Total station

Robotic or motorized total stations allow the operator to control the instrument from a distance via remote control. In theory, this eliminates the need for an assistant staff member, as the operator holds the retroreflector and controls the total station from the observed point. In practice, however, an assistant surveyor is often needed when the surveying is being conducted in busy areas such as on a public carriageway or construction site. This is to prevent people from disrupting the total station as they walk past, which would necessitate resetting the tripod and re-establishing a baseline. Additionally, an assistant surveyor discourages opportunistic theft, which is not uncommon due to the value of the instrument. If all else fails, most total stations have serial numbers. In the United States the National Society of Professional Surveyors hosts a registry of stolen equipment which can be checked by institutions that service surveying equipment to prevent stolen instruments from circulating. These motorized total stations can also be used in automated setups known as "automated motorized total station".

Function

Angle measurement

Most total station instruments measure angles by means of electro-optical scanning of extremely precise digital bar-codes etched on rotating glass cylinders or discs within the instrument. The best quality total stations are capable of measuring angles within a standard deviation of 0.5 arc-seconds. Inexpensive "construction grade" total stations can generally measure angles within standard deviations of 5 or 10 arc-seconds.

Angle measurement is typically performed by the operator first occupying a known point, aiming the head of the instrument at a target or prism which exists at either another known point or along an azimuth, which is to be held as a backsight — sighting with the reticle inside the eyepiece — then holding that line as an angle of 00°00‘̣00“̣. The operator then will turn the head of the instrument at a target or feature that is to be observed as a foresight and record the AR (Angle Right) from the backsight measured by the instrument in which a horizontal angle is produced. Angular error in the instrument as well as collimation error can be mitigated in many total stations by performing a set collection. This entails witnessing any angles recorded an equal number of times in both "direct" and "reverse" modes by sighting the observed backsight and foresights with the instrument facing the targets normally as well as with the scope flipped or "plunged" 180°. The recorded sets of angles taken from each target will be averaged together and a mean angle will be generated.

Distance measurement

Measurement of distance is accomplished with a modulated infrared carrier signal, generated by a small solid-state emitter within the instrument's optical path, and reflected by a prism reflector or the object under survey. The modulation pattern in the returning signal is read and interpreted by the computer in the total station. The distance is determined by emitting and receiving multiple frequencies, and determining the integer number of wavelengths to the target for each frequency. Most total stations use purpose-built glass prism (surveying) reflectors for the EDM signal. A typical total station can measure distances up to 1,500 meters (4,900 ft) with an accuracy of about 1.5 millimeters (0.059 in) ± 2 parts per million.

Reflectorless total stations can measure distances to any object that is reasonably light in color, up to a few hundred meters.

Coordinate measurement

The coordinates of an unknown point relative to a point with known coordinates can be determined using the total station as long as a direct line of sight can be established between the two points. Angles and distances are measured from the total station to points under survey, and the coordinates (X, Y, and Z; or easting, northing, and elevation) of surveyed points relative to the total station position are calculated using trigonometry and triangulation.

To determine an absolute location, a total station requires line of sight observations and can be set up over a known point or with line of sight to 2 or more points with known locations, called free stationing.

For this reason, some total stations also have a global navigation satellite system (GNSS) receiver and do not require a direct line of sight to determine coordinates. However, GNSS measurements may require longer occupation periods and offer relatively poor accuracy in the vertical axis.

Data processing

Some models include internal electronic data storage to record distance, horizontal angle, and vertical angle measured, while other models are equipped to write these measurements to an external data collector, such as a hand-held computer.

When data is downloaded from a total station onto a computer, application software can be used to compute results and generate a map of the surveyed area. The newest generation of total stations can also show the map on the touch-screen of the instrument immediately after measuring the points.