1. A method for measuring overlay error, comprising: acquiring an image of an overlay target in an active area of a substrate, the overlay target having a first target component on a first layer and an overlying second target component on a second layer, measuring light intensity along a first scan line of the image; calculating a first symmetry value of the first scan line using the measured light intensity along the first scan line; and calculating an offset between the first target component and the second target component using the first symmetry value.