Accurate direct georeferencing of aerial imagery in national coordinates

In aerial photogrammetry, data products are commonly needed in national coordinates, and, in practice, the georeferencing is often performed in the required national map projection frame directly. However, as a map projection frame is not Cartesian, some additional corrections are necessary in the georeferencing process to take account of various map projection distortions. This paper presents a new map projection correction method for the direct georeferencing of aerial images in national coordinates, which comprises of three consecutive steps: (1) a rough intersection to predict ground point coordinates in the Cartesian space; (2) calculating map projection corrections; and (3) a fine intersection. Benefiting from the explicit estimation of ground positions in the Cartesian space, our new method can directly adopt the accurate map projection distortion model that was previously developed for the direct georeferencing of airborne LiDAR data in national coordinates. Simulations show that the correction residuals of our new method are smaller by one order of magnitude than those of the previous best approach while their computational costs are at the same level, and even in an extreme scenario of 8000 m flight height above ground, the maximum error of our method is only several centimeters, which can be safely neglected in practical applications.