Editing Union army (section)

===Corps of Topographical Engineers===
{{main|United States Army Corps of Topographical Engineers}}
The Corps of Topographical Engineers had been established in 1831 with the mission of [[exploration]], [[surveying]], and [[cartography]], particularly in the [[American West]].  Topographical engineers (or "topogs") including [[John C. Frémont]], [[Howard Stansbury]], [[William H. Emory]] and [[Gouverneur K. Warren]] were instrumental in the westward expansion of the United States.<ref name="Baldwin27">Baldwin, p. 21-27</ref>  During the war, the civilian nature of the Corps' mission was largely suspended and it undertook the role of reconnaissance, construction of defensive works and supplying maps to the army.  In these conditions drawing a distinction between the two corps became increasingly impractical, until in March 1863 the Corps of Topographical Engineers was disbanded and its mission taken up by the Corps of Engineers.<ref name="Newell292"/>

The Topographical Engineers numbered forty-five officers before the Civil War.  In August 1861 an additional two lieutenant colonels, four majors and six lieutenants were authorized by Congress. There were no enlisted men in the Corps, although when the Corps was expanded Congress also authorized a company of engineers for the Topographical Engineers to be modeled after the Corps of Engineers.  However this company was never formed, and the actual size of the Corps shrank as a number retired, died, defected to the Confederacy or became general officers of the volunteers, until eventually the remaining officers were absorbed by the Corps of Engineers.<ref name="Newell292"/>

The most important role played by the Topographical Engineers, even after their merger with the regular Corps, was providing desperately-needed maps to army commanders.  Each field army headquarters established their own topographical departments under the supervision of engineer officers, which would provide the army with maps necessary for a given campaign.  Such departments themselves were staffed with teams of [[draughtsmen]] and assistants and stocked with [[printing press]]es, [[photographic]] and [[lithographic]] equipment.<ref name="Newell292"/>
;Leadership
The Chief of Topographical Engineers at the start of the Civil War was Colonel [[John James Abert]].  Colonel Abert had been responsible for lobbying Congress to make the Corps an independent branch of War Department and was appointed to lead his creation in 1838.  He retired in September 1861 and was replaced by [[Stephen Harriman Long]], who remained in the position until the Corps was disbanded.  Thereafter he continued to serve in the Corps of Engineers as the senior officer to the Chief Engineer.<ref name="Baldwin27"/><ref>Baldwin, p. 283</ref>