No fence, wall, hedge or screen planting of any kind shall hereafter be constructed or grown to exceed six feet in height within the area encompassed by the rear yard setback or the side yard setback to the front yard setback line, nor shall the same exceed four feet in height within the area encompassed by the front yard setback. In M, C-1, C-2 zones an additional two feet above the six-foot height limit is permitted for the purpose of wire security fencing. Fence height shall normally be measured from the natural ground level, provided, however, an alternate measurement may be authorized by the building official if special circumstances exist relating to the property.