Why so many US borders are straight

Lines on paper

A boundary is usually a river or lake, a divide between two drainage basins, or a line of latitude or longitude. Only the last depends entirely on surveyors to find it on the ground.^[1] For the western territories and states, Congress wrote territory and state lines into its acts as parallels and meridians.^[1]

For much of the 19th century those meridians were counted from Washington, not Greenwich. An 1850 act made the meridian of the Naval Observatory in Washington the American meridian for astronomy, and until the act was repealed in 1912 the north–south lines of Arizona, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, Utah, Wyoming and Dakota were written in degrees west of Washington.^[1]

Congress also owned the land itself. In 1785 surveyors started dividing public land into a grid of ranges and townships at a “point of beginning” on the north bank of the Ohio River, and the grid was later extended over every public-land state.^[1] Some state lines follow it directly: in 1889 Dakota Territory was split into North and South Dakota along the “seventh standard parallel” of the land survey.^[1]

Boxes on the globe

Colorado’s 1864 enabling act defines the state as the land between 37° and 41° north and between 25° and 32° west of Washington.^[1] Wyoming’s act of 1868 does the same between 41° and 45° north and 27° and 34° west of Washington.^[1] Utah would be a rectangle too if Wyoming did not cut a notch from its north-east corner, which leaves Colorado, Wyoming and Utah as the only states whose borders are all straight lines.^[2]

Even on paper they are not quite rectangles. Lines of longitude meet at the poles, so Colorado’s north border is about 35 km shorter than its south border.^[2] Other straight lines came out of politics: the parallel of 36°30′ was the Missouri Compromise line of 1820, north of which slavery was barred in the Louisiana Territory.^[3] When Texas sold its land north of that parallel to the United States in 1850, 36°30′ became the top of the Texas panhandle, with the 100th and 103rd meridians on its sides.^[1]

“Commencing at a point formed by the intersection of the thirty-seventh degree of north latitude with the twenty-fifth degree of longitude west from Washington; extending thence due west along said thirty-seventh degree of north latitude”^[1]

Straight lines, crooked surveys

Nineteenth-century surveyors had compasses and metal chains, and had to run straight lines through hard country whatever the terrain.^[2] Colorado’s line with New Mexico, run by E. N. Darling in 1868, has a jog of nearly half a mile near Edith. A later survey moved it, New Mexico sued, and on 26 January 1925 the Supreme Court ruled that Darling’s line, errors and all, was the border.^[1]

The Four Corners, where Arizona, Colorado, New Mexico and Utah meet, is the only point in the country shared by four states. It is where C. Robbins’s 1875 survey met Darling’s line, not the exact spot the laws describe.^[1][2] Counting every bend the surveys left, Colorado has not four sides but 697.^[2]

Surveys also made a few lines crooked on purpose. Missouri’s south border is 36°30′ except for the “bootheel” between the Mississippi and St. Francis rivers, said to have been added because a prominent landowner just south of the line wanted his plantation in the new state.^[1]