[tyb525]

Yes I would stay away from the string method on something that long, the string does stretch and makes it impossible to form a smooth arc.

What I've done in the past is layout the arc on Sketchup and take measurements on a grid layout, then transfer that to your wall/etc. I used 3" grid spacing for a 19' arch.

 

[tyb525]

Thanks I never thought of using that, I suppose I need to get some on the rare chance I need it.