Widely spaced contours, or an absence of contours, means that the ground slope is relatively level. The elevation difference between adjacent contour lines, called the contour interval, is selected to best show the general shape of the terrain. A map of a relatively flat area may have a contour interval of 10 feet or less. Maps in mountainous areas may have contour intervals of 100 feet or more. Elevation values are shown at frequent intervals on the index contour lines to facilitate their identification, as well as to enable the user to interpolate the values of adjacent contours.
Bathymetric contours are generally offshore since they show the shape and slope of the ocean bottom.
They are shown in blue or black. Bathymetric contours are shown in meters at intervals appropriate to map scale and coastal profile, and should not be confused with depth curves. Depth curves are shown along coastlines and on inland bodies of water where the data are available from hydrographic charts or other reliable sources. Depth figures, shown in blue along the curves, are in feet on older USGS maps and in meters on newer maps. Soundings, individual depth values, may also be shown.