were available (1887-1906, 1906-1935, 1935-1953, and 1953-1980). The princi- 

 pal advantage in using this instrument is the ability to check the accuracy of 

 every tracing by a compensating measurement, and of rectifying the result — if 

 necessary. Before any measurements were taken, the scales of the topographic 

 maps were adjusted to 1:24,000, and the maps were overlaid using latitude and 

 longitude coordinates. Areas of beach front erosion were determined by trac- 

 ing the change in beach front position from one map to the next. Similarly, 

 the areas of deposition were determined by tracing the figure (representing 

 the addition of material behind, as well as at the ends) of the island. This 

 method will be used in the following sections to calculate the amount of sedi- 

 ment eroded, or deposited by overwash and longshore drift, on this delta- 

 margin island. 



27. Certain limitations are inherent in the data set and they should be 

 noted at the outset. The US Geological Survey standard for horizontal accu- 

 racy states that at least 90 percent of the well-defined map points shall be 

 plotted correctly within one-fiftieth of an inch on the published map. Any 

 inaccuracies in the plotting will lead to small errors in the area calcula- 

 tions. Secondly, due to the lack of elevation data for Isles Dernieres prior 

 to 1980, all constants used in the calculations for the pre- 1980 intervals had 

 to be estimated using the known data. In the case of inaccuracies in the 

 quadrangles, the shoreline change is so great that the error involved is 

 rather insignificant when looking at trends. Isles Dernieres have been an 

 erosional headland for the entire study period, having the same sediment 

 sources and wave climate so that estimation of variables for the period prior 

 to 1980, while not desirable, provides acceptable values. When looked at in 

 the context for which they are intended (to define the trends of processes 

 controlling migration), the quantitative estimates of erosion and deposition 

 made using the variables are very useful and yield results which are 

 substantiated by geomorphic changes in the barrier complex. 



17 



