j) Level Line: It is line lying in a level surface; it is therefore, normal to the plumb line at all points.
ii) Horizontal Line: It is a straight line tangential to the level line at a point. It is also perpendicular to plumb line.
iii) Vertical Line:The direction indicated by a plumb line (the direction of gravity) is known as vertical line. This is perpendicular to the horizontal line.
iv) Bench Mark: These are fixed points or marks of known RL determined with reference to the datum line. They serve as reference points for finding RL of new points.