concept of image rectification

To derive geometric information from photographs is a task often set in architecture, preservation of historic buildings, building engineering and archeology. On the original photograph the shown objects normally appear distorted because of position and orientation of the camera. For instance on a photograph of a façade this effect becomes obvious at the converging vertical edges. Image rectification is the process by which the photograph is reorganized and a true-to-scale image plan is created. On the resulting image the vertical edges become parallel and the image has a defined scale.

distorted original		rectified image

To get a true to scale image plan a digital photograph and additional reference information on the object is required. That means, it is necessary to take a defined number of measurements from the object. These measurements can be control points or control distances. During the rectification process the reference measurements are assigned to the image. Using these assigned information the image can be transformed (rectified) arithmetically to a true to scale image plan. The plane where the reference measurements (control points, control distances) are located is called rectification plane. All parts of the object that are situated exactly in this plane are reproduced in true scale on the rectified image, elements that are not situated in this plane remain distorted.
example façade: You take a photograph of the relevant part of the façade. As reference measurements you choose points or distances that lie within the façade. They are measured by tachymeter, tape measure or hand held laser. On the rectified image all elements at the façade plane are shown in true scale, whereas a sloping roof or a balcony remains distorted. They can be rectified in a separate process.

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A comparison of Autodesk CAD Overlay and kubit PhoToPlan for the perspective rectification of photos in archaeology. At the FHTW in Berlin Dominik Westermann, graduate in the subject of restoration/excavation techniques, wrote his diploma dissertation on the theme of Photogrammetry and Digital Photography applied to the Technique of Excavation. For this he examined the practical application of rectifying individual images as a method of documentation on archaeological excavations. Amongst other things he compared the two rectification programmes PhoToPlan and CAD Overlay using a comprehensive practical test. The following article describes the results of the comparison.