David Thompson's 1797-98 Survey

David Thompson Takes an Observation

Joseph Tyrrell’s editing of David Thompson’s journals and eventual publication in 1916 included Thompson’s explanation for the survey he conducted in 1797\98. Thompson’s brief mistaken explanation for the survey, stating that it was conducted to survey the 49th parallel, can likely be attributed to his age and forgetfulness. The true purpose of the survey has its origins in the context of the American Revolutionary War and the much more complex and significant mission given to Thompson by the leadership of the North West Company (NWC).

The Revolutionary war between Britain and the United States ended with the Treaty of Paris signed in 1783, and Article 2 of the treaty stated that the boundary proceeded “… to the said Lake of the Woods; thence through the said lake to the most northwestern most point thereof, and from thence on a due west course to the river Mississippi; thence by a line to be drawn along the middle of the said river Mississippi until it shall intersect the northernmost part of the thirty-first degree of north latitude.” Clearly the British and American negotiators determination of the boundary did not include firsthand knowledge from the fur traders who knew the territory all too well. Benjamin Frobisher, of the Executive Council of Quebec, expressed his dismay since he and NWC partners knew that the Mississippi did not extend far enough north to be west of Lake of the Woods.

As a result of the flawed boundary, David Thompson was sought out to be the Company’s official surveyor, the first task of which was to peform a pre-emptive survey to determine the source waters of the Mississippi River since the “source, or head of the Mississippi was then unknown except to the Natives and a very few Fur Traders” and to “learn the true positions of their Trading Houses, in respect to each other.” Such a survey would at least provide some idea as to what a possible boundary would look like and have the affected North West Company posts ready for relocation outside American Territory. Performed in the fall of 1797 through to the summer of 1798, Thompson would survey and map the bend of the Missouri River, determine the headwaters of the Mississippi River and perform a near circumnavigation and survey of Lake Superior, the first ever completed. The survey and mapping of these areas became highly influential and widely copied all over the world.

His mapping of the Mississippi headwaters and the north bend of the Missouri River were particularly significant, as they filled in crucial gaps in the understanding of North American geography at the time and therefore, ultimately, a proper boundary. Nearly 20 years later, the knowledge that David Thompson gained of that territory was sought out following the War of 1812 and provided Thompson the key resume item to be appointed the principal British astronomic surveyor for Articles 6 and 7 of the Treaty of Ghent.

The maps from David Thompson’s surveys following his return to Assiniboine House in February 1798 included a manuscript map of the north bend of the Missouri River. This map eventually came into the possession of U.S. President Thomas Jefferson, who provided it to Lewis and Clark in 1804. It now resides in the U.S. Library of Congress and is referred to as ‘Bend of the Missouri River’. On the map is a notation written in pencil by President Thomas Jefferson, “Bend of the Missouri, Long. 101 25ʺ-Lat. 47 32ʺ by Mr. Thomson, astronmer to the N.W. Company in 1798.”

A map (16 x 31 inches at 300 dpi) containing a detailed account of the 1797/98 survey conducted by David Thompson that is suitable for printing or study can be acquired HERE.

The artwork of David Thompson Taking an Observation, C.W. Jefferys, C.W. Jeffery fonds, Library and Archives Canada, Accession No: 1972-026 X PIC 01406