"It's ideal for Headquarters, of course. My father put every security measure known to wizardkind on it when he lived here. It's unplottable, so Muggles could never come and call — as if they'd ever have wanted to — and now Dumbledore's added his protection, you'd be hard put to find a safer house anywhere." —Sirius Black
I understand that 12 Grimmauld Place is protected by the fidelius charm, and invisible to the neighborhood residents. It is explained in the books that the neighborhood residents just accepted that there was an error in the numbering of the homes on the street.
I assume (possibly incorrectly) that the fidelus charm gives it additional layers of security above and beyond the normal "unplottable" enchantments. But just how well protected is it?
Let's say one were to walk a measurement from 11 Grimmauld to 13 Grimmauld, and got a width of 200' - and then drove the same path with an odometer - would the measurements be the same? If one were to use maps to measure distance between points, would they match the same real-world measurements? What would keep a muggle from noticing an incredible logical discrepancy in measurements?