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Home / Total Station

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Total Station

📄 Leica Flexline Manual User Guide.PDF

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A total station (TS) or total station theodolite (TST) is an electronic/optical instrument used for surveying and building construction. It is an electronic transit theodolite integrated with electronic distance measurement (EDM) to measure both vertical and horizontal angles and the slope distance from the instrument to a particular point, and an on-board computer to collect data and perform triangulation calculations.

Robotic or motorized total stations allow the operator to control the instrument from a distance via remote control. This eliminates the need for an assistant staff member as the operator holds the retroreflector and controls the total station from the observed point. These motorized total stations can also be used in automated setups knows as Automated Motorized Total Station (AMTS).

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Function

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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 to 0.5 arc-second. Inexpensive “construction grade” total stations can generally measure angles to 5 or 10 arc-seconds.

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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 with an accuracy of about 1.5 millimeters (0.0049 ft) + 2 parts per million over a distance of up to 1,500 meters (4,900 ft).

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

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Coordinate measurement

The coordinates of an unknown point relative to a known coordinate 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 location, called free stationing.

For this reason, some total stations also have a Global Navigation Satellite System 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.

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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.

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Applications

Total stations are mainly used by land surveyors and civil engineers, either to record features as in topographic surveying or to set out features (such as roads, houses or boundaries). They are also used by archaeologists to record excavations and by police, crime scene investigators, private accident reconstructionists and insurance companies to take measurements of scenes.

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Mining

Total stations are the primary survey instrument used in mining surveying.

A total station is used to record the absolute location of the tunnel walls, ceilings (backs), and floors as the drifts of an underground mine are driven. The recorded data are then downloaded into a CAD program, and compared to the designed layout of the tunnel.

The survey party installs control stations at regular intervals. These are small steel plugs installed in pairs in holes drilled into walls or the back. For wall stations, two plugs are installed in opposite walls, forming a line perpendicular to the drift. For back stations, two plugs are installed in the back, forming a line parallel to the drift.

A set of plugs can be used to locate the total station set up in a drift or tunnel by processing measurements to the plugs by intersection and resection.

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Mechanical and electrical construction

Total stations have become the highest standard for most forms of construction layout.[according to whom?]

They are most often used in the X and Y axis to lay out the locations of penetrations out of the underground utilities into the foundation, between floors of a structure, as well as roofing penetrations.

Because more commercial and industrial construction jobs have become centered around building information modeling (BIM), the coordinates for almost every pipe, conduit, duct and hanger support are available with digital precision.

[clarification needed] The application of communicating a virtual model to a tangible construction potentially eliminates labor costs related to moving poorly measured systems, as well as time spent laying out these systems in the midst of a full blown construction job in progress.[citation needed]

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Meteorology

Meteorologists also use total stations to track weather balloons for determining upper-level winds. With the average ascent rate of the weather balloon known or assumed, the change in azimuth and elevation readings provided by the total station as it tracks the weather balloon over time are used to compute the wind speed and direction at different altitudes. Additionally, the total station is used to track ceiling balloons to determine the height of cloud layers. Such upper-level wind data is often used for aviation weather forecasting and rocket launches.

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Using a Total Station

For those who may wish a more detailed explanation of the use of the total station, the following description may be helpful.

A total station (Fig. 5) is a combination electronic transit and electronic distance measuring device (EDM). With this device, as with a transit and tape, one may determine angles and distances from the instrument to points to be surveyed. With the aid of trigonometry, the angles and distances may be used to calculate the actual positions (x, y, and z or northing, easting and elevation) of surveyed points in absolute terms.

A standard transit is basically a telescope with cross-hairs for sighting a target; the telescope is attached to scales for measuring the angle of rotation of the telescope (normally relative to north as 0 degrees) and the angle of inclination of the telescope (relative to the horizontal as 0 degrees).

After rotating the telescope to aim at a target, one may read the angle of rotation and the angle of inclination from a scale. The electronic transit provides a digital read-out of those angles instead of a scale; it is both more accurate and less prone to errors arising from interpolating between marks on the scale or from mis-recording. The readout is also continuous; so angles can be checked at any time.

The other part of a total station, the electronic distance measuring device or EDM, measures the distance from the instrument to its target. The EDM sends out an infrared beam which is reflected back to the unit, and the unit uses timing measurements to calculate the distance traveled by the beam.

With few exceptions, the EDM requires that the target be highly reflective, and a reflecting prism is normally used as the target. The reflecting prism (Figs. 5 and 6) is a cylindrical device about the diameter of a soft-drink can and about 10 cm. in height; at one end is a glass covering plate and at the other is a truncated cone with a threaded extension. It is normally screwed into a target/bracket on the top of a pole; the pointed tip of the pole is placed on the points to be surveyed.

The total station also includes a simple calculator to figure the locations of points sighted. The calculator can perform the trigonometric functions needed, staring with the angles and distance, to calculate the location of any point sighted.

Many total stations also include data recorders. The raw data (angles and distances) and/or the coordinates of points sighted are recorded, along with some additional information (usually codes to aid in relating the coordinates to the points surveyed).

The data thus recorded can be directly downloaded to a computer at a later time. The use of a data recorder further reduces the potential for error and eliminates the need for a person to record the data in the field.

The determination of angles and distance are essentially separate actions. One aims the telescope with great care first; this is the part of the process with real potential for human error. When the telescope has been aimed, the angles are determined. Only then does one initiate the reading of the distance to the target by the EDM. That takes only a few seconds; the calculations are performed immediately.

The total station is mounted on a tripod and leveled before use. Meanwhile, the prism is mounted on a pole of known height; the mounting bracket includes aids for aiming the instrument. The prism is mounted so that its reflection point is aligned with the center of the pole on which it has been mounted.

Although the tip of the pole is placed on the point to be surveyed, the instrument must be aimed at the prism. So it will calculate the position of the prism, not the point to be surveyed. Since the prism is directly above the tip, the height of the pole may be subtracted to determine the location of the point. That may be done automatically.

(The pole must be held upright, and a bubble level is attached to give the worker holding the pole a check. It is not as easy as one might expect to hold the pole upright, particularly if there is any wind; as a result, multiple readings may be required.

Because of that problem, the sighting method chosen at Pompeii was, if possible, not to begin by sighting on the prism itself but on the tip of the pole where it touched the ground. The angle from north would then be fixed and unaffected by the movement of the pole. Then the aim of the telescope could be raised to the level of the prism, adjusting only the angle of inclination.)

In Pompeii a Topcon total station was used,* and we quickly learned a few features of the equipment. (Mr. Eiteljorg had driven to Charlottesville to learn the idiosyncrasies of the instrument in May, but a malfunctioning battery cut the session short, and a few “simple” or “trivial” processes turned out to be neither simple nor trivial without practice.)

For instance, leveling the total station is more difficult than we had realized (and spongy soil is devastating, since the instrument is naturally unstable if its support is), and it depends upon accurate adjustment of the bubble level built into the instrument. We also learned that datum points were more difficult to select than expected, since they had to be repeatable; that is, we had to be able to find them again and again with absolute accuracy – this year and next.

When the instrument is set up and turned on, it sets itself to be pointing to zero degrees (north) when power is first supplied. The user must then re-set the instrument to zero degrees when it is actually pointing north; we learned that there is no secondary battery for back-up. When the battery dies, the instrument must be re-set for zero degrees.

Fortunately, these lessons came in the first day or two, and we had no more surprises. (One problem was on-going, however. There are two adjustment knobs for rotating within the horizontal plane. One rotates the telescope to make a sighting, with the readout of angles displaying changes. The other, however, permits the user to rotate the entire instrument and to keep the current angle unchanged during the process.

That effectively re-orients the zero or north setting. That can be very helpful when setting up or re-setting the instrument, but, of course, it can be devastating if one makes that adjustment by mistake and thereby changes the north setting. This particular instrument was designed in such a way that it was too easy to re-set the instrument when one only wanted to make a sighting.)

Since we were dealing with standing architecture, the prism pole was often inadequate for our work. The pole is designed to be placed on the survey point in a vertical position; it cannot be placed on a point on the face of a wall. In fact, a prism pole can rarely be placed against the face of a wall because of the bulk of the prism, the pole, and the target to which the prism is attached.

We devised two alternate methods for dealing with points on a wall. One involved the use of reflecting tape instead of the prism. Since we were working at such short range, bicycle reflecting tape would reflect the infrared beam well enough to permit the EDM to make a reading.

It was a bit slower than using the prism, but it worked. (Bicycle reflectors worked, but their back surfaces were not in the same plane as their reflecting surfaces; so the measurements they generated were from a point too near the face of the reflector by a few millimeters.)

The other method for dealing with points on a wall involved the use of the prism without its pole and target. We could simply position the prism against the point on the wall to be surveyed and take the shot. However, the prism is designed to work on the pole – to give a reading to the center of the pole rather than the back of the prism.

In this case, that meant that the prism gave a reading some 13 mm. behind the backmost point of the prism housing. We fashioned a shim with a 13 mm. thickness to attach to the back of one of the prisms (fortunately, we had two prisms). Then the prism could be placed against the point in question and a reading made.

The only problem – and the reason reflecting tape was sometimes preferred – was that the prism could not always be placed in a corner and sometimes could not be placed correctly while continuing to face the transit and EDM for reflecting the infrared beam.

When using reflecting tape or a prism without a pole, the tape or prism hides the point to be surveyed. So we aimed the telescope at the point to be surveyed before interposing either tape or prism and maintained the aim of the instrument while putting the tape or prism in position.

That reduced the possible error for angular measurement. In the case of the prism, after it was put into position, the transit operator would direct the person holding the prism so that it was aimed directly back at the instrument. (That would have required a walkie-talkie had we been working in a larger area.)

The survey information was recorded by hand, and the data were then entered into the AutoCAD model. We were able to use the data directly, no matter where the machine had been set up for a given session, thanks to an AutoCAD feature called the user coordinate system. Using that AutoCAD feature, each set of data could be entered accurately, regardless of the transit set-up point. (It is unnecessary to describe that process here, but a complete description is available from CSA.)

This process is not necessary if a data collector with the most modern of capabilities is available. (See The Ustica Excavations – A Total Station, AutoCAD at Work.) The data collector can automatically orient all new points to a pre-existing set of survey coordinates.

But the process we developed worked well and easily, and it gave us a check of our own accuracy as we manipulated the model and created the alternate user coordinate systems. We also put it into practice in a way designed to make it obvious to the user if he was not entering the data correctly. More important, we can use equipment with various levels of sophistication.

*The instrument used measures to within 5 seconds for vertical and horizontal angles. The electronic distance measuring device (EDM) measures to within 5 mm. and 3 parts per million; so the error will be no more than than the sum of 5 mm. and 3 parts per million of the measured distance from instrument to prism.

Instruments are available which measure to tighter tolerances, but for short-range work such as we were doing at Pompeii – nothing we measured was more than 100 m. from the instrument and most of the work was within 25 m. – the accuracy of the transit and EDM were more than sufficient. The EDM error at 100 m. would be no more than 5 mm. (3 parts per million at 100 m. adds less than a mm. to the maximum error).

At 100 m. an error of 5 seconds in an angular reading would make only a 2 mm. error in position; at 40 m., the angular error drops below 1 mm. For the vast majority of the work, then, the maximum theoretical error was the error of the EDM, 5 mm. Of course, human error may add to machine error.

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