Civil Engineering Reference
In-Depth Information
enough to be used for centering the tribrach over a point, the instrument level
must always be used for leveling the instrument. This means that each time an
instrument is set on a tribrach, the instrument person must fine tune the tribrach
by adjusting the tribrach leveling screws. For some instruments, the level is so
precise that the slightest amount of turn on a tribrach screw is enough to cause
the instrument to go off level. If you are having trouble centering the instrument
bubble, one method is to turn the screw a tiny bit then remove the hand from
the screw and wait a few seconds for the bubble to settle down. Sometimes, the
weight of the hand or pressure from the fingers is enough to disturb the level.
When leveling an instrument it is necessary to turn the instrument 180°, check the
bubble a second time and, if it is off to split the difference. If, after leveling the
instrument, the tribrach level is seen to be off, the tribrach bubble will need to be
adjusted.
Total stations and theodolites have two levels positioned 90° apart. One of the
levels is parallel to the telescope and the other level is 90° to the telescope. The
best way to level the instrument bubble is to align the level bubble parallel to two
of the tribrach leveling screws. One of the two screws can then be turned to center
the bubble. The third screw will then be turned to center the other bubble. This
method allows each level to be adjusted without affecting the other level. If you
find that, when you are adjusting one bubble and the second bubble moves, then
the level is not quite parallel to the two tribrach screws.
5.8 Corner Cube Prisms
Prisms are used to reflect light emitted from the total station EDM back to the
total station so that the distance between the total station and the prism can be
measured. Figure
5.9
shows a prism on a tripod. The prisms used for surveying
are
Corner Cube Prisms
. A unique feature of corner cube prisms is that even if
the prism is not perfectly aligned with the light source (EDM) it will reflect the
light beam directly back to the source. All prisms have a
Prism Offset
, sometimes
called a
Prism Constant
. The prism offset is a distance correction that must be
applied to all slope distances measured with a particular prism.
Light from the EDM has to travel on a path into and out of the prism in order
to be reflected back to the total station and this affects the prism offset. This can
be seen in Fig.
5.10
. The speed of light through the air and the glass of a prism
is different so this is an additional correction that must be made. The mounting
point of a prism in relation to the plumb line of the prism holder also affects the
prism offset so this must be taken into consideration when calculating the distance
between the prism and total station. This is shown in Fig.
5.10
as the vertical axis.
Prism manufacturers provide prisms with different offsets. Some prism holders
allow the offset to be changed. Normally, the prism offset is programmed into the
total station or data collector software so that distances are automatically adjusted
for the offset.
