MANUAL OF TIDE OBSERVATIONS 53 



LOCATION 



169. The selection of a location for a secondary tide station will 

 depend upon the purpose for which it is to be established. For the 

 hydrographic survey of any area there should be a principal station 

 somewhat centrally located which will be maintained through the 

 time the survey is in progress. If available, a standard automatic 

 tide gage should be installed at the principal station. If there is 

 already a primary tide station located within the area to be surveyed 

 this may serve as the principal station. 



170. Gages are to be established at other points in the immediate 

 vicinity of the soundings as the work progresses, the distribution 

 depending upon the change in the tide from point to point. In some 

 localities the tide may occur practically simultaneously with nearly 

 equal range over a large area. In such a case a single tide station 

 will serve for the entire area. In other localities the tide may change 

 rapidly in passing from one point to another and it may be necessary 

 to establish gages at close intervals. For the subsidiary stations, the 

 portable automatic tide gage will generally be found most convenient 

 to install and the records sufficiently accurate for the purpose. For 

 a very short series of observations, a plain tide staff to be read at 

 certain fixed intervals may be found sufficient. 



171. For sounding on outer coast. — For use in reduction of sound- 

 ings on offshore sounding work the tide record from a gage located in, 

 a harbor, such as at a primar;^ tide station, can be used with cor- 

 rections applied for difference in time and height. It is advisable, 

 however, to avoid a location well inside a river mouth or shallow 

 estuary or in a body of water having a narrow connection with the 

 sea. For use in connection with inshore hydrography along the 

 outer coast subsidiary staffs or portable gages should be established 

 where possible in the immediate vicinity of the work and connected 

 by means of comparison of simultaneous observations with nearby 

 stations where long series have been obtained, if available. 



172. Exposed channel approaches. — For surveys of exposed chan- 

 nel approaches, where the depths are near the draft of vessels and 

 where especially accurate soundings will therefore be required, for the 

 reduction of which the record from an inshore tide staff is not suf- 

 ficiently accurate, a temporary station should be established by pump- 

 ing down a pile or stake and several hours' tide observations obtained 

 on a portable gage or fixed staff simultaneous with the inshore sta- 

 tion. The observations at the temporary station should cover at 

 least a high and a low water. 



173. Abnormal tides due to configuration of shore. — In straits 

 connecting two bodies of water having tides of different ranges and 

 epochs of occurrence it usually happens that in portions of the straits 

 the tide varies rapidly from place to place. Hell Gate, East River, 

 N. Y., and the channel north of Vancouver Island, British Columbia, 

 are examples. When sounding in such straits, tide stations should be 

 established at frequent intervals. At times also appreciable differ- 

 ences both in time and height of tide occur on the different sides of the 

 same island in an archipelago. 



174. Upper reaches of tidal rivers. — In the narrow upper reaches 

 of tidal rivers a tide station is not representative of any considerable 

 area, and stations should be spaced sufficiently close together that 



