[shiroiushi]

>cities are (almost?) never in national parks.

Sometimes they are. See Washington, DC.

Anyway, the requirement is for determining if a photo was taken in a park or not. The resolution wasn't stated, however: just how accurate do we need to be? If I'm in a canoe in a river that borders a park, but the river isn't part of the park, but the shoreline a few meters away is, our algorithm might claim I'm in the park, when I'm really not. The requirement wasn't "somewhere near a park", but "in a park". Rivers change their courses over time, so some polygons aren't going to accurately describe this border.