206 Computing in Geographic Information Systems
of equal depth. These lines present a picture of the bottom. On some charts
depth contours are shown in solid lines; the depth represented by each line
is shown by numbers placed in breaks in the lines, as with land contours.
Solid line depth contours are derived from intensively developed hydrographic
surveys. A broken or indefinite contour is substituted for a solid depth contour
whenever the reliability of the contour is questionable.
Depth contours are labeled with numerals in the unit of measurement of
the soundings. A chart presenting a more detailed indication of the bottom
configuration with fewer numerical soundings is useful when bottom contour
navigating. Such a chart can be made only for areas ...