78 U. S. COAST AND GEODETIC SURVEY 



Zone, the ratio is 1.26, and this factor may be applied without mate- 

 rial error to the corrected mean range at any station on the Pacific 

 coast of the Canal Zone to obtain the spring range of tide. There- 

 fore to obtain the datum of mean low water springs at any station 

 in this vicinity, first compute the corrected mean range and half -tide 

 level by methods already described. The datum may then be ex- 

 pressed by the formula "Mean low water springs = half -tide level — 

 0.63 X mean range of tide." The factor may vary in other locations. 



TIDE REDUCERS FOR SOUNDINGS 



249. After the datum or plane of reference has been derived, the 

 tide reducers for soundings are obtained by subtracting the reading 

 corresponding to the datum from the recorded heights of the tide 

 taken at intervals during the time of the soundings. The differences 

 will in general be positive except when the tide falls below the datum. 

 The positive differences are to be subtracted from the soundings, but 

 when entered in the sounding books, the minus sign is usually omitted 

 for convenience. When the tide falls below the datum, the differ- 

 ence is to be added to the soundings and this difference must always 

 be prefixed by a plus sign when entered in the sounding record. De- 

 tailed instructions pertaining to the application of the tide reducers 

 to the soundings will be found in the Hydrographic Manual (Special 

 Publication No. 143). 



250. Graphic method. — When the reduced soundings are to be 

 given in integral feet, reductions for tide may be made easily and 

 rapidly from either the standard automatic tide-gage or the portable 

 automatic tide-gage marigrams by a graphic method described in the 

 following paragraphs. In using this method care should be taken, 

 however, to avoid confusion as to the times or the heights of tides, 

 especially when reading from a portable automatic tide-gage record 

 on which the curves representing the tide for different days are often 

 close together. At times it may be necessary to strengthen a faintly 

 traced curve of the record so that it may be sufficiently bold to be 

 readily seen through the transparent graphic scale. 



251. This scale is constructed on transparent tracing cloth or trac- 

 ing paper by ruling a series of horizontal lines spaced at intervals 

 representing feet in the same height scale as used on the marigram 

 and a series of vertical lines spaced at intervals representing hours 

 in the same time scale as used on the marigram. For the portable 

 tide-gage records the vertical lines may usually be omitted. The 

 horizontal lines are numbered upward from the bottom of the scale 

 + 3, +2, +1, 0, —1, —2, —3, —4, etc., according to the range of 

 tide, small figures being used for this numbering. A horizontal 

 line in red ink is now drawn on the tracing at the scale reading corre- 

 sponding to the value of the formula x—y — 0.7 ft., where ;r= height 

 of plane of reference (LW or LLW) above zero of the tide staff, 

 y= height of datum line of marigram as referred to zero of tide staff, 

 and the value "0.7 ft." represents the fraction at which the reduced 

 sounding changes by an integral foot. 



For the standard tide-gage record the value of "y" is the corrected 

 scale, setting as computed on form 465 (fig. 23). For the portable 

 tide-gage record the "0" of the marigram record may be taken as 

 the datum line and "y" becomes equal to zero. The red line will be 



