CHAP. IV.] THE MAIN TRIANGULATION 81 



used, and its position in order to fulfil the conditions specified 

 under the head of "Bases," must be considered, the base, 

 when extended, forming a side of one of the main triangles. 

 If it be possible to obtain the astronomical positions before 

 commencing the survey, a measured base is unnecessary. 



It is immaterial at what part of the survey the base is situ- 

 ated, but if it be near one end, a very satisfactory check on 

 the accuracy of the triangulation is obtained by comparing 

 the length of a side at the other extreme of the survey, derived 

 by calculation through the whole system of triangles, with the 

 length as deduced from a check base measured in its vicinity. 



It is generally a saving of time to measure the base at some 

 anchorage or harbour that requires a large-scale plan. The 

 triangulation involved in extending the base to connect it with 

 the main triangulation scheme can thus be utilised for both 

 purposes, and while the triangulation is being calculated and 

 plotted, the survey of the plan can be proceeded with. 



True bearings are observed at both ends of the survey, and 

 at as many main As near the coast as possible, the results 

 being subsequently compared. 



Astronomical observations for latitude are obtained at 

 observation spots near the extremes of the survey. The 

 meridian distance is run between them two or three times, and 

 the observation spots are connected with the primary tri- 

 angulation. They are usually disposed at intervals of from 

 100 to 150 miles. Thus, errors due to a triangulation carried 

 out Avith theodolites of moderate diameter do not accumulate 

 to any serious extent. 



If the survey be a very extended one, intermediate observa- 

 tion spots afford a satisfactory check, by comparing the posi- 

 tions as calculated in the triangulation with those obtained 

 by direct observation. 



A measured base is not essential. The length of any side Measured 

 in the triangulation being assumed as equal to a certain number ^*^® °°^ 



N6C6S- 



of " units," the lengths in " units " of the other sides may be sary. 

 determined therefrom. 



Calculating the distance in units from the observation spot 

 at one extreme of the survey to that at the other extreme, 

 and comparing the number of " units " thus found with the 

 distance in miles between the two observation spots derived 



6 



