The map is interesting for a variety of reasons, but even more so because of the story behind it:

William Smith was a guy who came from humble beginnings and became a land surveyor. His job put him all around the UK and he became a collector of fossils and sort of an amateur geologist.

Anywho, he noticed that the geology in the UK followed a certain pattern and had certain fossils in certain layers. He used this information and started to promote his ideas: In essence, he was able to predict what was underground using information he found above ground. This was contested for a variety of reasons by just about everyone. The church didn't like it because it implied an old earth and scientists at the time didn't like it because at that time science was a profession done by upper-crust douchebags (which Smith was not one of).

Well anyway, he made the above map himself and spent his own money to publish it because he was so convinced of his idea was right, and because people who mine for minerals would be greatly interested in this sort of thing, so there was a lot of money to be had.

Well, one of the upper crust douchebags (more specifically, one from the London Geological Society) doesn't like the fact that this workman was creating a map and doing a better job than them. So he blatantly copies the map, slaps the geological society logo on it, and sells it for less than Smith did. This bankrupted Smith and he went to debtors prison.

So many years later, he's a working old man still doing surveying, and he happens to do a job for someone in Parliament who also happens to be a geology enthusiast. Smith gives him a hand drawn map of his estate in the same style as the one that made him bankrupt. The landowner immediately recognizes the style from when he saw it many years earlier and is taken aback when he learns that the man who created it was still doing these tedious jobs instead of being honored and rich.

Anyway, because he's in Parliament, the guy pulls tons of strings and Smith finally gets recognized for the groundbreaking research and is honored before he dies.