A total station is an electronic theodolite and an electronic distance meter ( EDM ). This combination makes it possible to determine the coordinates of a reflector by aligning the instrument’s cross hairs on the reflector and simultaneously measuring the vertical and horizontal angles and slope distances.
A microprocessor in the instrument takes care of recording, readings and the necessary computations. The data is easily transferred to a computer where it can be use to generate a map. Wild, ‘ Tachymat ‘ TC 2000, and it is manufactured by M/s Wild Heerbrugg.
As a teaching tool, a total station fulfills several purposes. Learning how to properly use the total station involves the physics of making measurements, the geometry of calculations, and statistics for analysing the results of a traverse. In the field, it requires teamwork, planning, and careful observations.
If the total station is equipped with a data collector it also involves interfacing the data-logger with a computer, transferring the data, and working with the data on a computer. The more the user understands how a total station works, the better they will be able to use it.
- The rotation of the instrument’s optical axis from the instrument north in a horizontal plane : i.e. horizontal angle
- Inclination of the optical axis from the local vertical i.e. vertical angle.
- Distance between the instrument and the target i.e. slope distance.
The horizontal angle is measure from the zero direction on the horizontal scale. When the user first sets up the instrument the choice of the zero direction is made – this is Instrument North. The user may decide to set zero ( North ) in the direction of the long axis of the map area, or choose to orient the instrument approximately to True, Magnetic or Grid North.
The zero direction should be set so that it can be recover if the instrument was set up at the same location at some later date. This is usually do by sighting to another benchmark, or to a distance recognizable object. Using a magnetic compass to determine the orientation of the instrument is not recommended and can be very inaccurate.
Most total stations can measure angle to at least 5 seconds, or 0.0013888 degree. The best procedure when using a Total Station is to set a convenient ” north ” and carry this through the survey by using back sights when the instrument is move.
The vertical angle is measure relative to the local vertical ( plumb ) direction. The vertical angle is usually measure as a zenith angle ( 0 degree is vertically up, 90 degree is horizontal, and 180 degree is vertically down ), although one is also given the option of making 0 degree horizontal. The zenith angle is generally easier to work with. The telescope will be pointing downward for zenith angles greater than 90 degree and upward for angles less than 90 degree.
Measuring vertical angles require that the instrument be exactly vertical. It is very difficult to level an instrument to the degree of accuracy of the instrument. Total stations contain an internal sensor ( the vertical compensator ) that can detect small deviations of the instrument from vertical.
Electronic in the instrument then adjust the horizontal and vertical angles accordingly. The compensator can only make small adjustments, so the instrument still has to be well level. If it is too far out of the level, the instrument will give some kind of ” tilt ” error message.
The instrument to reflect distance is measure using an Electronic Distance Meter ( EDM ). Most EDM’s use a Gallium Arsenide Diode to emit an infra light beam. This beam is usually modulate to two or more different frequencies. The infra beam is emit from the total station, reflect by the reflector and received and amplify by the total station.
The receive signal is then compare with a reference signal generate by the instrument ( the same signal generate that transmits the microwave pulse ) and the phase-shift is determine. This phase shift is a measure of the travel time and thus the distance between the total station and the reflector.
Total Stations only measure three parameters :
- Horizontal Angle
- Vertical Angle
- Slope Distance
Let us use symbol I for instrument ( total station ) and symbol R for the reflector. In order to calculate coordinates or elevations it is first necessary to convert the slope distance to a horizontal distance. The horizontal distance is –
Hd = Sd cos ( 90* – Za ) = Sd sin Za
where Sd is the slope distance and Za is the zenith angle. The horizontal distance will be use in the coordinate calculations.
We can consider two vertical distances. One is the Elevation Difference ( dZ ) between the two points on the ground. The other is the Vertical Difference ( Vd ) between the tilting axis of the instrument and the tilting axis of the reflector. For elevation difference calculation we need to know the height of the tilting-axis of the instrument, that is the height of the center of the telescope, and the height of the center of the reflector ( Rh )
The way to keep the calculation straight is to imagine that you are on the ground under the instrument. If you move up the distance Ih, then travel horizontally to a vertical line passing through the reflector then up ( or down ) the vertical distance ( Vd ) to the reflector, and then down to the ground. This can be write as
dZ = Vd + ( Ih – Rh )
The quantities Ih and Rh are measure and recorded in the field. The vertical difference Vd is calculate from the vertical angle and the slope distance
Vd = Sd sin ( 90* – Za ) = Sd cos Za
where dZ is the change in elevation with respect to the ground under the total station. We have chosen to group the instrument and the reflector heights. Note that if they are the same then this part of the equation drops out. If you have to do calculations by hand it is convenient to set the reflector height the same as the instrument height.
If the instrument is at a know elevation, Iz , then the elevation of the ground beneath the reflector, Rz , is
Rz = Iz + Sd cos Za + ( Ih – Rh )