David Thompson made several trips to map western Canada. His first major exploration voyages, made under the banner of the Northwest Company, were intended to survey the border with the US from the Great Lakes to the headwaters of the Mississippi. The border had been disputed after the revolution and Jay's Treaty, and mapping the region was his first major contribution.
After Lewis and Clark's voyage into the Northwest on behalf of the US government, Thompson was commissioned to lead an expedition into the Northwest, with the hopes of finding the long sought-after Northwest Passage. (This had been one of Lewis and Clark's primary objectives as well.) In his search, he became the first European to travel the entire length of the Columbia River from the Rockies to the Pacific, a huge and lucrative breakthrough for the hotly contested fur trade and reason enough to pronounce his series of missions a success.
Along the way, he claimed territory in the region for Great Britain and announced the intention of the Northwest Company to build a fur trading depot close to the Snake River. But his lasting legacy, and a big part of his mission to the Northwest, was to map out huge swaths of Alberta and British Columbia.