I am not entirely sure about his post war life. Nathan Finegan's papers are at the Ohio State Historical Society, but they haven't been digitized. The inventory indicates that it is extensive. What I have found about him speaks to a multitalented mind.
This paragraph from the following article about Sherman's maps refers to Finggen's motley crew, but not by Merrill's colorful moniker.
"Thanks to Capt. William Merrill, chief topographer of the Army of the Cumberland, Sherman got what he needed, and then some. By the summer of 1864 Merrill had assembled a crack team who continuously improved Union intelligence through fieldwork, traversing the land and collecting local knowledge. As a result they simply knew the terrain better than their counterparts, and mapped it with more detail, giving Sherman a decisive advantage as he closed in on Atlanta. These maps have been ably collected in the Sherman collection at the Library of Congress, and testify to the extraordinary work done by Merrill and his men, as well as by the Coast Survey, the primary federal mapping agency."
Link
How the general used a surprisingly sophisticated set of data to plan his famous march across Georgia.
archive.nytimes.com
View attachment 592377
This well worn L.O.C. map is an example of the information that Nathan Finegan's motley crew collected.
"A REMARKABLE UNION ARMY FIELD MAP, PRINTED FOR SHERMAN'S OPERATIONS IN GEORGIA. Capt. Merrill, Chief Topographical Engineer of the Army of the Cumberland, had his own complete establishment for map production — a printing press, lithographic presses, and draughtsmen. Equally importantly, Merrill's assistant Sgt. N. Finnegan developed an extraordinary body of intelligence, drawing on spies, prisoners, refugees, peddlers, itinerant preachers and scouts, what Merrill called "his motley crew". All of this information was digested by Merrill day by day, until he was notified that the campaign would begin within the week. At this point the topographers finished their work, and two hundred copies were produced, mounted on linen for field use, and distributed to field commanders down to the brigade level. In five months Merrill and his men had produced a remarkably accurate map of country that lay mostly behind enemy lines.
Sothebys
In the L.O.C. Civil War maps of Georgia collection are maps with colored pencil notations. Such maps were returned to the topographical unit & corrected maps issued via its efficient printing office."
Sothebys
This link to the Library of Congress collection of Civil War maps is one you want to keep.
Link
As this instant I can share more:
Well… I'll be switched! It has been a while since I tried to research Sgt. Fenigan. Apparently I am not the only person to find him interesting.
Link
William Merrill elevated cartography to a science that helped Union commanders win the war
historynet.com
Link
On 14 August 1863, Maj. William E. Merrill, chief engineer for the Army of the Cumberland, issued an information map of the area of Cleveland, Tennessee, in preparation for a campaign against Chattanooga in the northern part of the state. The map represented a graphic terrain analysis for the...
www.dvidshub.net
Link
Browse antique maps and prints by Nathan Finegan. Rare and collectible cartography from Barry Lawrence Ruderman Antique Maps.
raremaps.com