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short-term changes should be filtered out of the record. The best approach is 

 to use only maps and aerial images from the same season, preferably 

 summertime. 



A crucial problem underlying the analysis of all historical maps is that they 

 must be corrected to reflect a common datum and brought to a common scale, 

 projection, and coordinate system before data from successive maps can be 

 compared (Anders and Byrnes 1991). Maps made before 1927 have an 

 obsolete latitude-longitude coordinate system (U.S. datum or North American 

 (NA) datum) that must be updated to the current standard of NAD 1927 or the 

 more recent NAD 1983. To align maps to a specific coordinate system, a 

 number of stable and permanent points or features must be identified for 

 which accurate and current geographic coordinates are known. These 

 locations, called primary control points, are used by computer mapping 

 programs to calculate the transformations necessary to change the map's 

 projection and scale. The most suitable control points are triangulation 

 stations whose current coordinates are available from the National Geodetic 

 Survey. 



Maps that were originally printed on paper have been subjected to varying 

 amounts of shrinkage. The problem is particularly difficult to correct if the 

 shrinkage along the paper's grain is different than across the grain. Maps with 

 this problem have to be rectified or discarded. In addition, tears, creases, and 

 folds in the paper maps must be corrected. 



Aerial photographs, which are not map projections, must be corrected by 

 optical or computerized methods before shore positions compiled from the 

 photos can be directly compared with those plotted on maps. The distortion 

 correction procedures are involved because photos do not contain defined 

 control points like latitude-longitude marks or triangulation stations. On many 

 images, however, secondary control points can be obtained by matching prom- 

 inent features such as the corners of buildings or road intersections with their 

 mapped counterparts (Crowell, Leatherman, and Buckley 1991). Types of 

 distortion which must be corrected include: 



• Tilt. Almost all vertical aerial photographs are tilted with 1 deg being 

 common and 3 deg not unusual (Lillesand and Kiefer 1987). The scale 

 across tilted air photos is non-orthogonal, resulting in gross displace- 

 ment of features depending upon the degree of tilt. 



• Variable scale. Planes are unable to fly at a constant altitude. 

 Therefore, each photograph in a series varies in scale. A zoom transfer 

 scope can be used to remove scale differences between photos. 



• Relief displacement. Surfaces which rise above the average land eleva- 

 tion are displaced outward from the photo isocenter. Fortunately, most 

 U.S. coastal areas, especially the Atlantic and Gulf barriers, are rela- 

 tively flat and distortion caused by relief displacement is minimal. 



Chapter 5 Analysis and Interpretation of Coastal Data 



