1216 HYDROGRAPHIC MANUAL PaGE 12 



The project instructions will ordinarily specify what tide station is to be used as 

 a standard for the project and where it is located if an existing station can be used, 

 or where it is to be located if it is to be established by the survey party. An inspection 

 of and a report on an existing standard gage may be required. 



Supplemental tide stations are generally required and the instructions will specify 

 how many of these are necessary and the desired locations. The minimum length of 

 time each supplemental tide station is to be maintained will be stated. A Chief of 

 Party is authorized to depart from the specified locations if, on examination, they 

 appear impracticable. 



Copies of the descriptions of previous tide stations in the area, with the eleva- 

 tions of the bench marks and the heights of the datum planes at each, will be furnished. 



After the receipt at the Washington Office of the records of the establishment of a 

 new station, including the level records and at least 2 weeks of satisfactory tide observa- 

 tions, a preliminary plane of reference can usually be derived for use in the determina- 

 tion of the tide reductions to be applied to the soundings. This plane of reference will 

 be furnished to the survey party on request, but the request should be deferred until 

 the datum is actually needed. 



All tide stations shall be established in accordance with the instructions contained 

 in 143 and in Special Publication No. 196, Manual of Tide Observations (revised 1941 

 edition), except as amended by the project instructions. 



1216. Shore Magnetic Observations 



The project instructions may require magnetic observations at certain existing 

 stations or at new stations to be established within the project area. The Washington 

 Office will either specify a standard station at which the instrument used for the mag- 

 netic observations shall be standardized at the beginning and end of the season, or 

 issue instructions to return the instrument to the Office for standardization. 



In the absence of any specific instructions, magnetic observations shall be made 

 with the compass declinometer, or equivalent instrument, at intervals of not less than 

 10 miles along the coast in areas where no magnetic anomalies are found, and at lesser 

 intervals in magnetically disturbed areas. Wliere a magnetic anomaly of more than 

 2° is discovered, a sufficient number of additional observations shall be made in the 

 area to delineate the extent of the disturbance. 



The magnetic observations should be made at, or in the immediate vicinity of, a 

 triangulation station, if possible. If the observations are not made at a triangulation 

 station, sufficient data must be given to determine the approximate position of the 

 magnetic station. An azimuth determined to the nearest minute is required at every 

 magnetic station. If one is already available, definite information should be given so 

 that it can be determined from data in the records at the Washington Office or from 

 the records of the current field season. If an azimuth is not available, one must be 

 observed to a natural object or a nearby recoverable station whose position is known. 

 Three-point fixes should not be used for determining the azimuth, if other methods 

 are available. If used, the stations must be positively identified, and the fix must be 

 strong. 



All magnetic observations shall be made in accordance with the instructions 

 contained in Serial No. 166, Directions for Magnetic Measurements, except that, if the 

 observations are made with a transit magnetometer, the special instructions which 



