v. Gruber, Otto. “Photogrammetric Equipment of the Graf Zeppelin on the Arctic Voyage of 1931 and Methods of Restitution of Observational Material Obtained”. The International Hydrographic Review, no. 1 (June 11, 2018). Accessed July 24, 2024. https://journals.lib.unb.ca/index.php/ihr/article/view/28567.