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Shoreline and coastal change maps that are constructed from historic maps 

 and photographs are subject to numerous sources of error. For example, 

 maps may not have common datums, may have different scales, may have 

 variable accuracy due to age or loss of accuracy in publication procedures, 

 and may be based on different projections, which in turn cause geometric 

 distortions. Ideally, shoreline change maps constructed from aerial photo- 

 graphs should be corrected for distortions caused by pitch, tilt, and yaw of the 

 aircraft. Difficulties in identifying common points over time, problems in 

 rectifying scale, and distortions near margins and corners are common. 

 Additional problems include the unavailability of photographs of the desired 

 vintage, scale, clarity, or resolution. Haze, fog, and cloud cover may obscure 

 ground features. Finally, the water level at the time that the photographs 

 were taken can greatly influence the position of the shorelines. 



Chapter 2 Secondary Sources of Coastal Information 



