136 U. §. COAST AND GEODETIC SURVEY 
These checked figures may then be readily summed directly from the sheet and the 
sums and means entered in their appropriate places. The DHQ and DLQ are then 
derived by subtracting the mean values of the high and low waters of the month from 
the corresponding values of higher high and lower low waters, respectively. Provision 
is also made on the back of the high and low water form for deriving mean values of the 
range of the tide and of the inequalities by use of the factors in Tables 5 and 6. 
Computation Forms 
Comparison of simultaneous observations.—In connection with a short series of 
observations the form illustrated in Figure 55 is useful, as all the results desired for 
datum planes are derived on a single sheet. It is designed to accommodate 7 days of 
observations, but it is to be noted that it is not necessary that these be consecutive 
days. In Figure 55 a week of observations at Falls Creek, Alaska, is compared with 
simultaneous observations at Ketchikan. . 
In the columns of heights and height difference the higher high and lower low 
waters are marked with a check, and from these the appropriate sums and means are 
derived. The back of the form carries instructions and explanations. For stations on 
the Pacific coast, where the datum on the charts is mean lower low water, the instruc- 
tions call for filling in all the items shown for Falls Creek. For stations on the Atlantic 
coast, where the plane of reference on the charts is mean low water, the instructions 
call for the omission of items (4) to (9), (14) to (19), (25), (26), and the computation of 
DHQ and DLQ at the bottom of the page. In this case the heights of all the high 
waters are combined into a single sum and similarly the low waters, the headings being 
made to read HW and LW respectively. 
The form is also adapted for the computation of datum planes where the tide is 
predominantly diurnal. In this case, it will be recalled, mean higher high water is the 
same as mean high water and mean lower low water the same as mean low water. The 
use of the form with predominantly diurnal tides is illustrated in Figure 56. ~ 
For completeness, the form also has columns for deriving mean values of the 
lunitidal intervals. For datum planes these are unnecessary, and have not been used 
in the illustrative examples of Figures 55 and 56. The columns of “Time difference,”’ 
however, are used with the determination of datum planes to make sure that correspond- 
ing tides have been used at the subordinate and standard stations. If the difference 
for any particular tide in either of the columns of “Time difference’”’ varies considerably 
from the other values in its column, it will call for examination, and if found due to 
disturbed conditions may necessitate the elimination of all the comparisons for that 
tide. 
With a series of tide observations covering several or more months the form illus- 
trated in Figure 57 is used. Any outstanding value in the column of differences or of 
ratios calls for examination of the corresponding values at both the standard and sub- 
ordinate stations and helps in the detection of errors or elimination of observations 
disturbed by unusual conditions. In Figure 57, twelve months of observations at 
Avila are compared with simultaneous observations at Los Angeles. The corrected 
values for the various tidal data are then immediately available for determining desired 
tidal datum planes. 
