Contour a line that joins a series of points all of which share an equal elevation (height) above a given vertical datum. A vertical datum can be ; 1.) Assumed, an example of an assumed datum could be using the first floor elevation of a house, assumed to be elevation 100.00. You could assume that the starting point in the middle of the a field might be elevation 100.00. 2.) Referenced to a published datum such a Mean Sea Level also know as National Geodetic Vertical Datum 1929. Within the United States, the National Geodetic Survey (NGS) is in charge of maintaining benchmarks with know vertical heights. The current datum that NGS uses is the North American Vertical Datum of 1988 (NAVD 1988).