Isohypses

Isohypses are lines on a map that connect points sharing the same elevation relative to a reference level, usually mean sea level. They form a class of isolines and are most familiar as contour lines on topographic maps. Isohypses are typically drawn at regular height intervals, such as every 10 or 20 meters, producing a graduated skeleton of the terrain.

The pattern and spacing of isohypses convey relief: closely spaced lines indicate steep terrain, widely spaced lines indicate gentle slopes. The shape of lines reveals features such as ridges, valleys, plateaus, and depressions. Labels on selected lines provide absolute elevations. In planimetric maps, an index contour—every nth line is drawn heavier and labeled to aid reading.

Production and data sources: isohypses are derived from elevation data gathered by land surveying, aerial photography, LiDAR, or digital elevation models. In traditional cartography, contours were drawn by field surveyors; modern maps often generate isohypses digitally and may be supplemented with shaded relief to emphasize terrain.

Applications: isohypses are used in geology, geomorphology, civil engineering, hydrology, urban planning, and outdoor recreation. They support watershed analysis, slope and aspect assessment, flood risk evaluation, and infrastructure design. The concept is closely related to hypsometry and to other isolines that map constant height or depth across a region.