Sentence:  The original base lines, which form the starting points of the survey, require to be measured by apparatus of the most delicate nature, since any error, committed in the early part of this work, will necessarily be multiplied throughout the series of <WSD>triangulations</WSD> which rest upon it. ... Nor is less precision necessary in the measurement of the angles of the primary and secondary <WSD>triangulations</WSD>, which serve as checks upon each other, since the position of any point in the former is determined by each of these series. 
Meaning of the word marked with <WSD> tags:  A technique in which distances and directions are estimated from an accurately measured baseline and the principles of trigonometry;  an instance of the use of this technique.