1. A method of obtaining a dimension of a target object in an image, comprising: receiving coordinates of a number of feature points in the image; receiving coordinates of at least one reference object in the image with a known dimension; performing a calibration to adjust the coordinates of at least one of the feature points, wherein the coordinates of at least one of the feature points are adjusted to increase an accuracy in determining the dimension of the reference object; and receiving coordinates of the target object in the image and determining the dimension of the target object based on the coordinates of the feature points.