Approximate Correction of Length Distortion for Direct Georeferencing in Map Projection Frame

Many geometric distortions, such as earth curvature distortion and length distortion, exist in the map projection frame. Therefore, in aerial photogrammetry, if direct georeferencing is performed in the map projection frame, the crucial work becomes compensating for the effect of the various geometric distortions. This letter mainly focuses on length distortion and proposes two new correction approaches: the changing image coordinates method and the changing object coordinates method. The experimental results show that the changing object coordinates method is less influenced by terrain fluctuation, and its correction accuracy is therefore commonly higher than the changing image coordinates method as well as two existing approaches (i.e., the changing flight height method and the changing focal length method).