> Is it? I'm no expert on parks, but surely some of them have borders along rivers. Many US states have such borders.

Depending on the country, but Australia has some [1]. I still think that there is a set of polygons that can be used to describe this border.

Not to argue against your point (I rarely get less than 4m of accuracy), but luckily

> but anywhere else, the accuracy isn't that great, especially around buildings. GPS accuracy is terrible in cities.

cities are (almost?) never in national parks.

[1]: https://www.nationalparks.nsw.gov.au/-/media/npws/maps/pdfs/...