BASE MAP ANALYSIS OF COASTAL CHANGES 

 USING AERIAL PHOTOGRAPHY 



by 

 Craig H. Everts and Deborah C. Wilson 



I . INTRODUCTION 



Time-sequence aerial photography is frequently used in determining past 

 shoreline positions. Often, this photography is the "only source of such infor- 

 mation. Most methods of determining shoreline position changes from aerial 

 photography require that measurements be taken directly from the photos. 

 Stafford (1971), for example, presents an excellent description of such a method 

 used in beach erosion surveys. The method requires that the absolute distance 

 scale be established for each photo (or in some cases, each set of photos), a 

 time-consuming and often difficult task. 



This report presents a technique in which such problems are minimized. A 

 master base map containing the exact location and outlines of various subaerial 

 reference points is constructed using the earliest and latest sets of aerial 

 photos available. Separate base maps for each in-between photo set are then 

 duplicated from the master. The image of the aerial photos is superimposed at 

 the map scale, and features of interest are traced on each base map. Distances 

 along specified lines drawn normal to shore on each map are then measured to 

 shoreline features. When a comparison between each base map is made, these dis- 

 tances provide shoreline change data through time. 



II. BASE MAP ANALYSIS TECHNIQUE 



Application of the base map analysis technique requires three main tasks: 



(a) the construction of the base maps, using a transfer instrument to super- 

 impose the image of fixed features on the aerial photography to the maps; 



(b) the superposition and recording of the desired shoreline and other features 

 from the aerial photography to the base maps; and (c) the reduction of data to 

 determine the required measurements. 



1. Construction of the Base Maps. 



a. Procedure . An accurate master base map, which is critical to the suc- 

 cess of the analysis, is constructed in four steps. The first step is to select 

 a suitable map scale, slightly larger than the scale of the largest scale aerial 

 photo set, to ensure that all photo sets are traceable on the base map. Mosaics 

 of the earliest (in time) and the latest sets of the largest scale aerial photos 

 are prepared, and the master base map is drawn on a sheet of high-quality, di- 

 mensionally stable paper cut at the appropriate length and width to include the 

 largest scale mosaic. 



The second step is to determine reference points, which are features common 

 to all photo sets, to establish the absolute location of the images of the 

 aerial photos on the base map. Reference points must be included on all photo 

 sets, and must be nonchanging through time; e.g., if a road corner is selected, 

 the road must not have been widened during the earliest to latest interval of 

 the photo sets. Cultural features such as road junctions make excellent ref- 

 erence points. The reference points must be point definable, flat lying, if 



