Yuma

The survey of the western end of the U.S.- Mexico International Boundary began near San Diego.  The first leg of the survey determined the exact position of the boundary at the Pacific Ocean and its position at the Colorado River and then drew a straight line between them (to read about this part of the survey, go to The Journal Of San Diego History 1973 article titled "Amiel Weeks Whipple and the Boundary Survey in Southern California" by Thomas Scharf.).  They then crossed into what would later become Arizona and continued the survey up the Gila River, which was the agreed-upon boundary in Arizona until the Gadsden Purchase of 1852. 

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