tilt errors, and greatly reduces relief displacement errors by proper reference 

 point selection. The scope is an optical train type viewing instrument which, 

 using the camera lucida principle, provides the superimposition of the image of 

 the photo on a horizontal plane (McGivern, Martin, and Benjamin, 1972). The 

 image of a photo, using a variable magnification system (optical zoom), a one- 

 direction magnification, and an image rotation system, is ratioed, deformed, 

 rotated, and transformed to establish the best fit with the planimetric base 

 map. 



2. Aerial Photo Analysis . 



This task involves the tracing of the desired beach, near shore and wave 

 features on the base maps by carefully matching reference points on the photo 

 to those on the maps. Using the transfer instrument to match the variations 

 in scale and tilt, thus eliminating errors, the photo image and the base map 

 reference points appear superimposed. Selected coastal features are then 

 traced from their superimposed images on the base map. For measurement ac- 

 curacy, a hard lead, sharp pencil is used to trace shoreline features (e.g., 

 solid line for water and shore structures, dashline for wetted bound, dash- 

 dot line for dune line) . Colored pencils are used for the other features not 

 requiring as exact a location (e.g., solid red line for submarine bar and 

 shoal position, blue dashline for breaking wave crests, solid blue line for 

 nonbreaking wave crests) . Care must be taken to work as much as possible from 

 the centers of each photo when tracing features on the maps. This reduces 

 errors due to tilt, and is the reason a large overlap area between photos is 

 important. 



3. Data Reduction . 



Measurements of shoreline features (waterline, wetted bound, and dune posi- 

 tion) are taken at the intersection of transects with the shore-parallel refer- 

 ence line on the base maps to the feature traced on the map. Any feature on the 

 base map can be measured, using the same procedure. Measurements may be taken 

 using an architect's scale with interpolations to 1/32 or 1/64 inch (0.012 or 

 0.006 centimeter) for an estimated accuracy of 0.003 to 0.0015 centimeter, 

 which is 2 to 1 meter on a scale of 1:10,000. 



III. DATA REQUIREMENTS AND SELECTION OF COASTAL FEATURES 



Accuracy in obtaining various data from aerial photos is based on the objec- 

 tive selection of beach and nearshore features with characteristics distin- 

 guishing these features from other coastal features. 



1. Shoreline Position . 



Changes to the shoreline position are determined by identifying shoreline 

 features such as the waterline position, the wetted bound position, and the dune 

 line position. Generally, the wetted bound is the best shoreline position 

 marker to use because it does not vary appreciably over a tidal cycle, and the 

 position is identifiable on most aerial photos. The following criteria should 

 be used to identify the shoreline position: 



