Geo-referencing and image rectification

The process of image rectification consists of determining the relationship between the original image pixels coordinates and the real world coordinates: latitude, longitude and elevation (see Fig. ^S1 for the case of Grand Popo, Benin). The process is accomplished by direct linear transformation using the ground control point’s coordinates⁷⁶ and radial distortion correction of the lens⁷⁸. The image distortion induced by the curve of the lens is corrected using the internal parameters of the camera (focal length and distortion coefficients) which are calculated using the least squares error minimization method.

Overall, the average horizontal error due to image rectification varies from a few centimetres on the beach to a few metres offshore⁴⁵. However⁷⁹, showed that the local video station settings (camera height and pointing angle) add errors of a few metres to rectification error, in addition to uncertainties related to camera movements due to atmospheric conditions⁸⁰.