Production of site imagemaps

The maps are based upon analysis of the water-level monitoring sites that are on the USGS Center for Water and Restoration Studies cooperative data-collection program and that are checked by field visit on an annual to monthly basis. The selection is further restricted to those sites that have had water-level data collected from site visits within the past 270 days. The basis for the retrieval is the second month prior to the date the maps are generated. For example, during April 2002, the target month was February 2002. For the sites, the following data are also retrieved from the project database:

1. County in which the monitoring site is located,
2. Water-supply system monitored by the site (aquifer for wells),
3. Latest available water levels at the sites for the end of the target month, and
4. Historical frequency statistics for the target month.

In addition to a large-area map showing the selected sites in South Florida, maps for each county, major aquifer, and the surface-water system are generated. The symbols for each site are determined by the type of monitoring system at the site, whether the site has recent data, and by comparing the latest available water-level value to the frequency statistics for the period of record (last 25 years).

Basis of computations:

• Water levels are measured in feet above the National Geodetic Vertical Datum of 1929 (NGVD1929). This datum is not identical to mean sea level, although for many purposes the two are similar in usage.

• The frequency statistics for the site are obtained as follows:

  1. The measured water-level values collected during the past 25 years are retrieved.

     The original choice of an early cutoff at 1975 reflects the limited extent of the ground- and surface-water networks in South Florida prior to the mid-1970s, and the change in USGS data storage methods circa 1974, making the data collected since then readily available in a computer-retrievable format. The project has retained the 25-year data window to provide a consistent statistical measure through time.

  2. Frequency statistics are obtained from the retrieved data set using a standard statistical analysis package. The resulting data are then stored in the project database.