In a more general context, the National Ocean Service (NOS) of the 

 National Oceanic and Atmospheric Administration (NOAA). U.S. 

 Department of Commerce and U.S. Army Corps of Engineer (US ACE) 

 Districts nationwide are converting to mean lower low water (mllw) as the 

 chart datum for navigational tidal waters of the United States. The NOS is 

 revising charts for the east and Gulf coasts with mllw as chart datum, a 

 several-year process. Tidal datums such as mllw are calculated and 

 published by NOS, and, by mandate of the Congress of the United States, 

 the USACE must reference this NOS chart datum. The Districts are 

 following guidance issued by Headquarters, USACE, in Engineer 

 Technical Letter (ETL) 1 1 10-2-349 (USACE 1993). Also, by 

 ETL 1 1 10-1-152 (USACE 1994), Districts are required to implement the 

 North American Vertical Datum of 1988 (NAVD 88) as a replacement for 

 the National Geodetic Vertical Datum of 1929 (NGVD 29). 



The NGVD 29, a standard geodetic reference for elevations, is 

 sometimes confused with or referred to synonymously as mean sea level 

 (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 there are many variables controlling 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. Thus. 

 NGVD 29 and NAVD 88 are not equal to msl, do not bear a consistent 

 relation to msl along a coast, and are fixed, whereas the elevation of msl 

 typically changes slowly with time. 



For several decades, CESWG has recognized that standard low-water 

 datums such as mean low water (mlw) and mllw that are defined in terms 

 of the daily phase of the tide as computed by NOS are not adequate as a 

 navigation datum within the waterways along the Texas coast. This 

 recognition owes to four observations: 



a. In the shallow inland coastal waters of Texas, the astronomical 

 tidal signal is very weak, and the seasonal change in water 

 elevation within waterways typically exceeds the range in daily 

 tidal elevation for long periods of time (as long as months). 



Chapter 1 Introduction 



