1 Answer

Bench mark (BM) is a permanent elevation reference point.  Can be refered to later to check on your notes if something doesn't jive, or layout other elevations later.  In any kind  of surveying establish SEVERAL BM's.  Also if direction is involved establish a minium of 2 direction ref's to refer to later.  Should always have more reference points than just the minimal.  Why?

Old joke about surveyors and equipment operators:

If lost in the desert, jungle, Greenland whever,  drive a reference stake in the ground, very neatly label it, tie a colorful ribbon around it so it can be seen by EVERYBODY and just wait.  In a little while a halfwit dozer operator will come along and knock it over.  You just been saved. 

P.S.  Most professiona lequipment operators are not hafwits.  They're usually pretty smart.