The rocks are prominent monoliths and may resemble hay-ricks, some of which are 1,500 ft (460 m) high, and some of which have springs on the top. They provide formidable sites for fortification because most are scalable only by steps cut in them. There is a famous droog at Tiruchiripalli in Tamil Nadu, also known as the Rock Fort. This is climbed by 434 steps cut into the rock face. Temples to Ganesha and Shiva have been built there. Droogs were used as sites for his theodolite by Colonel William Lambton while Superintendent of the Geological Survey of India in the 19th century. The survey was completed by Sir George Everest (pron eeve-rest)for whom Mount Everest is named. The survey, commissioned by the East India Company, laid the foundation for building the extensive Indian railway system.
Excerpt from "Gods, Guides and Gurus'" John Hatchard (2010) If the Madurai skyline was dominated by the huge Gopuram towers of the Meenakshi temple, Trichy’s was dominated by a spectacular granite outcrop crowned by a fort and a temple. The view from the top had to be spectacular so it was a place to visit. We had passed similar upstanding natural features on the way here, huge granite monoliths, smooth and rounded like an elephants back and spectacularly water stained. These outcrops, found all over southern India, were called droogs. In most places they provided easily defended sites where local chieftains could base themselves. Here in Trichy it was presumably the same story. As time went by temples were added and the place became more important.
These droogs proved of great use when, in the early 19th century, Colonel William Lambton became Superintendent of The Great Trigonometrical Survey of India. From a base line at St. Thomas Mount, Madras he worked inland and then took the survey south from Bangalore toward Cape Cormorin. He used both droogs and temple gopurams as high points on which to place the great 450 kilogram theodolite used to measure his angles. This survey, commissioned by the English East India Company, gave it an accurate account of the lands it now controlled and was the most remarkable exercise of its kind ever undertaken anywhere. There is a story that while attempting to establish the theodolite on a certain droog, Lambton ran into trouble with a local Muslim chief who believed he was really intent on spying on his harem.
Lambton’s successor, Sir George Everest (pronounced Eve-rest, as in Adam and Eve!), completed the survey after Lambton’s death in 1823 and took it, with great difficulty, through dense forests and across deserts up into the Himalayas. Another objective of the Survey was to determine the exact height of various peaks in the Himalayas about which there was much speculation. When the heights of the greatest peaks had been surveyed as accurately as possible, the highest peak was named for Sir George Everest. The Survey laid the ground for building the Indian railway and road system.