Hello Bose Yu..Good Evening.
Regarding on your site,The stone on your pictures is just a secondary marker.A perfect Hole or Spoke can be decipher Measure the Diameter and the Depth of the Hole.Use a Millimeter.
Then add to get the total. Example if the Diameter is 1 inch and the depth is 1 inch. 1 inch = 25.4 millimeter and so the Total is 50.8 millimeter..use a compass if the Hole is facing upward
then use a 90 degrees Bearing. If the Hole is on the side use also a compass.
Regarding on your Area,Prior on your digging make a survey ask for the Bench Mark.The Tower the tallest Tree or the Big Rocks that cannot be displaced by Human or Natural Calamities.
Nowaday actual survey by the Japanese team is observe.Like in the area of Calinan Davao City,to Upper Lacson I have seen an actual Japanese Woman Engineer on the site who did an
actual survey she have an Digital Transit with a team.In the area they put a Red Mark.
Back on your Area look for the Remark Cocunut Tree..Always use a compass...
Good Luck and Happy Hunting..