Lets work backwards a little bit. If you were going to build a new boat from plans (and I'll assume that it is a woodstrip boat) the key element in determining hull shape would be the station molds, sections of the boat probably taken at one foot intervals. From the station molds you should be able to develop 3 dimensional plans and make alterations from there. How do you develop station molds from an existing boat? It's not that hard to mark one foot intervals and scribe the shape of the boat onto cardboard templates. You will need to be sure to keep the templates aligned to each other in all 3 axes. So it will help to get the boat blocked up in a stable, level position.

Luckily, canoes and kayaks are small enough to work with cardboard templates. Builder of larger boats face different problems related to the size of the boat, but they use the same basic technique for documentating the shape of an existing hull. Wooden Boat magazine has published a number of articles on this procedure over the years, possibly even one on lifting the lines of a canoe.