How is "fair market value" determined for purposes of a tax deduction?

[Sylvia Castellanos]

This is a tax question.

There are a couple of vacant lots I own that I am thinking of donating to a non-profit, such as the land bank in the county where the properties are located. In deducting this from my taxes, I know I can deduct its “fair market value.” How is this defined? Is it acceptable to deduct its assessed value?

I am sure I will be talking to my accountant about it, but I would like to have some idea in the meantime.