trench maps

[waynepugh]

hi everybody,

just getting started and stuck at the moment, could anybody please help me by telling me what the BM references on a trench map are?

thank you

[johnreed]

Survey BMs are survey marks made by the Royal Engineers surveyors to record the height above the the Ordnance Datum. If the exact height of one BM is known the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling. Most BM are located on buildings or other semi-permanent fixtures. These also help in the accuracy in the fixation of the artillery Gun Batteries which in turn made for more accuracy in the fire of the battery. Further accuracy was improved with the use of Bearing Picket these are actual pegs put in the ground by the Royal Engineers Survey parties with a label attached giving the actual Grid Reference of the peg, the height of the peg and two or more bearing points to align the Director.

John

[Smithmaps]

Survey BMs are survey marks made by the Royal Engineers surveyors to record the height above the the Ordnance Datum. If the exact height of one BM is known the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling. Most BM are located on buildings or other semi-permanent fixtures. These also help in the accuracy in the fixation of the artillery Gun Batteries which in turn made for more accuracy in the fire of the battery. Further accuracy was improved with the use of Bearing Picket these are actual pegs put in the ground by the Royal Engineers Survey parties with a label attached giving the actual Grid Reference of the peg, the height of the peg and two or more bearing points to align the Director.

John

Well they say you learn something every day!

Guy