This layer represents locations of observations taken on the outbound journey of the Lewis and Clark Expedition as noted in the Lewis and Clark Journals. These locations were determined by James D. Harlan of the University of Missouri.

The Lewis and Clark National Historic Trail is approximately 4,900 miles long, extending from the Ohio River in Pittsburgh, Pennsylvania to the mouth of the Columbia River in Oregon, following both the outbound and inbound routes of the Lewis and Clark Expedition. The trail connects 16 states (Pennsylvania, West Virginia, Ohio, Indiana, Kentucky, Illinois, Missouri, Kansas, Nebraska, Iowa, South Dakota, North Dakota, Montana, Idaho, Washington, and Oregon) and was established by Congress in 1978 as part of the national trails system (NTS) as one of four original national historic trails. This trail commemorates the 1803-1806 expedition of Lewis and Clark and the Corps of Discovery, which explored the Louisiana Territory and beyond. This epic journey contributed to significant scientific knowledge and profound political, social, economic, cultural, and environmental changes to the lands and the peoples of the North American continent.