set on high ground and are tied to the tidestation near the water surface.Tide stations are classified as primary andsecondary. Primary stations require observationsfor periods of 19 yr or more to derive basic tidaldata for a locality. Secondary stations areoperated over a limited period (usually less than1 yr) and for a specific purpose, such as checkingelevations. The secondary station observations arealways compared to, and computed from, dataobtained by primary stations.A tide station is set up, and observations aremade for a period that is determined by a desiredaccuracy. These observations are compared witha primary tide station in the area and, then, arefurnished with a mean value of sea level in thearea.A closed loop of spirit levels is run from thetide station over the tidal BMs and is tied backto the tide station. The accuracy of this level linemust be the same as or higher than the accuracyrequired for the BMs.For permanency, tidal BMs usually are set insets of three and away from the shoreline wherenatural activity or future construction probablywill not disturb or destroy them.DATUMTidal datums are specific tide levels that areused as surfaces of reference for depthmeasurements in the sea and as a base fordetermining elevations on land. In levelingoperations, the tidal datum most commonlyused is the MEAN SEA LEVEL. Other datums,such as mean low water, mean lower low water,mean high water, and mean higher high water,are sometimes used, depending upon the purposeof the survey. Still other datums have been usedin foreign countries. When conducting levelingoperations overseas, you should check into thismatter carefully to avoid mistakes.Mean Sea LevelMean sea level (MSL) is defined as theaverage height of the sea for all stages ofthe tide after long periods of observations.It is obtained by averaging the hourly heightsas they are tabulated on a form similar to thatFigure 14-2.-Ways of using spikes as bench marks.14-3
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