Aquaculture Engineering

Adjustment of a level - Elimination of parallax

Unit 1- Surveying

Adjustment of a level

Elimination of parallax

Parallax is a condition arising when the image formed by the objective is not in the plane of the cross-hairs. Unless parallax in eliminated, accurate sighting is impossible. Parallax can be eliminated is two steps.

i) By focusing the eye-piece for distinct vision of the cross – hairs.
ii) By focusing the objective to bring the image of the object in the plane of cross – hairs.

Focusing the eye piece for distinct vision of cross-hairs
To focus the eye piece for distinct vision of the cross-hairs, point the telescope to words sky (or hold a piece of white paper in front of the objective) and move eye-piece in or out till the cross hairs are seen sharp and distinct.

Focusing the eye piece to bring the image of the object
The telescope is now directed towards the staff and the focusing screw is turned till the image appears clear and sharp. The image so formed is in the plane of cross hairs.