the string with the line level attached. Record the difference in elevation between each respective corner and the spike at the highest level. These initial depths then serve as reference points for all future measurements and will be designated on all floor plans and maps by the difference in depth in relation to the primary spike.
Periodically measure the depth of the test unit's floor using the line level as describe in step 2. If each corner was initially 5cm deeper than the primary spike and the depth for each layer is set at 10cm, then the first level is completely excavated when the floor at the primary spike is 10cm and all other corners reach the depth of 15cm.