Mean Sea Level

Mean sea level (MSL) is the average height of the sea for all stages of the tide after long periods of observations. It is obtained by averaging the hourly heights as they are tabulated on a form similar to that shown in Figure 5.

The heights on this form are added both horizontally and vertically. Enter the total sum covering seven days of record in the lower right-hand corner of the page. Find the mean for each calendar month by combining all daily sums for the month and dividing by the total number of hours in the month. Enter the monthly mean, to two decimal places, on the sheet that includes the record for the last day of the month. Yearly means are determined from the monthly means, and a mean is taken of all yearly means for the period of record. Use three or more years of records for a good determination of sea level. The actual value varies somewhat from place to place, but this variation is small.

For MSL determinations, use a station on the open coast or on the shore of bays or harbors having free access to the sea. Stations on tidal rivers at some distance from the open sea will have a mean river level that is higher than mean sea level because of the river slope. Note that mean sea level is NOT identical with mean tide level (MTL). The latter is derived from the mean of all high and low points on the tidal curve. But MSL is derived from the mean of a much larger number of points taken at hourly intervals along
the tidal curve.

The datum universally used in leveling is mean sea level (MSL), and it is the zero unit. The vertical distance of a given point above or below this datum then becomes the elevation of that point.