Angles and Bearings

Using a Theodolite to measure Horizontal Angles

Usually, when surveying, a point is located relative to another point by means of its polar coordinates. Thus, as well as needing a distance, an angle is needed.

Standard Equipment

As well as the Theodolite and its tripod, some way of accurately marking the points to be surveyed is required. This can be another tripod with a target placed upon it, or simply a wooden peg with a nail hammered into the top. The theolodite requires a skilled operator, and is generally a one man operation.

Measuring the Angle between two points
(as viewed from a third point)

  1. Set up the Theodolite on a known point.
  2. Select one of your marked points as your Reference Object, (RO). With the theolodite on face left, Sight onto the RO, and set the horizontal scale to a figure just above zero, say 000'00'12".
  3. Rotate the theolodite clockwise to sight onto your next point. Read and record the new bearing.
  4. Rotate the telescope in the vertical plane by 180'. This sets the instrument to face right. Now read and record the last bearing again. It should have change by 180 degrees.
  5. Rotate the theolodite anti-clockwise back to the RO. Read and record the bearing.
  6. Subtracting the RO bearing from the second bearing on both face left and face right should give a very similar result (accurate to ....). The mean of these values should be taken as the angle between the two points.
  7. For greater accuracy, Repeat the procedure from step 2 onwards, but this time, set a different bearing for the RO (say 090'00'00).

Reading Additional Angles

Imagine that whilst set up on one point, you wish to measure angles between more than 2 other points. This is simple. Use the same method as above, but include the following:-

Repeat step 3 as many times as you have extra points. Rotate the telescope vertically and read the last bearing again, as in step 4,

Then carry out step 5, but instead of swinging directly back to the RO, sight, read and record for each point in the reverse order to which you sighted them in step 3.

Now the angles between each set of points can be calculated in a similar way to step 6 - for the angle between two points of known bearing, subtract the lower bearing from the higher.

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