4.2 Basic Definitions

• Vertical Line
• Horizontal Line
• Horizontal Plane
• Level Surface
• Level Line: A line that lies on the level surface.
• Difference in elevation b/w 2 points: is the 
vertical distance b/w 2 level surfaces.
• Actual Line of Sight or Collimation: Neither 
horizontal nor level. Affected by atmospheric 
refraction.

4.2 Basic Definitions
• Bench Mark: A marked point whose elevation 
has been accurately measured.
• Height of Instrument: The line of Collimation 
above the datum after setting up the level above 
a certain point.
4.2 Basic Definitions

4.3 Basic Principles of a Level
Differential Leveling Requires:
a) a device which gives a truly horizontal line 
(the Level),
b) a suitably graduated staff for reading vertical 
heights (the Leveling Staff).
Level consists mainly of:
a) Telescope for sighting and
b) A leveling device for maintaining the line of 
sight in horizontal position.