# Mean Definitions for Land Surveyors

**mean error—**See *error, mean; error, average*.

**mean high tide**– See *mean high water.*

**mean high water**– (MHW) – The mean height of tidal high waters at the particular point or station over a period of time. For tidal waters the cycle of change covers a period of 19 years, and mean high water is defined as the average of the high waters over a 19-year period.

**mean higher high water** (MHHW)- The average height of all the daily higher high waters recorded over a 19-year period or a computed equivalent period. MHHW is usually associated with a tide exhibiting mixed characteristics.

**mean low tide**– See *mean low water.*

**mean low water** (MLW) – The mean height of all low waters a a particular point or station over a considerable period of time. For tidal water, the cycle of change covers a period of 19 years, and the mean low water is the mean of all low waters for that period.

**mean lower low water** (MLLW) – The average height of the lower low water at a place over a 19-year period. An approximation of this level, called lower low water datum, is used as a tidal datum in some areas, including the Pacific coast of the United States.

**mean river level**– The average height of the surface of a river at any point for all stages of the tide over a 19-year period usually determined from hourly height recordings. Unusual variations of river level due to discharge or runoff may be excluded in computation.

**mean sea level** (MSL) – The average height of the surface of the sea for all stages of the tide over a 19-year period, usually determined from hourly height readings. A determination of mean sea level that has been adopted as a standard for heights is called a sea level datum. The Sea Level Datum of 1929 (now the National Geodetic Vertical Datum of 1929) was used for many years as the official datum for vertical control in the United States. The vertical control datum now used is the North American Vertical Datum of 1988 (NAVD 88), which was established by adjusting leveling observations from Canada, Mexico, and the United States. See also *Datum, National Geodetic Vertical, 1929.*

**mean sea level datum**– See *datum, mean sea level.*

**mean sidereal day**– See *time, sidereal *(definition **2**)*; day.*

**mean solar day**– See *time, solar.*

**mean solar time**– See* time, solar.*

**mean-square error**– See* standard deviation.*

**mean sun**– A fictitious sun which is assumed to move around the equator at a uniform rate, to complete a full revolution in the same length of time required for the non-uniform motion of the apparent or true son- namely, 1 year. The mean sun is the basis for mean time, as kept by a clock. The mean sun is so placed that, on the whole, it precedes the true sun as much as it follows it. At any instant, the difference between the mean sun and the apparent sun is the equation of time, which varies between 0 and about 16 minutes in each direction.

**mean tide level**– The plane, or surface, that lies exactly half way between mean high water and mean low water (also termed half-tide level). Because of the lack of symmetry of the tidal curve, mean tide level is not exactly the same as mean sea level.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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