The Great Trigonometrical Survey, which aimed to measure the entire Indian subcontinent, was started by William Lambton. Here is a detailed explanation of the answer:

William Lambton:
- William Lambton was a British army officer and surveyor who initiated the Great Trigonometrical Survey in the early 19th century.
- He was born in 1753 and joined the East India Company's Bengal Army in 1773.
- Lambton was appointed as the Superintendent of the Trigonometrical Survey in 1802.
- He was known for his expertise in trigonometry and his meticulous approach to surveying.
- Lambton started the survey in the region of South India, specifically in the Madras Presidency.

The Great Trigonometrical Survey:
- The Great Trigonometrical Survey was a project undertaken by the British East India Company to map the entire Indian subcontinent.
- The survey aimed to measure the heights of mountains, the lengths of rivers, and the distances between various locations.
- The data collected from this survey was essential for cartography, military purposes, and the development of accurate maps of the region.
- The survey involved the use of trigonometry to measure distances and angles between various points.
- The surveyors used a chain of trigonometric stations, where each station was linked to the next by line-of-sight observation.
- The project was a massive undertaking and lasted for several decades, involving multiple surveyors and expeditions.

Significance of the Great Trigonometrical Survey:
- The Great Trigonometrical Survey was a significant scientific and engineering feat of its time.
- It provided accurate measurements and geographical data that greatly improved the understanding of the Indian subcontinent's topography.
- The survey played a crucial role in the mapping and exploration of India.
- It also helped in the demarcation of territorial boundaries, settlement of land disputes, and the planning of infrastructure projects like railways and canals.
- The data collected from the survey was used in the creation of the first comprehensive maps of India, including the iconic Great Trigonometrical Survey of India Map published in 1870.

In conclusion, the Great Trigonometrical Survey, which aimed to measure the entire Indian subcontinent, was started by William Lambton. His expertise in trigonometry and dedication to surveying laid the foundation for this ambitious project, which played a crucial role in mapping and understanding the Indian subcontinent's topography.