Optical-reading repeating and directional theodolites

In an optical-reading theodolite the glass circles are read through a microscope; there are no open verniers. Two types are used.

(i) Optical-reading repeating theodolite

• It has an upper plate and a lower plate, each with its own clamp and tangent screw, as in a vernier repeating theodolite.
• The graduated circle can be clamped to the alidade (upper part) or to the lower part. So an angle can be repeated (added) several times on the circle without resetting the reading.
• The circle is read through a micrometer or optical-scale microscope; both sides of the circle are brought together in the field of view (reducing eccentricity).
• It is used for the repetition method as well as the ordinary method.

(ii) Optical-reading directional theodolite

• The horizontal circle is fixed relative to the base during observation; it has only one clamp and tangent screw for the alidade (no separate lower motion), so repetition is not possible.
• Directions to each station are read from the fixed circle, and angles are found from differences of directions. This is the direction (reiteration) method.
• The circle is turned by a circle-setting screw to change the zero between sets.
• It gives more accurate results (reading to 1 second or 0.1 second) and it is quick, so it is used in triangulation.