David Thompson joined the Hudson’s Bay Company in 1784 as an apprentice. He developed his skills as a surveyor and map maker, but the company was more interested in procuring furs from the Indians and ordered him to stop his surveying work. In 1797 he joined the North West Company and during the next sixteen years he traveled through more than 55,000 miles from the Missouri River in the south to Hudson Bay in the north, the Mississippi River on the east and the Pacific Ocean on the west. 
     To determine the longitude, in the absence of accurate chronometers, Thompson observed the eclipse of Jupiter’s moons, or by measuring the angle of the moon against two stars. With “peg points” determined in this way he then completed the survey to fill out the detail. 
     In 1813 he created a chart approximately ten feet by six and a half feet tall. The original of the Map of the North-West Territory now hangs at the Archives of Ontario, Canada. Thompson erroneously included the “Caledonia River” between the Fraser and Columbia Rivers on his map. A year later he corrected the error. However, others who pirated his work continued to include the fictitious river form many more years. 
      From 1816 to 1827 Thompson served as chief astronomer and surveyor for the British team of the International Boundary Commission. After he resigned from the Boundary Commission he again turned to surveying the Muskoka Lake area in southern Ontario.
     The map on the stamp is not Thompson's Map of the North West Territory of the Province of Canada. It is rather a fictional map of the general area.

For further information about David Thompson see Stephen R. Brown, The Astonishing Astronomer of the Northwest," Mercator's World 6:2(2001)42-49.