Rectangle
Rectangular tops are the easiest to measure and the most affordable to make. Specify the corners by consulting this page.
Racetrack
Racetrack shaped tops can be specified simply by measuring the length and width. Check to make sure that the ends are true semi-circles (they normally are).
Surfboard
Three measurements: width, minimum length and maximum length are sufficient to uniquely define a surfboard shape: but a pattern is advisable.
Boat
Three measurements:length, minimum width and maximum width are normally sufficient to uniquely define a boat shape: but a pattern is advisable.
