The idea is that anyone copying the work of the publishers would be trapped in any legal action because they would copy the deliberate errors and be exposed. This is an age-old practice to keep the copycats at bay. Companies that create maps get their work pirated all the time. You might hire surveyors and draughtsmen, you might checks all of your spellings, you might get all of the towns and cities in the right place and another company comes along, say for example a tourist agency, and steals your work.
You cry 'Piracy!' and take them to court. "Prove it" they say "It's a map, it describes what is. Because there's a real world out there, obviously maps are going to be identical. So we're only guilty of describing the same world the other map described". Jurors think, "Hmm, sounds reasonable," and the pirates get away with it. Unless the mapmaker runs a little scam.
I am going to relate the fascinating story of what happened to a map published in the 1930s. The map-makers, Otto G. Lindberg and Ernest Alpers of the General Drafting Company of Convent Station, New Jersey, used an anagram of their initials, OGL and EA, to create the fictitious town of Agloe, New York. They sited it in a spot that they knew to be uninhabited 100m from the junction of Highway 206 and an, at-that-time, dirt road called Beaverkill Valley Road. So, were any plagiarist to copy their map, Agloe would in turn show up on the stolen property, and General Drafting Co. would have their proof.
|Google maps Street View couldn't get me any closer.|