A Geophysical Investigation of the Parade Ground at Fort Jefferson, Dry Tortugas National Park, Florida

Summary

In December of 2006, the Southeast Archeological Center (SEAC) conducted a ground penetrating radar (GPR) survey at Fort Jefferson in Dry Tortugas National Park (DRTO). The survey was designed to include all open and clear ground surfaces of the fort’s parade ground, and a limited area outside the fort’s walls in the campground and picnic area. The

goal of the project was to identify subsurface historic features that could be imaged using GPR and to provide maps of these features to the park to aid in the management and protection of buried historic fabric at the fort. Just

over fi ve acres (20,500 square meters) of Fort Jefferson’s parade ground, and 2,450 square meters (.61 acres) of land outside the fort’s walls, were surveyed with the GPR. The parade ground survey coverage represents 63.4 percent

of the full eight acres of interior space at the fort, but most of the unsurveyed property was covered with existing buildings, ruins, or impenetrable vegetation.

A variety of historic subsurface features no longer visible on the modern ground surface were revealed during the radar investigation. These included relict footpaths and roads, possible privies and/or cisterns, a probable grave, foundation remains of extinct buildings (the original lighthouse and other possible historic structures associated with the lighthouse keeper’s home and temporary structures that were in place during the construction of the fort), as well as a number of buried anomalies of unknown origin probably associated with historic refuse dumps. In addition, numerous historic and modern sewage, electrical, and water distribution utilities were identifi ed during the survey.

Prior to initiating the field portion of the radar survey, historic maps drawn during the construction and military occupation of the fort were collected and digitized into a geographic information system (GIS) that was used to predict the locations of, and later to interpret, anomalies identified during the GPR survey. Upon completion of the survey, highly accurate mapping of the fort’s interior and exterior was carried out using a global positioning system (GPS) and a total station. The map produced was used to further refi ne and adjust the original GIS, and provided an accurate base map upon which historic maps of the fort and the results of the GPR survey could be overlaid. The end result is a layered GIS database/map showing accurate locations of extant and relict historic features in real world coordinates that can be referenced on the ground and navigated to using a GPS unit.

This document includes a brief construction history of the surveyed areas at Fort Jefferson, maps and images produced by the GPR survey and GIS exercise, and an archeological interpretation of the data collected during the survey. The electronic data used in the creation of the GIS has also been provided to DRTO on a DVD so that it may be incorporated into the park’s resource management database.

Keywords

Spatial Coverage

Individual & Institutional Roles

Contact(s): Dry Tortugas National Park

Sponsor(s): National Park Service; U.S. Department of the Interior; Southeast Archeological Center

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