Georgism conflates made up “value” with market “value”. Ultimately some person has to assess the value of the unimproved land, meaning someone anointed to write down whatever number they want.
Value is subjective. There is no such thing as “unimproved land value”, unless you’re talking about the last sale price of a particular unimproved lot. Property taxes in general have this problem, but it’s very acute with unimproved land. Unimproved land is one of the most non-fungible goods there is.