Changes in the way things have been shown in maps

Surveying became reasonable accurate at the start of the nineteenth century. Since then, geographical features – coastlines, etc. – have been represented in much the same way on maps. A map of, say, Wales from 1820 is much the same shape as it is now, but noticeably different from the way it was drawn in 1780.

However, some things have changed significantly in the way they are represented in maps. This page links to maps of three such things.

I used to think that this happened gradually, as new information became available. However, in each of these three examples, there has only been one significant change. Before some date, maps copy the errors in other maps; after that date, they (mostly) use the new piece of information and get it right.