Every point on a contour line, therefore, has the exact same elevation. In the figures above the dashed lines on the lower surface correspond to the solid contour lines in the upper topographic map. Each line represents a 25ft difference in elevation. This is called the contour interval. There are four very important concepts to remember when reading or constructing a topographic map.
Contour lines connect points of equal elevation.
Different contour lines do not cross. The amount of space between contour lines is related to the steepness of a slope. Widely spaced contour lines indicate a gentle slope and closely spaced contour lines indicate a steep slope. V's which point upstream. In this figure the dashed lines represent streams.