While the line separating the United States and Mexico may seem pretty straightforward on paper, the ground truth isn't nearly that simple.
The U.S.-Mexico border was established by the Treaty of Guadalupe Hidalgo in 1848 and the Gadsden Treaty in 1853. These agreements allocated land to the two countries, but much more work had to be done to make the border a physical reality. Teams from both nations who surveyed, mapped, and placed landmarks along the border had to grapple with the challenges of desert and mountain landscapes as well as rivers that frequently changed course during heavy rains.
But even the lines in the sand drawn by these crews didn't settle once and for all where Mexico ends and the United States begins. Several conventions and two additional treaties have tried to resolve questions of where the border actually lies, particularly in instances where boundary rivers have changed course, transferring land across the border. The International Boundary and Water Commission, a bilateral organization created in 1889, has jurisdiction to settle such disagreements.
Technical quirks further complicate the precise location of the border. A total of 276 monuments, made of iron, granite, marble, and other materials and positioned with now outmoded technology, dot the line between the two countries. To the surprise of GPS hobbyists who trek along the border, newer, more precise measurements reveal that some of these markers are not exactly where the two governments thought they were. But even slightly misplaced monuments have the rule of law. Unless the two governments agree to move them to correct the errors, they stand as the official international boundary.