A Therion principle. Write once, use many times. Be cautious however and balance the 'centralised layout' idea against the contrary advantage of having each mine or 'collection of mines' in self contained folders that can be moved to different machines or environments without breaking links to outsourced locations. As most of 'my' cave systems are constrained to several square km in area, I have a single master layout for those at the top folder level. If I want to separate out just one cave from a sub folder and say give to someone else I only have to copy the 'central layout file' to that folder and one or two input statements to change to make the sub project work. There is a balance between totally centralised and having each subproject completely self contained if its folder is removed from its parent. A choice. See the links in my reply to Henry just now. Bruce
But as I just have finished the third therion mine map, and have many more I need to work onto stacked up during the last years (even such without shafts into them), I will outsource the layouts of the maps I made so far at once. Next step.
Best of all: if I combine both methods of linking stuff in, layouts and metapost code, I can now change my shaft symbols easily and centrally. This will make it very easy to re-view them very fast in many different maps, easily scriptable, and even better, change them in all maps at once if I made a workable version. Which may take a while, I fear.