Zone boundaries that are shown crossing lots are usually based on a topographic feature or a set measurement from a property line or topographic feature, such as the top of slope, middle of stream, 25 feet from top of bank, or 30 feet from the property line. When zone boundaries are shown crossing properties with no clear indication of the basis for the line, exact distances are to be determined by scaling the distances from the Official Zoning Maps, using the center of the zoning line.