Continuance

Continuance Fall / Winter 2003-2004

Explanation of Clark's Map

The map by Clark represents detailed survey information regarding the immediate area around the Confluence of the Ohio and Mississippi Rivers at present day Cairo, IL. On the right (flowing around C is the Ohio River, with A marking the confluence. Northwest of A is the Mississippi River. This map is thought to be the very first sketch survey map of the journey. This map and field notes provided the basis of official maps of the Lewis and Clark Expedition. Research continues to find the "point of beginning" and to establish a historical marker at the site. On the back of this map (enlarged on the back cover of "Continuance") Clark made the following notations, some of which are legible on the map. Clark's notes are in bold print, followed by explanations using A B C D E F on the map for reference points.

From the Point of the Mississippi & Ohio 25 poles from the highest Land on the Sand point

The primary location for the initial set-up of the survey equipment and subsequent survey appears to be at the waters edge at the southernmost tip of Illinois indicated on the map as a small circle. This point A at the water's edges is 25 poles [412.5 feet – in surveyor's language, a "pole" equals 16.5 feet and is the same unit of measurement as a "rod" in modern surveying terminology] up the bank to the highest land or "sand point" near where they probably camped.

From thence e to Signal on the opposite Side of the Ohio is N. 30 1/2 degrees E 149 poles and 32 po and 7/ 10 to the bank

They appear to have placed a "signal" or a tall post with an easily visible flag or marker across the Ohio River in shallow water near the Kentucky shore representing the shortest distance across the Ohio River in order to determine the width of the river at the Confluence. They used a surveyor's compass to identify the direction of this line referenced from true north. The angle measured counter-clockwise from due north to the above referenced signal is 30 1/2 degrees. The distance across the river was more difficult to obtain as it was basically impossible to stretch and control a "chain" consisting of a series of connected 8 inch long metal links totaling 66 feet, or a "set of chains" a third of a mile across moving water. They appear to have used trigonometry by establishing a line AE along the bank on the Illinois shore for a determined distance of 115 poles (190 feet) and then using an "octant" of a surveyor's compass to measure the angle at E between C & A and then an angle at A between C & E. Using books with trigonometric functions and charts, the distances of the lines EC and AC were calculated rather than directly measured. The distance from A to C was 149 poles or 2,485.5 feet, plus another 32.7 poles, or 539.55 feet up to the actual bank from the signal (flag). Therefore the actual distance across the Ohio River along line AC to B was 3,025.05 feet or 1,008.35 yards. Note: This is less, for some unknown reason, than the 1,274 yards across the Ohio indicated earlier in the Journal.

From the Said Beginning Cross the Mississippi to a Signal is S 33 degrees E

The "signal" appears to be adjacent to a "house" drawing on the map. This line represents the shortest distance or width of the Mississippi River from point A to point F. The angle of this line from A to F is referenced to true South instead of true North and is 33 degrees counter-clockwise from a south direction on the surveyor's compass. (235 poles) The distance across the river is 235 poles (times 16.5 feet, or 3,875 feet), or 1,292.5 yards. Note that this is also less than the 1,435 yards indicated in the Journal.