These are contour lines on a topographic map that are drawn bolder or thicker than the regular contour lines. A distinct elevation value is assigned to each of these enhanced lines, appearing on the map at consistent intervals. For instance, if the contour interval of a map is ten feet, every fifth contour line (representing a fifty-foot change in elevation) might be drawn in this bolder style. This visual distinction aids in rapidly determining changes in terrain and identifying specific elevations without needing to count each intervening contour line.
The use of these enhanced lines simplifies map reading and analysis, particularly in areas with closely spaced contours where individual lines can be difficult to discern. This approach enhances the efficiency of terrain assessment, enabling quick determination of elevation changes, identification of hills, valleys, and slopes. Historically, their introduction represented a significant advancement in cartography, streamlining the process of interpreting topographic information and rendering maps more accessible to a broader audience.
