52 U. S. COAST AND GEODETIC SURVEY 



record in accordance with instructions for comparative note in para- 

 graphs 120-126. 



164. Measurements. — The depth of the harbor bottom at the tide 

 fjtaff and at the float well as referred to the floor of the wharf or other 

 specified fixed point should be taken and reported at each inspection 

 in order that there may be a record of any change in the depth which 

 may occur from time to time. These depths need be given only to 

 the nearest foot or half- foot. Previous measurements of the relation 

 of the tide staff and float well to the wharf floor should also be 

 verified. 



165. Bench marks. — At the time of inspection search should be 

 made for all bench marks within a radius of 1 mile from the tide 

 station, their condition noted, and the old description revised if 

 necessary. (See par. 109.) If an old bench mark cannot be found, 

 a statement to that effect should be included in the record, together 

 with an explanation whether the failure to find the mark results 

 from an inadequate description, inaccessibility of location, or posi- 

 tive information of its destruction. If the number of bench marks 

 recovered is less than the minimum of five marks for a primary tide 

 station, the inspecting officer should arrange for the establishment of 

 such additional marks as may be necessary. Descriptions and reports 

 concerning bench marks are to be included in the leveling record, 

 form 258. 



166. Leveling. — On the occasion of each inspection the tide staff 

 must be connected by a double line of levels with not less than three 

 suhstanfial bench marks including the primary bench mark, and the 

 results must come within the limits of accuracy indicated in para- 

 graph 112. For procedure in regard to leveling to a portable tide 

 staff or to a tape gage, see paragraph 111. The height of a bench 

 mark above the datum of a tape gage of the type illustrated by figure 

 2 is equal to the height of the bench mark above the reading mark 

 of the gage plus the reading of the plane of flotation on the tape scale 

 extended. If the bench mark is below the reading mark, the differ- 

 ence in elevation is subtracted from the reading of the plane of 

 flotation. Form 258 is to be used for the leveling record. 



167. Recommendations. — A report of inspection should include 

 recommendations for such repairs or changes as may appear desirable, 

 with an estimate of the cost when possible. When repairs are 

 urgently required, the inspecting officer should give them immediate 

 attention, requesting by telegraph approval for expenditure of such 

 extra funds as may be needed. 



SECONDARY TIDE STATION 



168. Secondary tide stations include those which are operated over 

 a very limited period of time, the observations in general extend over 

 less than a year but in some cases covering more than a year. 

 Secondary tide stations are established for the purpose of obtaining 

 general tidal information for a locality and also to obtain specific 

 data for the reduction of soundings in connection with the hydro- 

 graphic surveys. Observations at a secondary tide station are not 

 usually sufficient for a precise independent determination of tidal 

 planes, but when reduced by comparison with simultaneous obser- 

 vations ^t a suitable primary tide station very satisfactory results 

 may be obtained. 



