You don't sign a "land contract" for this sort of thing.
It's called a ground lease, and if the county ever terminated it against the will of the other party it would cost them a staggering sum of money.
To illustrate how ridiculous this is ...
Can you imagine being the bank or commercial lender that extended a loan to build on a property that was subject to a 50-year (for example) ground lease, only to find that the owner of the land terminated the lease five or ten years into the deal? You'd never be able to borrow money to build anything on leased land if it were so easy to terminate the lease.
Yep, ground leases are typically 50 years.