I hope this question falls within the rules of this sub.
Most history books I read are full of maps to show where the major events being discussed take place. For example, where were the front lines on the western front in Oct 1914? Or, where was the Germany - France border in Aug 1914? How do historians know what these borders were? And once they know, how do they turn this into a graphic for their text?
I have a somewhat more specific question. I am collecting my great grandfather's war letters into a book to share with my family and I would like to include some maps showing where the letters were written (France and Belgium, WW1 Aug-Oct). In my day job (scientist) I am often editing other people's vector graphics to add my own annotations so I thought that maybe there was some well known historical database of open source maps that could then be edited. Otherwise, I would try and modify a modern map, but that leads me to my questions above? how do you know where the borders were?
thank you for your help and I apologise of this kind of question is outside the scope of this sub.
As a historian of cartography (science and landscape), I've had to deal with this on a few occasions. Sometimes, you end up having to try to figure out from textual descriptions where (using GIS) a boundary was supposed to be, and sometimes that wasn't where it actually was; it can vary by the way a society or government thought about belonging and territoriality, too. The specific question you raise is one of method and well in bounds here. It's also (WWI) one of the easier ones in terms of potential sources of data existing, although that's not to say it is easy to parse all of that. So the ease depends on the era, the sources, and how people think about boundaries, but for post-1700s Europe you can get pretty good textual and graphical data, even if the originals are distorted by our standards and would require updating.
For the First World War, all of the major combatants had general staff organizations. These organizations produced maps--usually overprinted on stock of a region, such as GSGS or TSGS maps in the UK--to keep GHQs, intelligence officers, and even local commanders in the know. That doesn't mean they were quite correct, but they were out there. In terms of the processes of mapping, various books by Peter Chasseaud are the place to start, I think; I'm partial to Artillery's Astrologers about the survey and mapping of the front lines, but he's got a carto-bibliography of trench maps (Trench Maps: A Collectors' Guide) as well. There, you can get information about maps that survive, many at the National Army Museum in London and the National Archives in Kew. For French and German resources, I'd have to dig a bit, but our WWI specialists probably know right offhand.
The British government has digitized some of these. WO 297 is a major group at Kew, I think, for the updates of the 1:20,000 series. [edit: but not all are digitized] These boundaries were fluid ones showing actual positions as far as were static, though they weren't; one can't automatically trust a map as a snapshot in those conditions. Legal boundaries, however, are pretty easy to find. Rarely, I've had to compile a map from reports together with aspirational (and inaccurate) maps of a given situation, but WWI was fairly well updated in terms of editions issued.
In terms of how we make them into book graphics, well, you usually hire a cartographer or graphic designer for that. They take the base material, guidance from you about what you want the map to accomplish, and then try to simplify the graphic so just enough is shown that it gives information and context without overwhelming the viewer. Universities often have such infographics and carto labs, but there are independent ones as well, and a publisher normally has a few in their address book if they don't have one in-house.
[edit: clarified a few things]