Basics of Total Station7.1 IntroductionThe most commonly used surveying instrument today is the total station (Fig. 7.1). A total station is a combination of an electronic theodolite, an electronic distance measuring device (EDM) and a microprocessor with memory unit. The electronic digital theodolite, first introduced in the late 1960s by Carl Zeiss Inc., helped to set the stage for modem field data collection and processing. With this device, one can determine angles and distances from the instrument to the 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) and the angle of inclination of the telescope (relative to the horizontal as 0). After rotating the telescope to aim at a target, one can read the angle of rotation and the angle of inclination from a reference point. All total stations have an electronic distance measuring device (EDM) and electronic angle scanning. The coded scales of the horizontal and vertical circles are scanned electronically, and then the angles and distances are displayed digitally. The horizontal distance, the height difference and the coordinates are calculated automatically and all measurements and additional information can be recorded.The electronic transit provides a digital read-out of those angles instead of a scale, which is more accurate and less prone to errors arising from interpolating between marks on the scale or from recording error. The read-out is also continuous, and hence, angles can be checked at any time.The EDM instrument transmits an infrared beam, which is reflected back to the unit with the help of a prism (for total internal reflection), and the EDM uses timing measurements to calculate the distance travelled by the beam. With few exceptions, the EDM instrument requires that the target be highly reflective, and a reflecting prism is normally used as the target.

Most of the total stations include data recorders. The raw data (angles and distances) and 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 recording errors and eliminates the need for a person to record the data in the field.The determination of angles and distances are essentially separate actions. One aims at the telescope with great care first. This is the part of the process with a real potential for human error. When the telescope has been aimed, the angles are determined. The observer then initiates the reading of the distance to the target by the EDM. This takes only a few seconds and the calculations are performed immediately.At present, total station is widely used in many survey sites .Sometimes it is not fully used since the users are unaware of its full operational capability. Total station used for levelling comes under the classification indirect levelling. It can maintain considerable accuracy and is hence used for many public works such as construction of roads, airports and harbours.The total station is mounted on a tripod and levelled before use (Fig. 7.5). 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 (Fig. 7.6). 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 and 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 can be done automatically. The pole must be held upright, and a bubble level attached to give the technician holding the pole a check (Fig. 7.7). It is not as easy as one might expect to hold the pole upright, particularly if there is any wind, and as a result, multiple readings may be required. Because of this problem, the sighting method chosen at such occasions is, 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.

Fig. 7.5 Wooden and aluminium tripods for total station

Fig. 7.6 Typical standard prism and sight

Fig. 7.7 Typical standard rod (with bubble level)7.2 Advantages of Total StationThe advantages of total station include:1. Quick setting of the instrument on the tripod using laser plummet2. On-board area computation programme to compute the area of the field3. Greater accuracy in area computation because of the possibility of taking arcs in area computation4. Graphical view of plots and land for quick visualization5. Coding to do automated mapping. As soon as the field jobs are finished, the map of the area with dimensions is ready after data transfer6. Enormous plotting and area computation at any user required scale7. Integration of database (exporting map to GIS packages)8. Automation of old maps9. Full GIS creation (using Mapinfo software)10. Local language support

7.3 Disadvantages of Total Station1. Their use does not provide hard copies of field notes. Hence, it may be difficult for the surveyor to look over and check the work while surveying.2. For an overall check of the survey, it will be necessary to return to the office and prepare the drawings using appropriate software.3. They should not be used for observations of the sun, unless special filters, such as the Troelofs prism, are used. If not, the EDM part of the instrument will be damaged.4. The instrument is costly, and for conducting surveys using total station, skilled personnel are required.The less expensive unit with a single prism reflector can measure distances up to 1000 m. Those in the higher price range are capable of measuring distances up to 2000 m, when single prisms are used. The accuracies of measurements with the less expensive instruments probably run about 6(5 mm 1 5 ppm) and the expensive total stations can run about 6(1 mm 1 1 ppm).----------------------------------------------------------------------------------------------------------------Conventional surveying has used analog methods of recording data. The present trend is to introduce digital surveying equipment into the field for a fully digitized work. Electronic total stations now perform the fastest digital data collection methods. Total stations have drastically increased the amount of topographic data that can be collected during a day. The method is well suited for topographic surveys in urban landscapes and in huge construction sites. Modern total stations are also programmed for construction stakeout and highway centreline surveys. Total stations have made trigonometric levels as accurate as the different differential level techniques in areas possessing large relief landforms. These instruments can quickly transfer 3D coordinates and are capable of storing unique mapping feature codes and other parameters. In the past, this could only be recorded on paper media such as field books. One of the best features of the total station is the ability to download data directly into a computer without human errors.Before the introduction of total station, a plane table has long been regarded as the best way to map a small area. The output of a plane table is just a low precision analog drawing. Since a map is drawn directly on a sheet in a fixed scale, there was no way to improve the quality.The advent of the total station survey has made it possible to accurately gather enormous amounts of survey measurements quickly. Even though total stations have been in use for more than 20 years, they are only now beginning to become popular among the surveying and engineering community. Over the last 10 years, total stations and data collectors have become common field equipment.In the early 1980s, surveying instrument manufacturers introduced what has become a true total station, redefining the term by creating an entirely electronic instrument. The read out on the display panels of a total station and the readout from the EDM are in a digital form. This feature eliminated reading errors, which can occur while using an optical theodolite. Also, with the introduction of the electronic theodolite came the electronic data collector, minimizing both the reading and writing errors. Since the data collector can be interfaced directly to a computer, errors which occur in transferring the field information from the field book to the computer are eliminated. At this point, one can measure a distance to a suitable range with an accuracy of better than 5 mm (plus 1 ppm), and angles can be turned with the accuracy of 0.5 sec, all accomplished electronically.A vast increase in productivity resulted due to the introduction of this modern electronic equipment. In most land surveying situations, the normal crew size can be reduced to two when equipped with an electronic total station. Since the data acquisition time is so fast, in some situations, three men are warranted when it is possible to utilize two prism poles. This often results in an overall reduction in man-hours spent on the job

9.2 Fundamental Parameters of Total StationThe fundamental parameters of the total stations are the parameters for calculation and the correction factors and constants.9.2.1 Parameters for CalculationA total station is a digital theodolite with an EDM and a microprocessor. The theodolite measures the horizontal angle (Hz) and the vertical angle (V) of the line of sight from the center of the total station to the center of a target on a point to measure. The center of the total station is at the intersection of the rotation axes of the horizontal and vertical circles. The center of a target is at the intersection of the axis of the centering rod and the axis of tilting. The EDM measures the slope distance between the center of the total station and the center of a prism (not the center of the target). The CPU calculates the coordinates (E, N, H) in a rectangular system of the point under the target, with reference to the total station coordinates (E0, N0, H0) using the measured polar coordinates (SD, Hz, V), instrument height, target height and several correction factors and constants (see Fig. 9.2).9.2.2 Correction Factors and ConstantsThe scale factor (in ppm) is a sum of atmospheric correction (D1 in ppm), in reduction to mean sea level (D2 in ppm) and the projection scale factor (D3 in ppm). The atmospheric correction factor is a function of temperature, atmospheric pressure and humidity. Input of atmospheric correction is usually required. Most user manuals have diagrams to estimate atmospheric corrections from the three parameters. New total stations can calculate atmospheric corrections. Using the formulae narrated below, the atmospheric correction can be calculated very easily. In any case, a thermometer and barometer are necessary to estimate atmospheric correction.

where, D1 = atmospheric correction in ppmP = atmospheric pressure (hPa)t = temperature (C) at the time of observationh = relative humidity (%)a = 1/273.16

Reduction to mean sea level (D2) and projection scale factor (D3) are necessary to map in scale on the earths spheroid surface at a particular geographic locality. If site accuracies are considered rather than geographic accuracy, reduction to mean sea level and projection scale factor need not be considered. Practically, the amounts of correction derived from these factors are less than 100 ppm.

The prism constant is to be added to the distance measured by the EDM. The path of the laser between the source in a total station and the optical center of a prism (reflector, target) (Fig. 9.3) is not the same as the true distance. The prism constant indicates the distance in millimeter to be added to (positive) or subtracted from (negative) the measured distance. Typically, the constant ranges from 0 to 50 mm. The mistake in setting these consonants results in much larger error than other factors. For example, 0 nun Leica prism constant is not applicable for the Sokkia total station.

Fig. 9.3 Different prisms used for total station surveysFor exact calculation of the distance and the height difference, the curvature of the earths surface and the refractive index of the atmosphere are to be taken into account. The users manual is to be referred for the reduction formulae employed for each model total station. The CPU is programmed to use these formulae to give coordinates of measured points. When one recalculates rectangular coordinates from polar coordinates, one should use these formulae, although the difference is not very big.

9.3 Precautions to be Taken While Using a Total StationThe following precautions need to be taken while using a total station:1. Always carry a total station in a locked hard case even for a very short distance. Take the total station out of the hard case only for fixing it firmly on a tripod for taking observations.2. Do not move or carry a tripod with the total station fixed on it, except for centering.3. Use both hands to hold the total station handle.4. Never release the handle before the total station is fixed with the tripods fixing screw.5. Set up the tripod as stable as possible.6. Always keep the top of the tripod, the bottom and top of the tribrach and the bottom of the total station clean and away from any shock and impact.7. Take maximum care when the tribrach is removed from the total station.8. Do not make the total station wet.9.4 Field EquipmentModern electronic survey equipment requires surveyors to be more maintenance conscious than they were in the past. They have to take care about power sources, downloading data, and the integrity of data, including whether or not the instruments and accessories are accurately adjusted and in good form. When setting up a crew to work with a total station and a data collector, it is helpful to supply the party chief with a checklist to help the crew maintain its assigned equipment and handle the collected data upon returning to the office. It is also important that each crew should be supplied with all necessary equipment and supplies. These should be stored in an organized and easily accessible manner.Preparing an equipment list carefully will assure the survey crew (two-person crew, consisting of a party chief/rod person and a notekeeper/instrument person) a sufficient equipment inventory to meet the general needs of boundary, layout, and topographic surveys. This procedure will confine what is needed to maximize productivity when using a total station with a data collector.The minimum equipment inventory required is as follows:1. Total station set 1. Total station instrument in a hard case2. Battery charger3. Extra batteries4. Memory module/card, serial cable5. Rain cover6. User manuals7. Tripod8. Tape measure2. Prism set 1. Prism2. Prism holder3. Centering rod3. Back sight set 1. Prism2. Prism holder3. Prism carrier (to be fixed on tribrach, with optical/laser plummet)4. Tribrach (to exchange prism carrier and total station)4. Data processing> 1. laptop computer with serial port or USB port2. Serial cable or USB-serial adaptor3. Terminal application4. Application programme: MS Excel,Adobe illustrator, Coordinate Converter, etc.5. Data backup device and media (zip, memory card, etc.)5. Survey tools 1. Stakes, nails, paint, marker2. Hammer3. Thermometer, barometer/altimeter4. A pair of radio (with hand-free head sets)5. Clipboard, field note, pen6. Compass7. GPSThe total station set, one set of the prism set, and survey tools are indispensable throughout. The data can be processed back in the office, but it is better to arrange all data and make a backup as soon as possible in the field itself, before forgetting mistakes and details. With one back sight set, measurement of a back sight and traverse will be very easy and accurate. To occupy a new point, simply exchange the part above the tribrach. The center position of the total station and the target center above the tribrach is the same after the exchange.With this equipment inventory, a two-person field crew will be able to handle most of the survey tasks that are routinely encountered in day-to-day operations. An additional tripod, plumbing pole, carrier, tribrach and reflector would give the crew even greater flexibility, and allow them to handle many projects more efficiently. It is also helpful for the field crew to have a convenient place to store their assigned equipment. The crews should be equipped with briefcase-sized cases that will hold three tribrachs, four reflectors with holders, three carriers and four target plates. A hard camera case or pistol case works well for this purpose. With all the components stored in one place, it makes the inventory of the equipment easy and reduces the chance of the equipment being left at the job site. This also allows for proper equipment maintenance.