Civil Engineering Reference
In-Depth Information
station, which allows the total station to automatically track the prism. The instrument
person must remain at the total station, while a second person walks around making
the locations. The advantage of a tracking total station is that the telescope tracks the
prism as it moves from one point to the next. This makes locations very fast because
the person at the total station does not have to aim and focus the telescope for each
measurement. It is very useful for locating features where it is necessary to take many
shots such as the edge of pavement and fences. For these locations, a person having a
prism mounted on a prism pole simply walks from one point to the next and signals
the instrument person when to take the shot. Measurements can proceed almost as
fast as a person can walk. Losing lock is not a problem because the person at the total
station can simply aim the total station at a point near the prism and it will automati-
cally lock onto the prism again. Tracking total stations are also useful for finding a
prism in dense undergrowth. There are times when the total station will lock onto the
prism when it is barely visible when looking through the telescope.
Many servo driven total stations do not have traditional horizontal and verti-
cal clamps to lock the telescope onto the target, only horizontal and vertical tan-
gent screws. The instrument person can simply move the telescope by hand in the
direction of the target. Once the telescope is pointed near the target, the tangent
screws can be used to fine tune the direction. The principle is similar to a friction
clutch in that turning the total station body and telescope by hand will overcome
the tangent screw clamping force. Some total stations have two speed tangent
screws which alternately allow course and fine adjustment.
Unlike transits and theodolites, all total stations require batteries in order for the
electronics to function. Some total stations have onboard batteries that fit into the
instrument. For other total stations, external batteries are required. Standard total
stations do not require much power to operate so they often use small onboard bat-
teries. Robotic total stations which have servo motors use quite a lot of power so
the batteries must have much greater capacity. Often, these batteries are externally
mounted and connected to the total station through a cable. Some total stations
have battery packs which can hold several batteries at one time. This is particularly
useful when a servo driven total station must turn many angles and measure many
distances during a day. Most of the batteries in use are rechargeable. Some of the
chargers operate from a conventional 115 V AC outlet and others from a 12 V vehi-
cle DC system. The DC chargers are most convenient as they allow batteries to be
charged in the survey vehicle in the field. Survey crews may have multiple batteries
so that one or more will always be charging in the vehicle while the crew is using
the total station. Some chargers and/or batteries may have temperature limitations
for charging so extremely cold or hot weather may prevent charging in the vehicle.
5.6 Tripods and Tribrachs
Tripods are used to support and position surveying instruments and prisms accu-
rately over a point. Tripods are also used to support GPS antennas. A tripod con-
