How to Check a Theodolite

How to check a Theodolite.

A theodolite should be checked at least once a month or before any precise work is to be carried out.

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The checks on a theodolite should comprise of a five-point check carried out in the following order.

1. Plate Bubble Check.

1.1.    Set up the instrument on a sturdy tripod.

1.2.    Turn theodolite until the bubble tube is parallel with 2 of the footscrews, and level the bubble carefully.

1.3.    Turn the theodolite through 90 degrees, and level the bubble using third footscrew carefully.

1.4.    Turn the theodolite back through 90 degrees, and re-level bubble if necessary. Keep repeating this process until you are confident the bubble rests in the centre.

1.5.    Then turn the theodolite through 180 degrees. The bubble should be central if correctly adjusted.

1.6.    If the bubble does not stay central, then you are best advised to have it adjusted, or repaired, and calibrated by your specialist survey equipment supplier.

2. Optical Plummet Check. (There are two types of optical plummets.)

2.1.    Fixed in Tribrach.

2.1.1.Having set up the theodolite and checked the Plate Bubble as described above, level the tribrach with theodolite or plate level over a point defined by a cross (the finer the cross the better).

2.1.2.With a sharp pencil trace the outline of the tribrach base plate on the tripod head.

2.1.3.Release the retaining screw slightly and carefully turn the tribrach through 120 degrees keeping within the marks on the tripod.  Re-level and sight the cross and note if the cross is not in the centre.

2.1.4.Repeat, turning through a further 120 degrees. The cross should be central each time.

2.1.5.If the cross is not in the centre each time then you are best advised to have it adjusted, or repaired, and calibrated by your specialist survey equipment supplier.

2.2.    Revolving Plummet. (Fixed in the theodolite or plate level.)

2.2.1.Having set up the theodolite and checked the Plate Bubble as described previously, level the tribrach with theodolite or plate level over a point defined by a cross (the finer the cross the better).

2.2.2.Level and centre over a point defined by a cross.

2.2.3.Rotate the theodolite through 120 degrees and observe the cross.  The cross should remain central.

2.2.4.Rotate the theodolite through a further 120 degrees and observe the cross.  The cross should remain central once more.

2.2.5.If the cross is not in the centre each time then you are best advised to have it adjusted, or repaired, and calibrated by your specialist survey equipment supplier.

3. Horizontal Circle.

3.1.    Set up and level the instrument.

3.2.    Select and sight a well-defined distant point, this should be at least 100m away and you should also be careful to ensure a clear line of sight (A cool overcast day would be ideal).  Read the Horizontal angle scale (H1) and note down your reading.

3.3.    Transit the telescope and re-sight the same point on the opposite face.  Read the Horizontal angle scale (H2) and note down your reading.

3.4.    Work out the difference between H1 and H2.  The difference between H1 and H2 should be 180 degrees.  However, your angular error should be less than the stated accuracy of the theodolite.

3.4.1.For example, H1 = 180 degrees 0 minutes 0 seconds.  H2 = 0 degrees 0 minutes 3 seconds.  Then (H1)-(H2) =179 degrees 59 minutes 57 seconds.  This is a 3 second inaccuracy.  If the theodolite is classed at 5 second accuracy, then the result would be acceptable.

3.4.2.If the result is greater than the accuracy stated on the theodolite then you are best advised to have it adjusted, or repaired, and calibrated by your specialist survey equipment supplier.

4. Vertical Circle.

4.1.    Set up and level the theodolite as before.

4.2.    Select and sight a well-defined elevated point.  Again this is best done on a cool and overcast day.

4.3.    Read the vertical angle scale (V1) and note down the reading.

4.4.    Transit the telescope and re-sight the same point on the opposite face.

4.5.    Read the vertical scale (V2) and note down your reading.

4.6.    V1 and V2 the abstract angles of elevation or depression should total 360 degrees.

4.6.1.For example, V1 = 100 degrees 0 minutes 0 seconds.  V2 = 260 degrees 59 minutes 57 seconds.  Then (V1)+(V2) =359 degrees 59 minutes 57 seconds.  This is a 3 second inaccuracy.  If the theodolite is classed at 5 second accuracy, then the result would be acceptable.

4.6.2.If the result is greater than the accuracy stated on the theodolite then you are best advised to have it adjusted, or repaired, and calibrated by your specialist survey equipment supplier.

5. Trunnion Axis.

5.1.    Set up and level the instrument as before.

5.2.    Sight a well-defined high level point with the centre of the cross hairs (like a steeple or a fixed radio mast.

5.3.    Depress the telescope and read a tape set (perpendicular on the floor) close to the instrument (T1) and record your reading.

5.4.    Repeat on the opposite face and read the tape again (T2).

5.5.    The readings T1 and T2 should be equal.

5.6.    If the result is greater than the accuracy stated on the theodolite then you are best advised to have it adjusted, or repaired, and calibrated by your specialist survey equipment supplier

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