defined in terms of daily highs and lows and does not include such 

 changes as a seasonal variation in water elevation. Note that Mn is not a 

 tidal datum because it is not an elevation or level, but a range. Mean Tide 

 Level (mtl) or "half tide level" is a tidal datum and is the arithmetic mean 

 of mhw and mlw. The mtl may lie above or below msl by an amount 

 depending on the relative amplitudes of the diurnal (having a period of 

 approximately 1 lunar day) components of the tide. 



Mean higher high water (mhhw) and mean lower low water (mllw) 

 are, respectively, the averages of the higher high water heights and lower 

 low water heights of each tidal day (or lunar day; 24.84 solar hr) observed 

 over the NTDE. For example, if there are two lows in a tidal day, the 

 lower of the two is used to compute mllw for that day. The mllw is 

 presently used by NOS as a nautical chart datum. 



Tide gauge and tide station 



A tide gauge, or water-level gauge in a more general sense, is an 

 instrument for measuring the rise and fall of the tide or water level. 

 Presently, acoustic gauges and pressure gauges are used as automated 

 water-level measurement systems because of their reliability and 

 capability to produce an electronic signal for convenient processing. 

 These measurement systems also require little power and are suited for 

 remote sites where the power unit must be self-contained. A tide staff is a 

 simple form of tide gauge and consists of a vertical graduated staff from 

 which the height of the water can be read visually. 



A tide station, or water-level station in a more general sense, is the 

 geographic location at which tidal or water-level observations have been 

 made. The defining aspect of a tide station is the presence of a system of 

 (tidal) benchmarks, which are fixed physical objects (typically, deep- 

 driven rods) that serve as a reference for a vertical datum. The tidal 

 benchmarks are central to water-level measurement because the relation 

 between elevation of the land and water can be tracked through time, the 

 benchmarks providing the fixed reference (in the absence of subsidence or 

 crustal movement in general). Water-level measurement without reference 

 to benchmarks cannot produce recoverable tidal datums. 



1 2 Chapter 2 Water Level Definition, Measurement, and Properties 



