In The Shawshank Redemption, Andy told Red about his dream of the pacific ocean and at last when Andy is about to break out he told Red to visit a specific hayfield near Buxton to retrieve a package.
Now, Andy has been in prison for more than 19 years and he does not have any information about the outside world. So how was Andy so sure that the tree was still there after 19 years?
I know Andy hid that cache after he escaped from Shawshank. But how was he sure about the tree when inside the prison?