B Automated feature extension to lengthen existing physical feature for block delineation U.S. Census Bureau C Nonvisible centerline of area enclosed by corridor boundary U.S. Census Bureau I Irregular feature extension, determined manually, to lengthen existing physical feature U.S. Census Bureau M Automated closure extension for cartographic products U.S. Census Bureau N Not a feature extension U.S. Census Bureau S Nonvisible separation line used with offset and corridor boundaries U.S. Census Bureau