By an Act of Congress, generally referred to as the Compromise of 1850, New Mexicos east boundary was set at the 103rd meridian. But where was it prior to that time under Spain and Mexico? While never officially established, an answer of sorts may be found by examining early maps.
Until the beginning of the 18th century New Mexico was considered to border on Florida. The boundary was generally drawn along the Sangre de Cristo Range and, by extension, along the Pecos River, even though both of these features were more or less erroneously located on contemporary maps. Guillaume Delisle, geographer of Louis XIV, in 1703 produced a map that incorporated information provided by French explorers of the Mississippi watershed. On it he put the mouth of the Pecos River in nearly the right longitude but had that river running almost due south, showing the east boundary of New Mexico in about the meridian of the present Texas city of Lubbock.
In 1718 Delisle mixed mapmaking with politics when he produced a provocative map that drew angry protests from Britain and Spain. He had labelled the entire area between the Pecos and the Appalachian Mountains "Louisiana", an exaggeration that was exploited by the United States in 1803, when it claimed an area several times as large as Spain intended to sell in the Louisiana Purchase. On the Delisle map also appeared for the first time the name Tiejas [Texas]. Spain (which did not recognize the Delisle map) and the U. S. were still arguing over the location of the boundary in 1806, when Zebulon Pike called the Sangre de Cristo Range, which he mistook for the continental divide, a natural boundary between the province of Louisiana and New Mexico.
Texas was not shown with definite boundaries on any map until 1833, when David H. Burr, geographer to the U. S. House of Representatives, produced a map that was based in part on Stephen Austins sketch of his Texas colony. Burrs map was copied in 1835 by J. H. Young and published in 1837 in Philadelphia by S. Augustus Mitchell under the title New Map of Texas with the contiguous American & Mexican States. It shows the following boundary for Texas: [description is mine]
From the mouth of the Sabine River upstream to the 32nd parallel and along it to the Red River. Thence along the Red River to the junction of the Prairie Dog Town Fork and the Pease River near the present city of Vernon. From there, the Red River (and the Texas boundary) is shown erroneously to continue east along the 34th parallel to its imagined source in the Sacramento Mountains of New Mexico. The west boundary of Texas then winds southerly along the crest of the Sacramento and Guadalupe Mountains, continuing through the Davis Mountains of Texas to the source of the Nueces River and along it to the Gulf of Mexico.
Both cartographers show a portion of the south boundary of New Mexico, Burr in latitude 32 degrees 12 minutes and Young from a point ten miles south of Paso del Norte (Juarez, Mexico) which he erroneously placed in latitude 32 1/3 degrees (its true location is in latitude 31-44). From there it follows the parallel east to the crest of the Guadalupe Mountains, thence along that crest to the NW corner of Texas, and along the erroneously located Red River to the 100th meridian, thence north along that meridian to the Arkansas River.
In March 1848 the Texas legislature organized "Santa Fe County", its east boundary drawn in a sweeping arc along the Pecos and Red Rivers to the 100th meridian, thence north to the Arkansas. The 100th meridian cropped up again in May 1850 at a convention held in Santa Fe to draft a state constitution. The delegates set the east boundary of the proposed state of New Mexico along the 100th meridian, perhaps in retaliation to the Texas claim of a boundary to the Rio Grande.
Throughout 1850, in often venomous debate in Congress, many boundary proposals were made and voted down. The measure that eventually became law was the work of Senator James Pierce of Maryland, who in August came up with a longitude 103 – latitude 32 proposal. It was signed by President Fillmore on September 9 and accepted by the Texas legislature in November.
A recurring problem was the fact that most members of Congress were woefully confused about the geography of the American Southwest. Here as elsewhere so many times afterwards, accurate maps and adequate surveys were only made after paper boundaries had already been fixed.