"Please note that this dataset is for noncommercial use only" -- quite limiting. Are you planning to create a product?

Yes I offer a commercial version at https://landrecords.us. (I did not want to link to the paid site on HN)

Could be interesting for finding winners and losers of a potential land value tax

This is really cool. What do you think the remaining ~1% is?

Some counties haven't digitized their maps, some require I go see them in person. There's one in particular that wants me to mail them a check for $10,000. Haven't done that yet...

The area inside the circles described by cloverleaf interchanges. ;)

Coverage is calculated by actual / expected # of parcels, not area. But good try :)