The most commonly used surveying instrument is the total station. A total station is the combination of an electronic theodolite, an electronic distance measuring device (EDM), and a microprocessor with memory unit. The original name of this instrument of this type was electronic tacheometer but HEWEET – PACKARD introduced the name TOTAL STATION.
With this instrument, we can determine angles and distance from the instrument to the points to be surveyed with the aid of trigonometry, the angles and distances may be used to calculate the relative position (EASTING, Northing and Elevation) of surveyed points.
All total stations have an electronic distance measuring device (EDM) and electronic angles scanning which measure the angles and distance and displayed digitally. The horizontal distance, the height difference and coordinates are calculated automatically and all measurements and additional information can be recorded.
The EDM instrument transmit an infrared beam, which is reflected back to the unit with the help of a prism (by total internal reflection), and the EDM used timing measurements to calculate the distance travelled by the beam. With the exceptions, the EDM instrument requires that the target be highly reflective and a reflecting prism is normally as the target.
Most of the total stations, include data recorders. The raw data (angles and distances) and the co-ordinates of points sighted are recorded along with same additional information. 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 data in the field.
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 optional capability. Total station used for levelling comes under the classification indirect levelling. It can maintain considerable accuracy and is hence for many public work such as construction of roads, airports and harbours.
The total station is mounted on a tripod and levelled before use. Meanwhile the PRISM mounted on pole (TARGET ROD) of known height. The mounting bracket includes the aids for aiming the instrument. The prism is mounted so that its reflection point is aligned with the centre of the pole on which it has been mounted. Although the tip of the pole is placed 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 up right, and a bubble level attached to give the technician holding the pole upright particularly if there is any wind and as a result at such occasions, the reading may be taken by not sighting at the prism but on the tip of the pole where it touches the ground. The angle from north would then be fixed and telescope could be raised to the level of the prism, adjusting only the angle of inclination.
2 PREPARATION FOR SURVEYING
2.1 Centring and Levelling of the Instrument
2.1.1 Setting up the instrument and the tripod
- Adjust the tripod legs so that a height suitable for observation is obtained when the
- Instrument is set on the tripod.
- Hang the plumb bob on the hook of the tripod, and coarse centre over the station on the ground. At this time, set the tripod and fix the metal shoes firmly into the ground so that the tripod head is as levelled as possible.
- If the tripod head is mislevelled by the action of fixing the metal shoes into the ground, correct the level by extending or retracting each leg of the tripod.
Note – Laser plummet model (The instrument is equipped with the laser plummet. The laser plummet is not set to be ON at factory shipping.)
- Turn on the laser plummet function by pushing the [Laser] key and [F4] [PLUM.ADJ] key.
- Match the position by the tripod so that the laser mark coincides with the ground mark.
- The centring knob is loosened, and the upper plate is pushed by the tip of a finger, and a centre mark is matched to the ground mark.
- Tighten the centring knob.
- Loosen the horizontal clamp screw, rotate the instrument by 90°, and confirm the vial of the circular vial is at the centre position. Correct the vial with the leveling screw when the vial comes off from the center position.
A)Levelling with the plate vial
- Align the plate vial in parallel with a line joining any two of the levelling screws. Then, adjust the two screws to centre the bubble in the plate vial.
– Turn two levelling screws in an opposite direction mutually in a way that the bubble moves from the side of the plate vial to the center.
B) Rotate the total station 90°.
- Use the remaining screw to centre the bubble in the plate vial.
- Rotate the instrument by 90° and 180° and confirm the position of the bubble in the plate vial. At this time, it is not necessary to adjust it if the bubble of the plate vial is in the vicinity of the centre.
C) Levelling screw
2.2 Eyepiece adjustment
The eyepiece adjustment is performed before target sighting.
- Remove the telescope lens cap.
- Point the telescope at a bright object, and rotate the eyepiece ring full counter clockwise.
- Look through the eyepiece, and rotate the eyepiece ring clockwise until the reticle appears at its maximum sharpness.
- When looking into the eyepiece, avoid an intense look to prevent parallax and eye fatigue.
- When it is hard to see the reticle due to poor brightness, press [ILLU] + [F3] [RETICLE] to access the RETICLE INTENSITY ADJ. screen. Use the left and right arrow to adjust the reticle intensity.
2.3 Target sighting
2.3.1 Target sighting by Manual focus
- Loosen the telescope clamp and horizontal clamp screws.
- Point the telescope at the target using a collimator.
- Tighten the above two screws.
- Adjust the eyepiece.
- Look through the telescope and then rotate the Focus ring and stop it where the target can be clearly seen and the target image does not move in relation to reticle even if your eye is vertically and horizontally moved.
- Align the reticle accurately on the target using telescope and horizontal tangent screws.
2.4 Attachment and detachment of tribrach
The tribrach of LTS-202N and LTS-205N can be detached from the instrument, if required when replacing the instrument with a prism for example.
- First loosen the recessed screw with a screwdriver, then rotate the locking knob until the arrow points upward, and lift the instrument up.
- Mount the instrument on the tribrach with the guide marks coinciding, and rotate the locking knob until the arrow points downwards.
- The guide and guide mark must be fitted to attach the instrument.
- When the tribrach does not need to be attached or detached, or the instrument is to be transported, tighten the recessed screw with a screwdriver to fix the locking knob.