However, when digitizing cliffed shorelines, control points at about the 

 same elevation as the feature being digitized must be selected. 



• Radial lens distortion. With older aerial lenses, distortion varied as a 

 function of distance from the photo isocenter. It is impossible to cor- 

 rect for these distortions without knowing the make and model of the 

 lens used for the exposures (Crowell, Leatherman, and Buckley 1991). 

 If overlapping images are available, digitizing the centers, where distor- 

 tion is least, can minimize the problems. 



Fortunately, most errors and inaccuracies from photographic distortion and 

 planimetric conversion can be quantified. Past shoreline mapping exercises 

 have shown that if care is taken in all stages of filtering original data 

 sources, digitizing data, and performing distortion corrections, the resulting 

 maps meet, and often exceed, National Map Accuracy Standards (Crowell, 

 Leatherman, and Buckley 1991). 



Topographic and Bathymetric Data 



Introduction 



The analysis and examination of topographic and bathymetric data are 

 fundamental in many studies of coastal engineering and geology. When 

 assembling bathymetric surveys from a coastal area, a researcher is often 

 confronted with an immense amount of data that must be sorted, checked for 

 errors, redisplayed at a common scale, and compared year by year or survey 

 by survey in order to detect whether changes in bottom configuration have 

 occurred. This section will discuss three general aspects of geographic data 

 analysis: 



• Processing of bathymetric data using mapping software. 



• Applications and display of the processed results. 



• Error analyses. 



Bathymetric data processing - data preparation and input 



Most historical bathymetric data sets consist of paper maps with printed or 

 hand-written depth notations (Figure 43). Occasionally, these data are 

 available on magnetic media from agencies like NOAA, but often a reseacher 

 must first digitize the maps in order to be able to perform computer-based 

 processing and plotting. If only a very limited region is being examined, it 

 may be more expedient to contour the charts by hand. The disadvantage of 

 hand-contouring is that it is a subjective procedure. Therefore, one person 

 should be responsible for all the contouring in order to minimize variations 



Chapter 5 Analysis and Interpretation of Coastal Data 



109 



