I think you would have to take the current market value of the land, then subtract an estimate of what it would cost to re-do the improvements at today's prices

I think you would have to take the current market value of the land, then subtract an estimate of what it would cost to re-do the improvements at today's prices. The insured value of structures may be an easy way to get at that estimate. For instance, a lot with a house on it that costs $250k, when the structure would cost $180k to reconstruct from scratch, implies that an adjacent empty lot of the same size would be $70k. But that's not entirely unimproved. There still may be water, power, telecoms, and sewer utility hookups, which certainly qualify as improvements. So you estimate the cost to re-wire/re-pipe the entire neighborhood from the utility, and divide that across all the lots, weighted by area. And being close to a road is an improvement... Professional appraisers would most likely just make a plausible-sounding number up, and collude with one another to ensure consistency over an entire county. It's the easy way.