The way maps were made, copied and delievered at the start of the war was very labor intensive. A map maker would draw with ink on paper a detailed map. A wood cutter would engrave an exact copy of the map onto a block of wood and using a printing press print individual maps for distribution.
By 1864, a large format camera was used to photograph the original map, and a developer would use the negitive to reproduce the photograph onto large photosensitive papers. This could be done quickly and the endlessly (as long as you had enough paper). This was a very expensive and highly technical process at the time, but it could be done in the field much quicker than using a wood cutter to painfully reproduce the original map for printing.
The beginning of the photo copy machine.