67. Other sources of potential error also were considered. The most 



difficult of these to determine accurately was the location accuracy of the 



MHWL on the source surveys and maps, on either (a) the early surveys prior to 



approximately 1930 and (b) the group of maps based on photogrammetric surveys. 



In discussing the early surveys, Shalowitz (1964, p 175) has stated: 



The accuracy of the surveyed line here considered is 

 that resulting from the methods used in locating the 

 line at the time of survey. It is difficult to make 

 any absolute estimates as to the accuracy of the early 

 topographic surveys of the Bureau. In general, the 

 officers who executed these surveys used extreme care 

 in their work. The accuracy was of course limited by 

 the amount of control that was available in the area. 



With the methods used, and assuming the normal control, 

 it was possible to measure distances with an accuracy 

 of 1 meter (Annual Report, U. S. Coast and Geodetic 

 Survey 192 (1880)) while the position of the plane- 

 table could be determined within 2 or 3 meters of its 

 true position. To this must be added the error due to 

 the identification of the actual mean high water line 

 on the ground, which may approximate 3 to 4 meters. 

 It may therefore be assumed that the accuracy of loca- 

 tion of the high-water line on the early surveys is 

 within a maximum error of 10 meters and may possibly 

 be much more accurate than this. This is the accuracy 

 of the actual rodded points along the shore and does 

 not include errors resulting from sketching between 

 points. The latter may, in some cases, amount to as 

 much as 10 meters, particularly where small indenta- 

 tions are not visible to the topographer at the 

 planetable. 



The accuracy of the high-water line on early topo- 

 graphic surveys of the Bureau was thus dependent upon 

 a combination of factors, in addition to the personal 

 equation of the individual topographer. But no large 

 errors were allowed to accumulate. By means of the 

 triangulation control, a constant check was kept on 

 the overall accuracy of the work. 



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 photography was at 



least maintained by reducing most of the 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 



53 



