HISTORICAL SHORELINE MAPPING .2. APPLICATION OF THE DIGITAL SHORELINEMAPPING AND ANALYSIS SYSTEMS (DSMS DSAS) TO SHORELINE CHANGE MAPPING IN PUERTO-RICO

A new, state-of-the-art method for mapping historical shorelines from maps and aerial photographs, the Digital Shoreline Mapping System (DSM S), has been developed. The DSMS is a freely available, public domain software package that meets the cartographic and photogrammetric requi rements of precise coastal mapping, and provides a means to quantify a nd analyze different sources of error in the mapping process. The DSMS is also capable of resolving imperfections in aerial photography that commonly are assumed to be nonexistent. The DSMS utilizes commonly av ailable computer hardware and software, and permits the entire shoreli ne mapping process to be executed rapidly by a single person in a smal l lab. The DSMS generates output shoreline position data that are comp atible with a variety of Geographic Information Systems (GIS). A secon d suite of programs, the Digital Shoreline Analysis System (DSAS) has been developed to calculate shoreline rates-of-change from a series of shoreline data residing in a GIS. Four rate-of-change statistics are calculated simultaneously (end-point rate, average of rates, linear re gression and jackknife) at a user-specified interval along the shoreli ne using a measurement baseline approach. An example of DSMS and DSAS application using historical maps and air photos of Punts Uvero, Puert o Rico provides a basis for assessing the errors associated with the s ource materials as well as the accuracy of computed shoreline position s and erosion rates. The maps and photos used here represent a common situation in shoreline mapping: marginal-quality source materials. The maps and photos are near the usable upper limit of scale and accuracy , yet the shoreline positions are still accurate +/-9.25 m when all so urces of error are considered. This level of accuracy yields a resolut ion of +/-0.51 m/yr For shoreline rates-oi-change in this example, and is sufficient to identify the short-term trend (36 years) of shorelin e change in the study area.