Major James Rennell FRS FRSE FRGS (3 December 1742 – 29 March 1830) was an English geographer, historian and a pioneer of oceanography.

His father, John Rennell, an officer in the Royal Artillery, was killed in action in the Low Countries in July 1747 during the War of the Austrian Succession.

[2] His mother Anne subsequently married Mr Elliott, a widower with children of his own and unable to care for additional ones, leading to Rennell being brought up by a guardian, the Rev.

[2] Rennell entered the Royal navy as a midshipman at the age of fourteen under Captain Hyde Parker, who was in the process of commissioning the frigate HMS Brilliant (launched in October 1757) at the beginning of the Seven Years' War.

In 1760, he went out to East India, and served in HMS Grafton under Captain Hyde Parker during the three following years, when he saw some active service, including a cutting-out expedition at Pondicherry.

He soon mastered the theory and practice of marine surveying, and, on account of his proficiency in this regard, Parker lent his services to the East India Company.

He served for a year on board one of the company's ships bound to the Philippines, with the object "of establishing new branches of trade with the natives of the intervening places".

Rennell accompanied the hydrographer Alexander Dalrymple and drew several charts and plans of harbours on journeys in the schooner Cuddalore (1759–62), the London (1762–63) and the Neptune (1763–64).

Owing to the friendship of an old messmate Mr Topham, who had become the governor's secretary, he was chosen for survey duties and was initially commissioned as an ensign in the Bengal Engineers, dated 9 April 1764.

[3] The headquarters of the surveyor-general were at Dacca, and in the successive working seasons he gradually completed his difficult, laborious, and dangerous task.

[8] Economic pressures further forced a reduction in pay and there was dissatisfaction among the native ranks on the question of double batta (part payment).

The mapping project was originally a general survey of newly acquired lands, but the job soon gained a wider scope under Warren Hastings, who was appointed as Governor-General in 1773.

As for Rennell's part in this, his project was carried out much like a military survey, searching for safe passage through territory, with information gathering a secondary object.

The remaining fifty-three years of his life were spent in London, and were devoted to geographical research chiefly among the materials in the East India House.

He was interred in the nave of Westminster Abbey, and there is a tablet to his memory, with a bronze bust by Ludwig Hagbold, near the western door.

[17] In 1851, botanist Pieter Willem Korthals published Rennellia, a genus of flowering plants from Indo-China and western Malesia, belonging to the family Rubiaceae and it was named in James Rennell's honour.

[18] James Rennell is credited with the original mapping of the Mountains of Kong, supposedly located in the western part of Africa, based on information supplied by explorer Mungo Park.

This range was intended to support his own theory on the course of the Niger River, and eventually led to a major impact on the mapping industry to include the mountains.