Electronic Theodolites, Electronic Tacheometers, Total Stations and Data Collectors

ELECTRONIC THEODOLITES, ELECTRONIC TACHEOMETERS, TOTAL STATIONS & DATA COLLECTORS Electronic theodolites operate in a manner similar to that of optical theodolites or even vernier transits. Angle readouts are to 1 second, with accuracies from 0.5 to 10 seconds; digital readouts (LED or LCD) eliminate the guessing and interpolation of circle and micrometer settings associated with scale and micrometer theodolites. Electronic theodolites have zero-set buttons for quick instrument orientation after the backsight has been set; horizontal angles can be turned left or right, and repeat-angle averaging is available on some models. Some models also include horizontal and vertical collimation corrections, and vertical circle readings referenced to zenith, horizon, or percent slope format. The most significant characteristic of electronic theodolites is their ability to be interfaced electronically to data collectors and to computers, permitting a quick, error-free transfer of field data to the computer. Electronic tachometers (ETI) combine electronic theodolites with EDM instruments both of which are interfaced to a data collector. In this configuration it is called a total station. These electronic tachometers can read and record horizontal and vertical angles together with slope distances. The microprocessors in the ETI can perform a variety of mathematical operations (e.g., averaging multiple angle measurements; averaging multiple distance measurements; X, Y, Z coordinate determination; remote object elevations (heights of sighted features); distances between remote points; adjustments for atmospheric conditions; and so on. In addition, attribute data such as point numbers, point codes, and comments can be included with the recorded field measurements. The data collector is usually a hand-held device connected by cable to the tachometer, although some manufacturers have the data collector included as an integral component of the instrument. The Lietz Set 3C has an on-board data collector with operation controlled via a remote-control device. This configuration has two positive aspects: 1. There is no cable to become entangled 2. There is no need to touch the instrument, and perhaps disturb it, in order to control the measurement and collection commands. Some of the data collectors described here are obviously capable of doing much more than just collecting data. The capabilities vary a great deal from one manufacturer to another. Similarly, the computational capabilities of electronic theodolites themselves also vary widely. Some electronic theodolites simply show the horizontal and vertical angles together with the slope distance, whereas others also show the resultant horizontal and vertical distances. The Leitz SET 3 has the additional capability of being programmed (independent of the data collector) to determine remote object elevation and distances between remote points. Providing theodolites with greater computational capabilities means that the surveyor could then use a less sophisticated (less expensive) data collector. Some surveyors prefer the simpler 1 equipment, wishing to perform data adjustments on the office computer prior to the computation of X, Y, Z coordinates. Future trends seem to be in the area of lower-cost electronic theodolites and EDMs interfaced to simple data collectors. Adjustments and coordinate computations can be accomplished on office computers. These simple electronic tachometers will be affordable for all surveyors. Early models of electronic theodolites used the absolute method for reading angles; that is, the instruments (e.g., Zeiss Elta 2) were essentially optical coincidence instruments with photoelectric sensors being used to scan and read the circles. Later ETI models (e.g., Wild T-2000, Lietz Set 3C, Geodimeter 460, Kern El, and Zeiss Elta 4) employ an incremental method of angle measurement. These instruments have glass circles that are graduated into unnumbered gratings. The number of gratings involved in a measurement is determined from whole-circle electronic scanning. Circle imperfections are thus compensated for, permitting higher precision with only one circle setting. The distance measurement is obtained using electro-optical range finders. Many ETIs have coaxial electronic and optical systems, thus permitting simultaneous electronic and optical pointing. The on-board microprocessor in the ETI monitors the instrument status (e.g., level or plumb orientation) and controls the angle and distance data acquisition and processing. In addition to computing horizontal distances and differences in elevation, most ETI microprocessors will also compute coordinates. ETIs that have automatic data collection capability for angle and distance measurement are called total stations. In addition to the fully automatic instruments described here, there are a wide variety of instruments that have some of the total station characteristics. That is, some instruments have automatic distance recording and others have automatic distance and vertical angle recording. In each case, the non-recorded data must be entered manually into the electronic keyboard. Most ETIs are designed so that data stored in the data collector can be automatically downloaded to the computer via an RS 232C interface with appropriate transfer software. The data can be adjusted by the computer (mainframe, mini or micro) and can then be printed out or graphically portrayed by an interfaced digital plotter. The chief attributes of an ETI system are the speed and ease of data collection and processing, and the elimination of many of the usual opportunities for mistakes and errors. The only disadvantage in the use of ETIs is the lack of hard-copy field notes that can be scanned and checked in the field. Although individual lines of data can be recalled in the field from the data collector, the overall sense of the survey must wait for the computer printout or digital plot. Recognizing this danger, ETI surveyors carefully design their survey implementation in order to minimize errors and mistakes (e.g., use of rigidly specific techniques and redundant measurements); and they design the computer system output so as to highlight possible discrepancies (e.g., "extra" cross sections, profiles, and plan views). Any apparent output inconsistencies are quickly investigated in the field; the computer graphics and printouts can in 2 some cases be available within 24 hours of the completion of the fieldwork. One data collector (Leitz) can be directly connected to a dot matrix printer, permitting a quick analysis of the field data. ELECTRONIC TACHOMETER OPERATION Typical field surveys require the acquisition of horizontal angles, vertical angles, and slope distances from the instrument station to any other point; in addition, survey attribute data such as point numbers and point identification codes are also required. All these data can be quickly captured by the tachometer, or entered via the keyboard. Figure 1 shows a sketch of a control traverse in an area requiring a topographic survey. 3 Figure 2 Typical Data Collector Codes 4 Figure 2 will be used to illustrate typical procedures employed when using electronic tachometers. Initial Data Entry 1. Temperature: (degrees F or C). 2. Pressure: in. Hg. 3. Prism constant (-0.03 m is typical for many instruments). 4. Degrees or gon (grad) selection. 5. Foot or meters selection. 6. Some instruments also provide for these additional entries: a. Sea-level correction. b. Curvature and refraction settings. c. Number of measurement (distance or angle) repetitions for each sighting. d. Choice of direct and reverse positions. e. Automatic point number increments. After the initial data have been entered, and the operation mode selected, the collector program will prompt the operator for all entries in sections B and C (see below). Instrument Station Identification Entries 1. Height of instrument. 2. Station identification number (e.g., #111, Figure 1). 3. Station identification code (see identification dictionary, Figure 2, for example, 02 (CM) concrete monument. 4. Coordinates of instrument station (northing, easting, elevation). 5. Coordinates of backsight station 114 (northing, easting, and elevation) or a reference azimuth to the backsight station. 6. Note: Some instruments do not permit entry of coordinates, relying instead on the computer program to prompt for these entries later. 5 Data Collection Entries 1. Sight BS at station 114, zero horizontal circle; most tachometers have a zero-set button. 2. Enter code 20 (BS), Figure 2. 3. Measure and enter the height of the reflector (HR). 4. Press the appropriate measurement buttons: distance, horizontal angle, and vertical angle. Press "record" button after each measurement. Some instruments record the three measurements after pressing one button, in the "automatic" mode. 5. After the station measurements have been recorded, the collector will prompt for the station point number (e.g., #114) and the station identification code (e.g., "coordinate monument"-#08, Figure 2 dictionary). 6. If appropriate, as in traverse surveys, sight in the FS at station 112 (use operation code 30, Figure 2). Update parameters if necessary (e.g., HR, temperature, etc.). Press the measure buttons and record. Identify point number (112) and point code (e.g., "coordinate monument"-#08, Figure 2 dictionary). 7. While at station 111, any number of intermediate sights (IS) (use operation code 40, Figure 2) can now be taken to define topographic features being surveyed. The

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