Tax assessor info is not good for determining the land value of the type of property you are refering to,
You should contact an appraiser familiar with the area where the property is located. There are two common methods used to determine the land, or site value. Sales comparison method and extraction method.
The sales comparison method would involve finding comps of similar land properties, with or without similar amenities such as size, location, utilities (existing or available), usage, accessability, terrian, and perminant affixed improvements such as fencing, landscaping, driveways, building foundations, etc., and making necessary market adjustments for notable differences and market area concerns such as environmental and/or external obsolesences.
The extraction method would involve appraising the property "as is" using comps of similar properties with similar amenities, including structures similar, lesser then, or greater then the existing structure of the property being appraised, and making necessary market adjustments for differences including quality and condition, and developing the cost approach to determine the replacement/reproduction cost of improvements then extracting out the land value, which would be the difference between appraised value and replacement/reproduction cost less, depreciation of existing useful improvements.