Figure 14-1.-Common types of bench mark construction
and application.
and are generally marked to show the elevation
above sea level. When the elevation is not
marked, you can find out what it is by
contacting the government agency that originally
set the BM. Just be sure you give them the
same identification number as the one on
the marker. The type of standard bronze
markers used was discussed in chapter 11 of
this training manual.
BMs may be constructed in several ways.
Figure 14-1, view A, shows brass shaft stocks in
the tops of permanent horizontal control points
(monuments). Sometimes, monuments of this type
are also used for vertical control BMs. Original
BMs may be constructed in the same manner.
When regular BM disks are not available, brass,
not steel, 50-caliber empty shell casings may be
used. The shank of the empty shell casings should
be drilled crosswise and a nail inserted to prevent
its being pulled out or forced out by either
expansion or contraction.
For short lines and a level circuit of a limited
area, any substantial object may be used for
vertical control BMs. The remark in the field notes
should bear the proper identification of the BMs
used.
Figure 14-1, view B, shows a mark like those
commonly used on tops of concrete walls,
foundations, and the like. Lines are chiseled out
with a cold chisel or small star drill and then
marked with paint or keel. The chiseled figures
should be about the same size as the base area of
the rod. Preferably, they should be placed on
some high spot on the surface of the concrete
structure.
A spike may be driven into the root of a tree
or placed higher up on the trunk of the tree when
the limb clearance allows higher rod readings.
Figure 14-2, view A, shows the recommended way
to do this. The rod should be held on the highest
edge of the spike, and the elevation should be
marked on the blazed portion of the tree. Figure
14-2, view B, shows a spike driven on a pole or
post that also represents a BM. Drive the spike
in horizontally on the face of the post in line with
the direction of the level line. For the reading,
hold the rod on the uppermost edge of the spike.
After the elevation has been figured, mark it on
the pole or post for future reference.
Stakes driven into the ground can also be used
as TBMs, especially if no frost is expected before
they are needed. A detailed description of these
points is just as important as one for a monument
station.
In most permanent military installations,
monument BMs are established in a grid system
approximately one-half mile apart throughout
the base to have a ready reference for elevations
of later construction in the station. Generally,
these BMs are fenced to mark their locations. The
fence also serves to protect them from being
accidentally disturbed.
BM systems or level nets consist of a series of
BMs that are established within a prescribed order
of accuracy along closed circuits and are tied to
a datum. These nets are adjusted by computations
that minimize the effects of accidental errors and
are identified as being of a specific order of
accuracy.
In certain areas, TIDAL BENCH MARKS
must be established to obtain the starting
datum plane or to check previously established
elevations. Tidal bench marks are permanent BMs
14-2
