mean tide level (MTL). MTL is calculated as the mean of the differences between high water 

 and low water. 



In Florida, MHW surveys to be filed with the State involve coordination with the Florida 

 State Bureau of Survey and Mapping. The Bureau maintains a list of relations between 

 NGVD 29, the fixed land datum, and MFFW along the coast. These relations and other guidance 

 are provided to the surveyor, who can then locate the MHWL by an accurate beach-profile survey 

 that is connected to NGVD 29. 



2.4.2. High-Water Line (HWL) 



Historical data generated by the U.S. Coast and Geodetic Survey (USC&GS, predecessor 

 organization to the present NOS) in its survey of the coast performed in the 1800s, and in coastal 

 topographic surveys performed to present, identify a shoreline position as an interpreted HWL. 

 The authoritative reference on the meaning and procedures of measuring the HWL is Shalowitz 

 (1964), who was educated both as an attorney and engineer and was employed by NOS. 



Quoting Shalowitz (1964, pp. 171-172), The most important feature on a topographic survey 

 is the high-water line. It is the line that is used on the nautical charts of the Coast Survey as the 

 dividing line between the land and water; the line that indicates whether the coast is building out 

 or receding... Further, From the standpoint of the surveyor, the high-water line is the only line 

 of contact between land and water that is identifiable on the ground at all times and does not 

 require the topographer being there at a specified time during the tidal cycle, or the running of 

 levels. The high-water line can generally be closely approximated by noting the vegetation, 

 driftwood, discoloration of rocks, or other visible signs of high tides. 



The HWL is, therefore, not the shoreline defined by MHW, as sometimes marked on charts 

 and maps published by the NOS and the U.S. Geological Survey (USGS). Instead, it is the 

 shoreline mapped at the time of predicted MHW, which includes meteorological effects such as 

 setup, set down, and runup because of waves. USC&GS topographers and topographers today 

 doing routine wide-area shoreline-position surveys (such as by Global Positioning System (GPS) 

 techniques) refer measurements to the HWL at the time of MHW in the field. 



The HWL inferred from aerial photographs might be either the instantaneous intersection of 

 land and water at the time of MHW or the boundary between aeolian and waterborne deposits 

 determined by visual interpretation of a discontinuity in color or geomorphology (Anders and 

 Byrnes 1991). Mapped shoreline positions related to water level at the time photography was 

 flown (other than MHW) may be poor estimates of the HWL and inconsistent with the historical 

 database. Byrnes, Mc Bride, and Hiland (1991) discuss origins and treatment of various types of 

 shoreline-position data. 



Chapter 2 Background 2-13 



