A few months back when Apple announced its new AR Kit, Niantic confirmed Pokemon GO will be one of the games that will support it.
Starting today models running iOS 11 (iPhone 6s and above) will support Pokemon GO AR+. With AR+, Pokemon will be displayed in the correct scale, so you can walk up to a Snorlax to have a rough idea of how huge it is.
If a trainer is getting too close, a Pokemon may flee if it becomes aware of that. Try tapping the tall grass nearby to make the Pokemon reappear. Catching Pokemon in close proximity will give trainers an easier time making Great and Excellent throws. Trainers may also earn an Expert Handler Bonus, awarding even more XP and Stardust.