Retracing the Past: Recovering 19th Century Benchmarks to Measure Shoreline Change along the Outer Shore of Cape Cod, Massachusetts

Article excerpt

Introduction

In 2007, the Provincetown Center for Coastal Studies (PCCS), in conjunction with the Cape Cod National Seashore (CCNS), began a project to survey and profile the outer shores of Cape Cod, Massachusetts (Figure 1). The outer or back shore of Cape Cod, from North Chatham to Long Point in Provincetown, is a single, self-contained sedimentary system, consisting of material supplied by glaciers that retreated about 15,000 years ago. These glacial deposits have been sculpted continuously by the sea and wind with sediments of the associated upland and marine land forms moving to produce a dynamic and constantly changing coastal environment. Significantly, the majority of sediment movement within this system occurs offshore or onshore within 1.25 miles (2 km) of the shoreline.

The primary objective of the ongoing PCCS survey work is to retrace similar work of the U.S. Coast & Geodetic Survey (Coast Survey) (1) performed between 1887 and 1889. Consisting of 229 profiles spaced at 300-meter intervals and extending approximately 0.5 - 1.25 miles (1-2 km) offshore, the Coast Survey work provides an accurate and comprehensive source of 19th century spatial data, including the location of dunes, bluffs, beaches, and nearshore sediment features. When completed, the results of the PCCS survey will be compared to those of the Coast Survey to measure changes over the entire coastal system through time and quantify the direction, volumes, and rates of sediment movement. This century-scale information will provide a foundation for future resource management decisions dealing with the impacts of climate change and sea level rise along this dynamic area of the coastal zone (Giese and Adams 2009).

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Reliable comparisons of spatial data from different periods of time, however, require accurate translation to common horizontal and vertical spatial reference systems. Since Coast Survey work was referenced to the mathematically defined New England Datum, the position of each transect can be translated to the North American Datum of 1983 (NAD83) using documented adjustments to values of latitude and longitude (Giese and Adams 2007). In contrast, elevation data depicted on Coast Survey profiles was referenced to a local mean sea level datum calculated from a series of tidal observations conducted in 1887 at the beginning of the survey. As a local tidal datum with no documented mathematical adjustment, translation to the North American Vertical Datum of 1988 (NAVD88) depends on the ability to recover reference points or benchmarks for which contemporary elevations can be determined. In the absence of benchmarks, comparisons of elevation data would necessarily require general and broad assumptions regarding datum differences, increasing potential uncertainties associated with numerical study conclusions (Byrnes et al. 2002; Gibbs and Gelfenbaum 1999). The goal of this study, therefore, is to recover multiple benchmarks established by the Coast Survey for the purpose of estimating the vertical relationship between 19th century and contemporary shoreline profiles.

Based on information contained in its Annual Reports (Marindin 1889; Marindin 1890), the Coast Survey established twenty six (26) benchmarks along the outer shores of Cape Cod between 1887 and 1888. Although many of these reference points were located on lighthouses, lifesaving stations, boulders, and other prominent features, a review of point descriptions indicated that significant changes in the physical and social landscape over the past 120 years would make recovery of many individual benchmarks difficult due to the disappearance of many reference features.

With only century old descriptions to rely upon, an accurate 19th century base map of the Outer Cape, based on Coast Survey topographic sheets of the 1850s-1880s, was developed to assist with benchmark recovery. Using 19th century reference features depicted on the base map, approximate benchmark locations could then be plotted from the descriptions contained in the survey reports and contemporary coordinate values of these locations generated to facilitate field reconnaissance. …