Basic Components of Level - Levelling, Surveying and Levelling | Surveying and Levelling Notes- Agricultural Engg - Agricultural Engineering PDF Download

Basic components of level :
1. Telescope – to provide a line of sight
2. Level Tube – to make line of sight horizontal
3. Levelling head – to bring the bubble of tube level at the centre of its run.
4. Tripod – to support the above three parts of the level.

TELESCOPE :  Telescope is an optical instrument used for magnifying and viewing the images of distant objects.  It consists of two lenses.  The lens fitted near the eye is called the eye piece and the other fitted at the end near to the object is called the objective lens. The objective provides a real inverted image infront of the eye piece at a distance lesser than its focal distance.
Two essential conditions are involved. :  
i)  The real image of the object, must be formed.
ii) the plane of image must coincide with that of cross hairs.
Focusing of Telescope :  The operation of obtaining a clear image of the object in the plane of cross hairs is known as focusing.

1. Diaphram :  A frame carrying cross hairs usually made of either silk thread or platinum wire and placed at the plane at which vertical image of the object is formed by the objective. Vertical hair of the diaphram enables the surveyor to check the verticality of levelling staff whereas horizontal hairs are used to read the staff graduations.
2. Level Tube :  Also known as Bubble  Tube consists of a glass tube placed in a brass tube which is sealed with plaster of paris.  The whole of the interior surface or the upper half is accurately ground so that its longitudinal section, is an arc of a circle.  Level tube is filled with either or alcohol, the remaining space is occupied by an air bubble.  The centre of air bubble always rest at the highest point of the tube. Outer surface of the bubble tube is graduated in both the directions from the centre. The line tangential to the circular are at its highest point i.e. the middle of tube is called the axis of bubble tube.  When the bubble is central the axis of bubble becomes Horizontal. The level tube is attached on the top of Telescope by means of capstan headed nuts.

3. Levelling head :  Levelling head generally consists of two parallel plates with 3 foot screws.  Upper plate is known as Tribrach and lower plate is trivet which can be screwed on to the tripod.  Levelling head has to perform three distinct functions :
i)  to support the telescope
ii) to attach the level to the tripod
iii) to provide a means for level (foot screws)