1437 HYDROGRAPHIC MANUAL PagE 34 



bottom, directed vertically upward, to obtain an echo from the water surface, thus 

 measuring the rise and fall of the tide. 



1437. Tide Observations by Radio 



Tide observations, especially in flood-control work, are also obtained by means of 

 an automatic gage that broadcasts the height of the tide by radio to the survey party. 



144. Compass Deviations 



For the accuracy of hydrographic surveys and the safety of the survey vessel it 

 is essential that the errors of magnetic compasses be kept to a minimum and that 

 their amounts be accurately known (see 4414 and 4415). 



Each survey ship and auxiliary vessel equipped with a magnetic compass shall be 

 swung to determine the compass errors, which shall be compensated for, so far as 

 practicable, at the following times: 



(a) After any extensive lay-up period in port, before proceeding to sea. 



(b) On the working ground at the beginning of each season's work. 



(c) During the field season whenever there is evidence of an important change in the deviations. 



The ship shall be swung and the compass compensated according to the instruc- 

 tions in Special Publication No. 96, Instructions for the Compensation of the Mag- 

 netic Compass. The results of the ship swing are recorded on Form 354, Observation 

 of Compass Deviations, and computed on Form 355, Computation of Compass Devia- 

 tions, and Form 356, Analysis of Compass Deviations. 



The deviations of the magnetic compass used in hydrographic surveying shall be 

 entered on page 1 of the first volume of the Sounding Records of each hydrographic 

 survey and, if changed during the survey, the new values shall be entered in the 

 appropriate volume, with the date of their applicability given (see 819c). 



A deviation table on Form 261, containing the most recent values, shall be posted 

 in the pilothouse or chartroom of every survey vessel. 



145. Weather 



A study of the meteorological conditions is of paramount importance, not only for 

 the safety of the vessel, but also for efficient planning of the survey operations (see 1333). 

 For the safety of the vessel, the regular Government forecast will usually suffice. 

 Major weather bulletins are issued daily via the United States Naval Radio Stations 

 and many special local broadcasts are made by certain specified radio stations. 

 Condensed data pertaining to the broadcasts are contained in chapter 1 of each 

 United States Coast Pilot. Additional information on weather l^roadcasts will be^ 

 found in the publications issued by the United States Weather Bureau, and in Hydro- 

 graphic Office Publication No. 206, Radio Weather Aids to Navigation (1941). 



1451. Transmission of Weather Reports 



Regular weather reports should not be sent from vessels of the Coast and Geodetic 

 Survey unless specific arrangements have been made with the United States Weather 

 Bureau. When engaged in offshore operations or when surveying in isolated localities, 

 the Chief of Party shall ascertain from the Weather Bureau if regular reports are 

 desired and, if so, he shall collaborate in furnishing them. Such reports should be 



