Complex sentence: Before the introduction of official maps of public paths in the early 1950s, the public did not know where paths were, and the Open Spaces Society helped the successful campaign for paths to be shown on Ordnance Survey maps.
Split into simpler sentences: Before the introduction of official maps of public paths in the early 1950s, the public did not know where the paths were. The Open Spaces Society helped the successful campaign for paths to be shown on Ordnance Survey maps.