A circular target on a plane is projected as an ellipse onto an oblique image. This paper reports an experiment conducted to evaluate measuring methods of elliptical target location by using simulated images of various sizes, eccentricities, and rotation angles of elliptical targets. Three popular measuring methods in digital photogrammetry : unweighted centroid method using a binary image created by thresholding (BCM), weighted centroid method (WCM), and least squares matching (LSM) were investigated. The experiment results show that BCM is robust against flatness and rotation of a target, and image noise. However, BCM cannot produce more accurate measuring results than WCM and LSM. Meanwhile, the results demonstrate that WCM is robust against flatness and rotation of a target, and WCM can achieve the most accurate measurement when a target image has no image noise. However, the measuring error of WCM would be proportional to the magnitude of image noises, and WCM would be unable to provide the best measuring results when a target image has larger image noises. The results indicate that LSM is rather robust against image noise, but very sensitive to flatness and rotation of a target. Moreover, the results show that BCM using an enlarged target image by bi-linear interpolation should be a good measuring method on accuracy and robustness against flatness and rotation of a target, and image noise.
