To correctly determine the model, the lengths and beam and other measurements are sometimes the only means to narrow the choices down. This must be combined with the HULL ID and SERIAL NUMBER and OBC RATINGS.
The CENTERLINE length is just that - the length down the center, perpindicular to the transom and measured at the level of the deck.
The GUNWALE length is measured around the curve of the deck and gunwale covering boards. Basically at the rub rail.

Hope this is of help.
Andreas