BLDGBLOG recently mentioned a feature that some commercial maps use to protect their intellectual property: trap streets, “deliberate cartographic errors introduced into a map so as to catch acts of copyright infringement by rival firms”.
In other words, if a competitor's map includes your "trap street"—a geographic feature that you've simply invented—then you (and your lawyers) will know they nicked your data, gave it a quick redesign and tried to pass it off as their own.
But this strategy of wilful cartographic deception is not always limited to streets: there can be trap parks, trap ponds, trap buildings.
What other circumstances might someone have used a technique like this? Imagine an earlier age where accurate maps are difficult to make and would be very valuable information both to a state and its enemies. So the canny vizier inserts some bogus features – a lake, a mountain pass, a village - that act as a kind of watermark. Now people heard asking for directions to Fictionville come to the attention of the intelligentsia, who would want to know how they came into possession of their map.
On a related note, there are other ways that a bogus feature can appear on a map, both accidental and as a prank. A fine example of a prank was Beatosu and Goblu, OH, each an invention of Peter Fletcher, then chairman of the Michigan State Highway Commission. Mr Fletcher was an alumnus of the University of Michigan and inserted the two fictitious Ohio towns into the Michigan highway maps as a retort to a friend from Michigan State University. The towns’ names were contractions of “Go Blue” and “Beat OSU” (as in Ohio State University).