Currently containing over 1900 images, this set of map images (US File Unit RG 77: Civil Works Map Files) was recently announced on a blog of the National Archives (US). From the website:

The records in this series comprised the main map collection for the Corps of Engineers during the nineteenth and early-twentieth centuries. They include manuscript maps forwarded to headquarters by Corps of Topographic Engineers and Army Engineer surveyors and cartographers in the field, and published editions of selected maps.