possible, and all at about the same elevation. A tower or smokestack is inap- 

 propriate because when it is not in the center of the photo it will appear 

 tilted (relief displacement error) , making the exact location at ground level 

 difficult to determine. Finally, reference points should be chosen close to 

 the shore and in sufficient numbers so that at least three are present for each 

 aerial photo analyzed. The reference points are then drawn on the master base 

 map. 



After the reference points are located on the master base map, a straight 

 line, approximately parallel to the shoreline, is drawn the length of the study 

 area. This line is then used in the third step of locating and numbering tran- 

 sects on the map. A series of lines, or transects, at right angles to the 

 shore-parallel line, are drawn from the line across the shoreline. The spacing 

 interval of the transects depends on the study requirements. For straight 

 coastlines with no structures, an interval of 1 kilometer should be sufficient 

 for most coastal engineering needs (Everts and Czerniak, 1977). When structures 

 are present and near inlets, river mouths, or headlands, a transect spacing of 

 0.5 kilometer or less is probably warranted. In some cases, such as in a groin 

 field, one or more transects in each compartment may be required. Measurements 

 will be made from the intersection of the transects and the shore-parallel line 

 to the desired coastal feature, e.g., the shoreline, on each base map. Thus, 

 the measurements made along exactly the same line on each map eliminate errors 

 due to nonidentical measurement points. 



When the master base map has been completed and checked, the final step of 

 preparing a base map from the master for each in-between photo set is accom- 

 plished. Copies are made and then checked for exactness of scale between each 

 copy. 



b. Instrument Requirements. An important requirement for the construction 

 of the master base map, as well as for the photo analysis and data reduction, 

 is an instrument which allows the image of small-scale features on aerial photos 

 to be superimposed and viewed on a base map. The instrument should also have 

 the capability to rectify slightly oblique images (tilt) to a vertical equiva- 

 lent and to magnify or reduce imagery to the scale of the base map. 



Scale variations, which are the largest sources of error in aerial photog- 

 raphy, occur when the aircraft either fails to hold a constant altitude during 

 a single flight or maintains different altitudes on successive flights. Thus, 

 an error results when a comparison is made between two photos of slightly dif- 

 ferent scale showing the same location. Tilt errors occur when the optical 

 axis of the camera is tilted from true vertical as the photo is taken, causing 

 a variation in scale. . Relief displacement errors occur where the images of 

 objects above the mean surface elevation are projected outward from the center 

 of the photo and the images of those below the mean surface elevation are dis- 

 placed inward of their true location (Stafford, 1971, p. 29). When maximum 

 elevation differences are less than 6 meters, as they are along much of the 

 U.S. Atlantic coast, relief displacement errors are small. These errors can 

 be further reduced by using only the central part of the photo for measurement 

 purposes (possible when photos show at least 30 to 40 percent overlap, as indi- 

 cated in Stafford, 1971, p. 29) and by selecting flat- or low-lying objects as 

 reference points. 



The Bausch and Lomb Zoom Transfer Scope meets the requirements for applica- 

 tion in the base map technique. Use of the scope nearly eliminates scale and 



