What Value do I use for land and property when I started to depreciate a rental property? Value at purchase or value by County assessor in year when I started renting it?

I purchased a property in 2008 and then rented it out in 2011. I want to make sure my depreciation is correct. I know that you can only depreciate the buildings and not the land. My Question is what value do I put for the buildings and land in Turbo Tax. is it the value of the building and land when I purchased in 2008 or the value when It was assessed by the County assessor in 2011? 

The value was lower in 2011 than when I purchased so I need to know which one to use. 

Thanks!

Dean