Contour lines are the primary two-dimensional graphic tool used to express three dimensional ground forms. Contour lines show land forms and the relationships of land forms.

Definition of Contour line: A line drawn on a plan which connects all points of equal elevation above or below a known or assumed reference point or plane, usually mean sea level (MSL).

Use of Contour Lines: Contour lines are used to study proposed changes in land form and eventually to guide and direct the work of grading contractors in executing grading projects.

Contours show land forms, for example, a hill, a valley, ridge, etc. They show the relationships of land forms, for example, the connection of a hill to a valley, to a stream and finally, to the ocean. Contours are shown two-dimensionally, the scaled distance between them is exactly the same in the field.

All grading plans have a vertical contour interval which remain the same of the entire drawing. This interval stands for the vertical distance between contours.

Two items of information that can be determined on contour maps visually; elevations of particular spots and relative steepness of adjacent areas. (Closer lines mean steeper land). By using simple arithmetic, exact steepness or grade of slopes can be computed.

Both proposed and existing contours are shown on the same grading plan. By showing both on the same drawing, it is possible to understand the exact location of the work to be performed and the exact amount of proposed grading quantities of cut or fill or combination thereof.