... Sighting the telescope, first at the tree and then at pole B, you measure the angle A of the triangle ABC, equal to the difference between the numbers you have read from the azimuth plate.. ... Why triangles? ... One large surveying project of the 1800s was the " Great Trigonometric Survey " of British India. ... Everest was the one who commisioned and first used those giant theodolites; they are now on display in the Museum of the Survey of India in Dehra Dum. ... Everest . ...