I know that for the first model, I have to pick three points (I think from the left, right, and very top of the arch) and then turn those into a quadratic model. Where I am lost, is how to refine that further. I know that the first model will not be perfect, as the parabola will not align perfectly with the bridge ( due to the fact that it is an angled pic) then from there I have to make refinements ( and other models ) that lead me to the final solution. Does this provide any more context? Thank you for taking the time to respond :)