76 U. S. COAST AND GEODETIC SURVEY 



241. Form 248 (fig. 27) is designed to bring out the individual dif- 

 ferences between the tides at the two stations compared, the accuracy 

 of the resulting means depending somewhat upon the uniformity of 

 these differences. If any single difference varies greatly from th& 

 apparent average of its group, and an examination of the original 

 data fails to show an error, the difference should be rejected from the 

 sum and the fact indicated by encircling the rejected value. For 

 stations where the diurnal inequalities are desired, the higher high 

 waters, lower high waters, higher low waters, lower low waters, and 

 their differences are summed and averaged separately; and all the 

 spaces in the bottom portion of form 248 are filled in. If the diurnal 

 inequalities are not needed, as is generally the case for stations on the 

 Atlantic coast, all high waters and corresponding differences may be 

 summed and averaged without distinction and likewise all low waters 

 and differences, the notations at the bottom of the columns being 

 corrected to read IIW and LW. The mean high and low water 

 heights for (A) station willthen be entered directly as items (10) and 

 (11) in the form, and the mean differences from the last two columns 

 as items (20) and (21). The form will then be completed as far as 

 necessary to obtain the results desired. 



TIDAL DATUMS 



242. A tidal datum is a plane or surface which may be defined l:y 

 the tides and which is used as a reference for heights or depths. The 

 principal tidal datums now in use by this Survey are (1) mean sea 

 level^ the datum of the first-order level net and in general use as a 

 reference for heights ; (2) mean low water, the datum of soundings on 

 charts of the Atlantic coast of the United States; (3) mean lower low 

 water, the datum of soundings on charts of the Pacific coast of the 

 United States, Alaska, Hawaii, and the Philippine Islands; and (4) 

 m^ean loio water springs, the datum for the Pacific coast of the Canal 

 Zone and in more or less general use as the datum of charts published 

 by foreign countries. 



243. Mean sea level. — Mean sea level may be defined as the average 

 height of the sea for all stages of the tide. It is obtained by averag- 

 ing the hourly heights as tabulated in form 362. The heights in this 

 form are summed both vertically and horizontally, and the total page 

 sum covering 7 days of record is entered in the lower right corner of 

 the page. For a continuing series of observations, the mean for each 

 calendar month is obtained by combining all the daily sums for the 

 month and dividing by the total number of hours as indicated at the 

 bottom of the form for months of different lengths. The monthly 

 mean carried to two decimal places is entered at the bottom of the 

 sheet containinfi: the record for the last day of the month. 



244. Form 472a provides for the compilation of the monthly means 

 and the computation of the yearly means from the same. It also 

 provides for an accumulative mean combining all yearly means up 

 to date. The precision of an independent determination of mean sea 

 level depends largely upon the number of years of observations. In 

 general a series covering not less than 3 years should be obtained for 

 an independent determination of the datum. For a shorter series of 

 observations, the sea level as directly obtained should be reduced by 

 comparison Avith simultaneous observations provided there is a pri- 



