# [PERSON] Glacier Status Analysis Based on Historical Data and Moving Detection Using RPAS
## 6 Data Processing
### Processing of Historical Image Data  
#### 6.2.1 Historical Aerial Images from the 1930s  
The set consists of several oblique aerial images in the form of scanned photocopies. The original images were taken by the Fairchild F-8 photogrammetric camera, which was released in 1930. Original images from the Fairchild F-8 camera were taken on \(5^{\prime\prime}\times 7^{\prime\prime}\) film. The focal length was 240 mm (12\({}^{\prime\prime}\)). We found only this information about the photos. The fiducial marks were very difficult to find, and the frame data were unusable. It was uncertain whether the photographs were complete in the original format. Fortunately, the center of the image was highlighted by a puncture and a mark in the photographs. All scanned paper photocopies had to be transformed into detected centers, fitted and cropped according to poorly visible fiducial marks to the same format. Projective transformation was used, which got the best results (Figure 15).  
Only eight historical images were selected, from which a project was created to process image information into a 3D model. It was necessary to define at least the basic parameters instead of the elements of internal orientation. A focal length of 240 mm was used, and the pixel size (14.2 \(\upmu\)m) was derived from the size of the scanned photographs in pixels and lines based on the original image size of \(5^{\prime\prime}\times 5^{\prime\prime}\). The additional part of the film was used for frame information, such as time and photo number. Metashape software was used. Automatic processing was not possible due to the low quality of scanned paper photocopies and the unknown internal and external parameters of the chosen photogrammetrical images. It was necessary to find suitable tie points manually; in the end, 25 tie points were used for a correct calculation. A sparse cloud was calculated, and the points were filtered using a \"gradual selection\" procedure, which filtered out points with error of bigger than ten pixels. This parameter was set empirically so that approximately 30% of all points were deleted from the processing. The camera parameters were recalculated until stable results were obtained. Some pictures did not have a regular photogrammetric overlap; they were taken by hand and it was almost always a pair of similar pictures. From the photo-flight sequence of the Knud Rasmussen Glacier, all images were finally successfully processed. An illustrative model of the historical status was calculated (Figures 16-19).  
Figure 15: (**a**) The Fairchild F-8 camera, (**b**) a historical photo with the pilot and photographer in the book. The photographer holding a Fairchild F-8 camera [7].  
Figure 16: The created three-dimensional (3D) model of Karale (**left**) and Knud Rasmussen glacier mouths (**right**) with tie points and photo positions; 1.23 million faces.  
Figure 17: A detail of the 3D model—the Knud Rasmussen Glacier in the 1930s.  
Figure 18: An orthographic view of the 3D model shows glacier mouths; it can be joined with satellite and drone images. Unfortunately, the images were taken at a low angle oblique, which caused a considerable amount of hidden parts due to the perspective (white areas).  
An interesting result is the fact that the face of the Knud Rasmussen Glacier changed little over time compared to the other, dramatically receding glaciers, such as Jakobshaven Glacier on the west coast [20; 21], including right next to the mouth of the Karale Glacier (the glacier on the left part of the model).