Time did not allow me to answer all of the questions of the American History Class at Olean High School, but I did respond to them by e-mail.  The first and second question were:

What information do you use to draw the maps?  Do you use photos along with studying the landscape?  What else do you use?  How do you decide if the information is reliable or legitmate?

I had to give a rather long answer to the question:

1. I use contemporaneous photographs, taken within days of some of the major battles (Antietam, Gettysburg e.g.).  I got a Cornell agronomist (a crop expert) to study the panoramic photos of these fields to determine the crops in the fields. It was easy to figure out what type of fencing there was, where the orchards were, the extent of the woodlands etc. Essential to the use of these photographs was the 1970’s work of William Frassanito, who painstakingly determined the point-of-view and location of each extant wartime photograph of Gettysburg and Antietam.
2. Then I used regimental histories – each regiment would occupy a small sector of the battlefield and their accounts and maps  would show their particular sector in great detail. The soldiers’ diaries and letters (this was probably the most literate war in history) also contain many references to their immediate surroundings. Many of them were themselves farmers and they would frequently comment on the crops and fields, fences, barns and orchards. The most poignant research items were “burial” maps. Dead men were often buried right where they had fallen by their fellow soldiers, who were usually their townsmen, friends, even relatives. They would then send the family a very detailed map so they could journey to the battlefield, locate the body and bring it home for burial.
3. On the major battlefields such as Gettysburg, Antietam, Stones River in Tennessee, the National Park Service has tons of data that they gladly make available to a researcher.
4. As I mentioned in the class, aerial photographs are very revealing if you know what you’re looking for. If I have a reasonably good Civil War era map and it shows a road or a lane that’s vanished, or a barn or a house, if you look very carefully, you can find a footprint or a trace of it. Also, the skeleton features in the landscape – the rivers, streams, hills, principal roads etc. – are obviously extant features on todays extremely accurate USGS maps. So those features are immediately drawn on my map so I immediately have a framework to go by. And the maps drawn and used by the actual Civil War armies, while not terribly accurate in terms of exact distances etc., are full of the sort of information the armies needed. They needed the name of residents along the roads because there were no route markers and there were not normally formal names for a given road or lane so the surest way to get reliable directions was to get pointed toward the Smith’s house or the Jones farm. Documenting the residents also gave the armies a idea of the local population. The more residents there were, the more resources there were for the armies to live off. (Union General Sherman set off across Georgia on November 16,1864 saying, “Where a million Georgians can live, my army will not starve.) The armies also had to know where they could find corn fields, wheat fields, orchards, hay fields, wells, springs etc. because an army on the march needed to feed 30, 40, 50 thousand men  and thousands of horses and mules. A typical mule would drink 10 gallons of water a day. Multiply that by 10,000! A Civil War army (and it didn’t matter whether they were friends or enemies, Union or Confederate) would devastate any countryside they marched through. They would take all the food for themselves and all the forage for their animals, drink the wells dry, destroy the fields they camped on, empty the barns, clear out the larders, tear down all the fences (to make their camp fires to brew their coffee) and raise havoc generally. But the armies need to live off the land is great for mapmakers because their military “route” maps detailed this sort of food and forage information so the armies knew where they could march and fend for themselves.
5. A great aid in mapping Gettysburg was the work of John Bachelder, who arrived in Gettysburg immediately after the battle. Bachelder marched up and down the field, mapping it, and he had as a resource the thousands of wounded soldiers, Union and Confederate, remaining behind, who he interviewed. His map, a birds-eye view of the field, was very reliable and valuable.
6. There were also quite accurate published county maps available of some areas, including Gettysburg, published prior to the Civil War that I could make use of. (They were also made use of by the armies and the generals.) Also, soon after the war, the U.S. War Department prepared surveys of many of the major battlefields. These maps were much more detailed and accurate than the maps made under wartime conditions. (The U.S. hired former Confederate mapmakers to work on these surveys. One Rebel named Blackford, who’d sworn he would die rather than live again under the stars and stripes, found himself, within a month of Appomattox, working for the U.S. Army mapping the battlefields.)
7. Finally, most every Civil War battlefield has people who have devoted their lives to studying “their” battle. They know every nook and cranny of the field and like nothing more than to guide someone around the field, point out sites, recommend books and resources, provide access private lands and to other knowledgeable locals, and in some cases they fix parking tickets.
8. There are other resources but the above provides a pretty good idea of what’s out there. I decide on the reliability of the information based on the resource I used. Photographs, for example, are irrefutable – at least back then they were, before photoshop – and other data would often be corroborated by some other data. And then you have to wonder why anybody would mislead about the existence of a stone wall or the presence in their front of a rye as opposed to a wheat field.