The Strand magazine once ran the following anecdote:
When a memorandum passed round a certain Government department, one young pedant scribbled a postscript drawing attention to the fact that the sentence ended with a preposition, which caused the original writer to circulate another memorandum complaining that the anonymous postscript was "offensive impertinence, up with which I will not put."
However "informal" ending a sentence with a preposition might be, it is not nearly as bad as twisting the syntax to avoid the purported error at the expense of clarity and euphony.
I would stress, thought, a map does not depict "wheres" -- it depicts "whats".
"Where" refers to the spatial relationship between two things. Consider:
"What is this place?"
"It is the house where I was born."
In the question, "what" refers to an object. Any sort of description of the object would be an appropriate response.
In the answer, the house is described in spatial relationship to another thing, the event of the speaker's birth, so "where" is used.
"Where is this place?"
"It's in downtown Petaluma."
The question is asking for spatial location. A description of the significance of the place or its history or anything else would not be responsive. You have to discuss its location.
A map does not (of itself) answer a where question. It shows some physical thing. The fact that the thing is a possible location means nothing. The question "Where are you tattooed?" could be answered with "On my ass", but if you saw a photo of my ass, you would not ask, "Where is this a photo of?" You'd have a lot of questions, but not that one.
The correct way to ask the question is either
What is this a map of?
Of what is this a map?
whichever you find sounds more pleasing.