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 each with a known dimension; performing a calibration to increase an accuracy in determining the dimension of the at least one reference object by adjusting the coordinates of at least one of the feature points based on a determined dimension and the known dimension of the at least one reference object, wherein the determined dimension is obtained based on coordinates of the feature points and coordinates of the at least one 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 number of feature points after the calibration, wherein the method is performed by execution of computer-readable program code by a processor of a computer.