Mapa de los Estados Unidos de Mejico [Map of the United States of Mexico] by J. Disturnell, 1847

Negotiators of the Treaty of Guadalupe Hidalgo agreed to use an 1847 map by J. Disturnell as the reference for the new boundary between the U.S. and Mexico. They found that the map had some serious errors, depicting El Paso and part of the Rio Grande many miles from their true locations, which complicated the negotiations.

Geography and Map Division, Library of Congress