image to x-y coordinate

i have an image which used to get a boundary of one object which represent a 3-degree polynomial curve BUT the curve in x-y x coordinate doesn't look like the boundary of the object (the slope of the x-y curve is more than the real object) that's because the the scale from image pixels to x-y is different, so HOW CAN I FIX THIS SCALING ISSUE? AND IN GENERAL when moving from point 1 to point 2 in x or y axis is that represent 1 cm displacement?!!

Answer 1

[bym]

you need to calibrate the camera

http://www.vision.caltech.edu/bouguetj/calib_doc/