Well I already pointed out that if you're within a couple meters of the boundary, you won't have good confidence because of this fact.
>and a park boundary is a well defined hard line polygon.
Is it? I'm no expert on parks, but surely some of them have borders along rivers. Many US states have such borders.
>Being close to the border changes nothing, I can just add a buffer outwards the park polygon to account for that.
That doesn't account for the 2m accuracy. What if I'm standing exactly 1m from the boundary when I take the photo? You have no idea if I'm really in the park or not from the GPS data.
I also have serious doubts about your 2m accuracy claim, based on personal experience. Maybe if you're standing in a wide-open desert with nothing around you, but anywhere else, the accuracy isn't that great, especially around buildings. GPS accuracy is terrible in cities.