Geographic Information System

GIS is a computer system capable of assembling, storing, analyzing, and displaying geographically referenced information. Locations of geographic features are stored in layers called coverages. Geographic features are represented in the coverages as polygons, lines, or points. Attributes of geographic features and related information are stored in files called tables. Attribute tables contain one record for each feature in the coverage. For example, in a coverage representing wells, the attribute table contains one record for each well. An attribute table may contain information such as well identification number, depth, and other construction data. Data tables may be independent of a coverage but, because GIS is a relational data base, can be linked to the attribute table using a common attribute, such as well identification number. In this way, multiple observations (records) describing a single geographic feature can also be stored in the data base. For example, in a water-level data table, multiple records of historical water levels can be stored for each well. Because each of the multiple records contains the well identification number, which is the common variable, the well coverage attribute table can be related to the data table.

GIS can also be used to compare features and to analyze data between coverages. For example, a polygon coverage representing a basin boundary can be used to identify wells within the basin. GIS can also be used to display coverage features and data in the form of maps and other graphics.

All the coverages in the Santa Clara-Calleguas Basin data base are in the Universal Transverse Mercator (UTM) projection, zone 11 (Snyder, 1987). The units of the projection are in meters. The UTM projection was selected because the single UTM zone encompasses the entire RASA study area for southern California. The origin of the UTM coordinate system is defined by the equator (Y direction) and the central meridian (X direction) of the UTM zone. The coordinates for the coverages are stored as single-precision values that allow as many as seven significant digits for each coordinate. As a result of the distance from the equator north to the study area, the coordinates in the Y direction are large and software limits on the number of significant digits can cause a loss of precision. To maintain precision, a value of 3.5 million meters was subtracted from all coordinates in the Y direction during the projection process.

The GIS software ARC/INFO was used during this study. ARC/INFO version 4 on a PRIME mini computer was used at the start of this study. At the completion of the study, ARC/INFO version 7 on Data General Aviion 300 workstations was being used.

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