Surveying the line. The line was surveyed in three stages– the KS-OK border in 1857, the NM-CO border in 1868, and the UT-AZ line in 1901. In theory the latitude should have been easy: find the 37th parallel using astronomical methods and then move methodically west (or east), measuring every mile with a 66-foot surveyor’s chain (80 chains to the mile). Longitude, however, was a lot more difficult, in the early days. In 1881 surveyors used telegraph signals to compare solar noon in Cincinnati with with solar noon in Las Animas, CO— and then chained 73 miles south to the 37th parallel in order to establish the Colorado-New Mexico-Oklahoma corner.

Now, having established a starting point– the CO-NM-OK corner, for example– a surveyor would aim a transit due west, mark a point a mile away, move to that point, align the transit with the previous point, rotate the transit 180 degrees and project a straight line due west again, scribing a line straight across the great plains. In practice, all three of the major surveys suffered from errors that even today show up in detailed maps. Two-thirds of the way across the NM-CO border, surveyor Ehud Darling realized he had drifted three quarters of a mile south. He adjusted his line north but never went back to correct his slanted line. This error eventually prompted a lawsuit: New Mexico took Colorado to court to recover the skinny wedge of land along the border; New Mexico lost, but the final border wasn’t declared settled until 1960.

Pre-survey planning. The map details below indicate how little was actually known about the landscape upon which the border lines were to be drawn.

Above: ‘Map of the Territory of the United States from the Mississippi to the Pacific Ocean’ ordered by the Hon. Jeff’n Davis, Secretary of War… compiled by Lieut G. K. Warren, Topl Engrs, … 1855. The red line is drawn on the 38th Parallel. The map appears to be incomplete: territory west of the Sangre de Cristo mountains– what would become the 37th parallel borderlands– evidently had not been explored when the decision to draw the borders at 37 degrees north latitude was made. Source: Schulten, Susan, Mapping the Nation. See mappingthenation.com.