2. 4. 1. Reference Datums 



A description of vertical datums pertinent to this study is presented in this section. 



National Geodetic Vertical Datum. The National Geodetic Vertical Datum of 1929 

 (NGVD 29) is a standard geodetic (related to the shape of the earth) vertical datum used by the 

 USACE and other agencies. NGVD 29 is a fixed vertical datum (sea level) observed at 26 

 primary tidal stations around the United States and Canada in 1929 (Shalowitz 1964). Therefore, 

 in the absence of accidental or other mechanical movements of the survey benchmarks, 

 NGVD 29 benchmarks are fixed through time and, therefore, form a convenient reference system 

 for civil engineering works. 



Construction Datums. On coastal engineering and other civil engineering projects, it is 

 often convenient to establish a local construction datum to which project measurements can be 

 referenced. The construction datum can itself be referenced to NGVD 29 or another datum. The 

 USACE construction datum along the Brevard County coast lies 1.9 ft below NGVD 29. 



Tidal Datums. Reference datums can be defined in terms of the phase of the tide and are 

 then called tidal datums. Tidal datums change slowly with time because of global sea-level 

 fluctuations and changes in local conditions, such as those associated with subsidence and water 

 and oil extraction from the ground or sea bottom. The NOS has the Federal mission of 

 determining and publishing tidal datums. This mission is accomplished by establishing a series 

 of permanent benchmarks on land, called tidal stations, and measuring the water level at fixed 

 intervals (typically, 6 min) with respect to the benchmarks. Water-surface records from short- 

 term stations, typically deployed from 3 months to 2 years, are then referenced to long-term tidal 

 stations with gauges that operate more than 19 years. In the 1970s, the State of Florida 

 undertook an extensive tidal measurement program in cooperation with the NOS. This 

 information is available for Brevard County. 



NGVD 29 is sometimes confused with or referred to synonymously as MSL. The datum 

 MSL is defined by NOS as the average of the hourly values of water-level readings of a specific 

 19-year tidal epoch called the National Tidal Datum Epoch (NTDE), presently 1960 to 1978. 

 However, because many variables control water level, and because a geodetic datum represents a 

 best-fit surface over a broad area and not to a specific area, NGVD 29 is not, in general, equal to 

 MSL. The geodetic datum can deviate from MSL by 1 ft or more, depending on location. 



The tidal datum MHW determines the boundary between State of Florida submerged bottom 

 lands and privately held uplands. The intersection of the land and sea at the elevation of MHW is 

 called the mean high-water line (MHWL), denoting the MHW shoreline. MHW and mean low 

 water (MLW) are, respectively, the averages of all the high-water heights and low-water heights 

 observed over the NTDE. The mean range of tide is the mean of the differences in height 

 between high waters and low waters over the NTDE. A tidal datum close to the value of MSL is 



2-12 



Chapter 2 Background 



