Lela Kahl, Age 12, Of San Diego, Calif., for her question:

How do they measure land elevations?

Surveyors and engineers must know the exact 1evel of the ground to chart maps and properties, to build roads and buildings, dams and bridges. They begin to measure the ups and downs of an area, its dips and rises, from a permanent marker called a bench mark. It may be a concrete post with a brass top, a fire hydrant or a metal marker in a road or street.

The original bench mark was figured by geographers on the basis of its exact height above or depth below sea level. It is used as a starting point to measure the elevation of other points in the neighborhood. Each point figured from the first bench mark becomes another bench mark to be used by surveyors and engineers needing to know the elevation farther afield or farther down the road