Much of surveying uses optical means to observe measurements, and the type of optical instrument used is the telescope. Both the optical level - used for direct levelling, and the theodolite (used for the measurement of angles and in many cases distances as well) use the same type of telescope. The type of telescope is internally focussing and contains a cross-hairs (reticule, diapraghm) against which the user can read a measurement. Survey telescopes are generally of about 32X magnification.The main components within the telescope tube are:-

• Objective lens - gathers the light and concentrates the light towards a point at the cross-hairs.
• Focussing lens- A concave lens that can move forward and backward to permit the light rays to concentrate to a sharp point at the cross-hair for the observance of objects at any distance from the telescope.
• The cross-hairs ( reticule, diapraghm).
• The eye-piece (ocular). Permits the user to bring the cross-hairs into focus. The user adjusts the eye-piece first before attempting focus on the object to be measured.

Parallax

In order to take a measurement correctly the image of the focussed point must fall sharply on the plane of the cross-hairs. The phenomenon that occurs if this is not the case is called parallax and is a significant cause of measurement error in surveying. Mouse-over the image above for animation!