40 U. S. COAST AND GEODETIC SURVEY 



five bench marks. For a primary tide station which is to be main- 

 tained over a long period of years ten substantial bench marks may 

 be considered as adequate. 



107. Primary bench mark. — At each princij)al tide station there 

 is usually one bench mark, known as the "primary bench mark," 

 which is selected for its stability and convenience of location and is 

 used in checking the elevation of the zero of the tide staff from time 

 to time, so that if any change takes place the proper allowance may 

 be made in the tabulations. The stability of the primary bench 

 mark is in turn checked by levels to a number of other bench marks 

 which are so located that their elevations will not be likely to be 

 changed by a common cause. 



108. Basic bench mark. — The basic bench mark used by the Coast 

 and Geodetic Survey consists of one of the standard disks set in 

 top of a concrete monument which extends about 2 feet above the 

 ground. The monument contains the inscription "United States 

 Coast and Geodetic Survey Basic Bench Mark" on one side and 

 rests upon a reinforced concrete foundation with an enlarged base 

 extending a number of feet below the ground. 



109. Descriptions. — Full descriptions of all bench marks must be 

 carefully prepared and forwarded to the oflSce with form 258, leveling 

 record — tide station. The descriptions should be clear and distinct 

 and sufficiently complete to enable the bench marks to be readily 

 recovered and identified. If a standard disk is used, the description 

 must include a definite statement whether the number of the mark, 

 the year of establishment, and its elevation have been stamped in 

 the metal. When a bench mark is located on a building, the street 

 and number should be given when possible, or the name of th6 owner. 

 When not on a prominent structure a bench mark should be refer- 

 enced by distance and direction to several prominent objects. A 

 sketch which will aid in locating or identifying the bench mark is 

 desirable. 



110. Leveling. — At the time of the establishment of a tide station 

 and at intervals thereafter the tide staff must be connected with the 

 bench marks by levels. If possible, all bench marks within a radius 

 of 1 mile of the tide station should be connected with the tide staff 

 at intervals not greater than 5 years, but it is desirable that the 

 staff be connected with not less than three bench marks including 

 the primary bench mark each year or at more frequent intervals, in 

 order that any change in the elevation of the tide staff may be de- 

 tected without unnecessary delay. It is also desirable that bench 

 marks of other organizations which may be in the vicinity should be 

 connected from time to time with the bench marks of this Survey. 



111. The graduating lines on the tide staff now in use have a 

 width of about 0.01 foot. The middle of each line is to be taken as 

 the reference' for heights on the staff. If a portable tide staff is used 

 at the station, the leveling rod should be held on the flat top of 

 the brass plate on top of the tide staff support, and the reading on 

 the tide staff to which this plate, corresponds must be entered in the 

 record. This staff reading is determined by the position of the 

 metal stop attached to the back of the staff. It is important, how- 

 ever, that the staff be placed in its support to see that this stop comes 

 in actual contact with the plate on the support without interference 



