Historical Shoreline Change: Error Analysis and Mapping Accuracy

Abstract

The United States Federal Emergency Management Agency (FEMA) is assessing technical methodologies and procedures for the collection, analysis, and computation of coastal erosion rates. This assessment is being performed to determine the feasibility of generating such data for use as a basis for administering Section 544 of the Housing and Community Development Act of 1987 (commonly known as the Upton-Jones amendment to the National Flood Insurance Program). It is likely that the methodology selected will involve the use of historical shoreline data compared with current shoreline information. This fundamental approach of predicting shoreline location based on extrapolation of past changes has been utilized by several states in developing data to support setback programs. Requirements contemplated for data development include: (1) standardization of a methodology for developing erosion rate data, (2) consistency in application, (3) capability of being applied in a timely manner, and (4) accuracy commensurate with program needs. This paper largely addresses the last issue by examining sources of errors inherent in the raw data. Source data include historical and recent National Ocean Service (NOS) T-sheets (produced ca. 1840's to present) and air photos (taken ca. late 1930's to present) as well as any other types of accurate map and photographic data. A study of this nature requires extensive preparation of the required source material prior to digitization. For example, the high water line (HWL) or bluff line must be identified and annotated on the air photos and maps. Furthermore, the source material must undergo extensive pre-digitization and post-data compilation accuracy checks and reliability assessments necessary to screen inaccurate maps and air photos. For example, older historical maps with obsolete coordinates must be updated to current standards and analyzed to insure that distortion has not rendered the map inaccurate and unusable. In addition, computerized techniques (e.g., Metric Mapping, analytical stereoplotter) or manual techniques (e.g., Zoom Transfer Scope) must be used to correct for distortion inherent in aerial photography.