A 1099S is generated from the sale of real estate.  Real estate can mean a house, a barn, an outhouse, or even just land which is what an easement is.  

The proceeds just reduce the basis of the property, so nothing needs to be reported unless they exceed the basis.  But IRS might be looking for something anyway.  I would report it on Schedule D, with the basis being the same as the proceeds, and attach a statement explaining the basis of the property before and after the easement was sold.  No one at IRS will read the statement but it will be useful to the taxpayer and future preparers, if they remember what happened.