Method and apparatus for detecting container movement by a crane

A tag can receive first and second wireless signals respectively defining first and second transmission fields that overlap. When the tag moves from the first field to the second field through the region of overlap, the apparatus transitions only once from recognizing the tag is in the first field to recognizing the tag is in the second field. According to another aspect, first and second signposts are supported at spaced locations in the region of a crane, and respectively transmit first and second wireless signals that are different, and that respectively define first and second fields of transmission. According to yet another aspect, a signpost transmits wireless signals having a transmission field, and a system senses positional information regarding a tag movable relative to the signpost, the system using the positional information to determine a location of the tag in relation to the transmission field.