88 



MEAN SEA LEVEL 



159. Use. — This datum is the basic plane of reference and the zero of 

 the ordinates of the harmonic components of the tide. It is deter- 

 mined by averaging the observed hourly tidal heights, measured from 

 a fixed bench mark, over a sufficient period of time. Because of the 

 variation in the density of the waters of the oceans with changes in 

 their temperature and salinity; because of the variation in the mean 

 barometric pressure upon them; and because of the effect of winds, 

 evaporation, and precipitation; mean sea level at different tidal sta- 

 tions may not be on precisely the same geodetic level surface. Thus 

 mean sea level at Balboa, at the Pacific entrance to the Panama Canal, 

 as determined from observations extending over 25 years, is nearly 0.7 

 foot higher than at Cristobal at the Atlantic entrance. In general, 

 however, mean sea level at tidal stations which have a free connection 

 with the sea, when determined from observations extending over a 

 number of years, are so nearly on the same level surface that the 

 difference between the elevation of any point on land above mean 

 sea level at one station, as determined by a line of levels from that 

 station, and the elevation of the same point above mean sea level at 

 another station, is within the error inherent in long lines of levels. 

 Mean sea level is therefore the standard reference datum for land 

 elevations. At tidal stations on tidal rivers, or on land-locked bays 

 and sounds with restricted entrances, the mean tidal height may be 

 above mean sea level and is more correctly designated as mean river (or 

 bay) level. 



160. Fluctuations in mean sea level. — Small fortnightly, monthly, 

 and semiannual fluctuations of mean sea level result from the long 

 period harmonic components established by the attraction of the sun 

 and moon (par. 71). These are, however, completely overshadowed 

 by the disturbances resulting from storm tides, and smaller systematic 

 meteorological disturbances. 



161. Storm tides. — Occasional violent fluctuations of the water 

 levels at a tidal station result from strong onshore or offshore winds. 

 When these are of hurricane velocity, the water may be raised many 

 feet. In the long run, storm disturbances raise (or lower) both the 

 high and the low waters by substantially the same amount, and may 

 be considered, therefore, as affecting primarily the heights of mean 

 sea level. 



162. Systematic meteorological variations in mean sea level. — Lesser 

 atmospheric disturbances produce a less apparent, but more con- 

 tinuous, variation in mean sea level. The seasonal variations in the 

 density of the water on the continental shelf and in the mean baro- 

 metric pressure over wide areas of the oceans, with concurrent varia- 

 tions in the prevailing winds, and perhaps other meteorological causes, 



