The border between the United States and Mexico has a long and often tumultuous history. After the end of the Mexican American War in 1848, the Treaty of Guadalupe Hidalgo gave the U.S. over half of Mexico’s land. This area eventually became all of California, Nevada, Utah, and Texas, and parts of Colorado, Arizona, and New Mexico.

The land that Mexico gave up in this treaty was still relatively unmapped when the treaty was signed, so the Treaty of Guadalupe Hidalgo also called for commissions from both the United States and Mexico to map the boundary and place permanent markers on the ground. The U.S. and Mexico Boundary Commissions mapped this boundary between 1849 and 1857, producing over fifty maps in the process.

During survey negotiations, the United States-Mexico boundary was marked by negotiators on a map. The surveyors’ job, then, was to find this drawn boundary on the land itself. The treaty writers understood that they could not define the boundary line exactly where the surveyors would end up placing it on the ground, so they decided that the surveying commissions would have the final decisions as to its exact placement. Because the commission included members from the United States as well as Mexico, the results and conflicts often reflected varying politics as each side vied for as much land as possible.

The Treaty of Guadalupe Hidalgo did describe some of the boundary areas, following both natural rivers and geometrical lines. These geometric lines were nearly always drawn by a treaty committee with no personal knowledge of the area’s geography, meaning that surveyors often found it difficult to access the land in order to conduct a survey and place boundary monuments.

However, river-based boundaries were not perfect either. The precise location of river banks is likely to change over time, and in several occasions differing lines following the various river channels could be interpreted as the true boundary. In fact, the Rio Grande became a point of contention for several decades following the completion of the boundary surveys, and an International Boundary Commission was created in 1889 to resolve problems related to changes in the Colorado River and the Rio Grande.

Another issue complicating the matter was the map upon which the treaty writers based their decisions; much of the area had not been properly mapped before this time, and the treaty writers were relying on an old and incorrect map. In many places, the treaty writers wrote that the boundary would be drawn at a particular point shown on the map, but they did not ensure that this map actually coincided with geographical reality. The map they used did not correctly show the location of several existing towns or the route of the Rio Grande, which were used as important points to describe the location of the boundary.

These issues caused problems almost as soon as the surveyors began their work. The boundary was particularly questioned to the west of the Rio Grande. The result, known as the Bartlett-Garcia Conde compromise, was soon embroiled in controversy too. In California, the surveyors began entangled in a political battle over the port of San Diego, which was wanted by both sides. In 1853, a new U.S.-Mexico treaty known as the Gadsden Purchase was signed to further describe the boundaries, removing several sources of controversy and clarifying what the surveyors had already accomplished.

The surveying of the U.S.-Mexico boundary, extending from the Gulf of Mexico to the Pacific Ocean, was completed in 1855, Once this fieldwork was complete, the then produced the first maps of the surveyed boundary line; these were completed in 1857.

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