I am faced with the need to draw the outline of an elliptical arch with a known span and height - at full scale on a job site. I know the method of using the foci and a loop of string or wire and the method of "cheating" an ellipse with two or more circular radii. The method I am searching for uses "trammels(?)" - pieces of wood - and I believe requires knowing only the overall length and width of the ellipse. Can anyone help?