-k- 



-H- 



-\- DIGITIZED VALUES 



-1-- CORRECTED VALUES 

 ' ADJUSTED TO TRUE 

 VALUES FOR 

 INTERSECTIONS 



Figure 19. Digitization procedure for correcting 

 shoreline position locations when original shore- 

 line movement map distortions exist (Everts, 

 Battley, and Gibson 1983) 



58. Accuracy of the high-water line on early topographic surveys of the 

 Bureau was thus dependent upon a combination of factors , in addition to the 

 personal equation of individual topographers, but no large errors were allowed 

 to accumulate. By means of triangulation control, a constant check was kept 

 on the overall accuracy of the work. 



59. On aerial photographs, the MHW line is located to within 0.5 mm at 

 map scale (USC&GS 1944) . This translates to less than 5 m on the ground for a 

 map scale of 1:10,000 or 9.99 m on the ground for a map scale of 1:20,000. 

 Since the great majority of source maps were of a larger scale than the 

 1:24,000 base maps, the 0.5 -mm accuracy of source maps made using aerial pho- 

 tography was at least maintained by reducing most source maps to the common 

 base scale of 1:24,000. Present NOS survey maps are even more accurate. In a 

 recent shoreline mapping project in the State of Florida using NOS charts, 



36 random features such as road intersections and shoreline features, includ- 

 ing points of marsh, were scaled from maps compiled from aerial photography. 

 These features were located by field traverse, and geodetic coordinate values 

 compared. The check revealed a maximum error of ±3.0 m. This accuracy is not 

 claimed for all surveys, but it does serve as an indicator of accuracy of sur- 

 veys conducted by NOS . 



60. The last source of potential error in map production is conversion 

 of digitized values to GP's. Digitizing equipment automatically recorded 

 1,000 coordinate values for every inch of shoreline traced, which were 



45 



