2.10: Rules of Topographic Maps

Rules of Topographic Maps

 Contour lines connect points of equal elevation; therefore, every point along a contour line is the exact same elevation.

  • Contour lines always separate points of higher elevation (uphill) from lower elevation (downhill).
  • Contour lines never cross or intersect. A point on the surface of the earth cannot be at two different elevations. An exception to this rule is, an overhanging cliff, and will be represented as dashed contour lines. 
  • Contour lines never split or divide. An exception to this rule is, a vertical cliff where contour lines appear to merge together, but are actually stacked on top of each other.
  • Contour lines always close to form a shape, usually an irregular circle. Note that sometimes contour lines extend beyond the area on a map so you may not see the entire closed circle (Figure 2.16).
Topographic map illustrating the proper closed shape.

Figure 2.16: Close the contour shape or run the contours to the edge of the map (CC-BY 4.0; Chloe Branciforte, own work).

  • Closely spaced contour lines indicate a steeper slope. Contours which are further apart indicate a gentle slope (Figure 2.17).
  • A hill is represented by a concentric series of closed contours (Figure 2.17). The “rule of halves” can be used to interpolate Links to an external site. the high points: add half of the contour interval to the elevation of the highest contour line.
Contour lines in map and profile view.

Figure 2.17: Contour lines which illustrate steep (left) vs. gentle (right) slope (CC-BY 4.0; Chloe Branciforte, own work).

  • Depression contours are indicated by hachure marks on the downhill side (Figure 2.18). The “rule of halves” can be used to interpolate the low points within craters: subtract half of the contour interval from the lowest depression contour.
A hill and depression in map-view.

Figure 2.18: Left: Concentric series of closed contours represent a hill. Right: Concentric series of closed contours with hachure marks represent a depression (CC-BY 4.0; Chloe Branciforte, own work).

  • Contour lines “V” upstream when crossing a stream.  The point of the “V” points uphill (Figure 2.19).
How a stream interacts with contour lines.

Figure 2.19: Contour lines “V” upstream, with the “V” pointing uphill (CC-BY 4.0; Chloe Branciforte, own work).