ABSTRACT
Mathematically, a contour line (or isoline) represents a function of two spatial coordinates along which it has a constant value. The best-known example is a two-dimensional map view of the topographic (land) surface where the land surface elevation is a function of X and Y coordinates. A topographic contour line connects all points of equal elevation above a given level (called datum, usually mean sea level). Successive contour lines on a topographic map are drawn for equal difference in elevation between them. This difference is called contour interval. Figure 4.1 illustrates the concept of contour lines applicable to any surface. In general, contour lines can be thought of as intersections of stacked horizontal planes with a real or hypothetical surface. Common examples of surfaces of hydrogeological interest include the water table of an unconned aquifer (real physical surface), the potentiometric surface of a conned aquifer (imaginary surface), and the top surface of a conning layer (real physical surface).
