How to estimate the accuracy for an estimated tine segment when actual line is known?

Suppose I have a set of XY coordinates (pink dots including noise)which is a measured values of a line segment (Black line). After estimating the best line (Orange line) for the measured data points, how can I calculate the accuracy of the fitted line with the actual one.

[Walter Roberson]

Replaced with better http://uk.mathworks.com/matlabcentral/answers/257380-how-to-estimate-the-accuracy-for-an-estimated-tine-segment-when-actual-line-is-known