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Chart of the Northern and Northwestern Lakes

Edward Molitor
U.S. Corps of Engineers (War Department)

Precision Via Triangulation
Maps for navigating the Great Lakes were far more accurate than those made for traveling by land.

The importance of waterborne transportation in the late nineteenth century—and the inherent dangers posed by rocky shorelines and narrow passages—called for highly accurate nautical charts. In fact, the U.S. Lake Survey conducted far more rigorous surveys than those of the General Land Office.

The Lake Survey used an array of precision instruments and employed triangulation to form the geographic framework of its maps. This technique allowed geographical locations to be determined with precision for the first time.

The first step in the mapping process involved establishing a baseline. Two of the baselines for the Great Lakes region can be seen at Chicago and Duluth. Baselines are established by determining two points and then precisely measuring the distance between the points. Surveyors determine the position of a third point by calculating the angle it makes with each end of the baseline.

Once the first triangle is produced, surveyors can use any two points of that triangle to continue this process, called triangulation. These calculations are repeated until the whole area to be mapped is surveyed.

The lake survey was begun in 1841 by the Corps of Topographical Engineers and was completed in 1882 by the U.S. Army Corps of Engineers, which merged with the Topographical Bureau during the Civil War. Most of the officers detailed to this duty were West Point graduates, trained in mathematics and the use of survey instruments. This triangulation became part of the general geodetic structure of the nation.