2211 HYDROGRAPHIC MANUAL PaGE 70 



In planning the establishment or extension of a scheme of coastal triangulation, 

 there are certain requirements, common to all projects, which may be enumerated as 

 follows: 



(a) Efficiency in reconnaissance. The scheme should be the strongest practicable under existing 

 conditions. 



(6) Economy in stations and signal building. The scheme should not contain more stations than 

 needed to provide the required control. The stations should require a minimum of signal building. 



(c) Permanence of stations. 



(d) Accurate and systematic observing. 



(e) Coordination of triangulation with other ship or shore party operations. 



(/) Accessibility, not only for the observing party, but for the topographic party and those other 

 organizations and individuals who, at some future time, may wish to make use of the stations and lines 

 established. 



ig) Provision for future lateral expansion of the scheme and for possible additional stations inside 

 the figures themselves. 



2211. Principal Scheme 



The principal scheme of triangulation shall be carefully planned. New triangula- 

 tion, except in areas where a geodetic datum has not been established, must be started 

 from a previously established line of first- or second-order triangulation. All necessary 

 lines of the principal scheme must be observed, if practicable. Superfluous lines must 

 not be observed, however, for not only is valuable observing time thus lost but the 

 resulting triangles are useless and complicate the office adjustment. 



In the principal scheme, an occasional unoccupied station may be tolerated, but 

 should be avodied if practicable. Too many concluded angles in the principal scheme 

 are objectionable. 



The computation of triangles must be kept up with the observations in order to 

 verify their accuracy by the length and side checks. The field computations through 

 the determination of geographic positions, with accompanying list of geographic posi- 

 tions, should be forwarded to the Washington Office with the other records. 



There is no restriction on the number of unoccupied stations and concluded tri- 

 angles which may occur in the location of supplemental or third-order stations, provided 

 the length and side checks are satisfactory. 



2212. Spherical Excess 



Spherical excess is often a factor in the computation of triangles. The ratio of 

 spherical excess to the area of a triangle is 1 second for 75 square statute miles. For 

 preliminary use in closing triangles, the spherical excess can be computed from the 

 graphically scaled lengths, in statute miles, of the base and altitude of a triangle by 

 applying the formula: 



base X altitude 



Spherical excess (in seconds) =- 



150 



For the computation of triangles, spherical excess is computed by logarithms and for 

 second- and third-order triangulation need be carried only to tenths of seconds. (See 

 specimen of computations, page 94, Special Publication No. 145.) 



2273. Observing Weather 



Consistently good results can be obtained only by the use of the best grade of 

 instruments in the best observing weather. It is wasted effort to attempt observa- 



