David Thompson (1770-1857) fur trader, astronomer and surveyor, mapped more of North America than anyone else. By horseback, canoe, dog sled and on foot, he travelled some 90,000 kilometres (55,000 miles), equivalent to circling the globe twice. – Ontario Government Archives
David Thompson is the premier explorer and surveyor of North America. From 1792 to 1812, David Thompson mapped the country west of Hudson Bay and Lake Superior, across the Rocky Mountains to the source of the Columbia River, and followed the length of the Columbia to the Pacific Ocean. – The Fur Trapper
Variously described as an English explorer and a Canadian explorer, cartographer, and surveyor, David Thompson was born in London to a Welsh father, but lived in North America from age 14 on. He was trained as a surveyor by the Hudson’s Bay Company but ditched them to go work for the North West Company, apparently so he could keep on exploring rather than having to move into a management job in the fur trade.
Although he died ten years before Canada became a country, his explorations and map-making opened up the West to further exploration, white settlement, and the eventual formation of Canada. And he did most of his work with a limp from a leg fracture, and blind in one eye (likely from looking at the sun through surveying instruments without proper eye protection).
He had an affinity for the Aboriginal peoples of the West, spending one winter with a Peigan elder, learning several Aboriginal languages, and marrying a Metis woman, Charlotte Small. They had 16 children together.
You can read some details of his trips and work here.