Converting contour-line data into data sets for a multilayered aquifer using a geographic information system

Data sets that define the altitude of the base of basin-fill units in the west Salt River Valley were developed for the National Water-Quality Assessment program using a geographic information system. Data that define the land surface and the base of each basin-fill unit within a multilayered aquifer were compiled into a series of raster-data lattices. The lattice of each basin-fill layer was constructed using contour lines from a published hydrogeologic report. The land-surface lattice was constructed from a:250,000-scale digital elevation models of the area. The resulting raster-data set was queried to define the altitude of the base of each basin-fill unit at specified locations. Using a computer script to be run within a geographic information system, a table was produced that provided information that related the altitude of the bottom of the wells to the altitudes of the bases of the basin-fill units. A comparison of the altitude at the bottom of wells with the range in altitude between each basin-fill unit base made it possible to begin to determine the basin-fill unit in which wells were completed.