A first orthomosaic

The historical aerial photographs as digitized during the COBECORE serve multiple purposes. Small individually georeferenced photos can serve as a reference to the state of the canopy at the end of the 1950s. The structure of the canopy has been shown to be linked to standing biomass of the forest, and hence this is a quantitative measure to be derived from these images.

In addition, when mapping the whole extend of a region we can easily determine what the land use at the time, and contrasting this with the current status of the forest we ccan map land-cover and land-use change (LULCC).

For the latter approach we will rely on stitching through structure from motion (Sfm) and the generation of an orthomosaic or orthophoto. An ortho photo corrects remote sensing data so the data represents a perfectly downward looking image, free from any perspective distortion due to topographic relief, lens distortions and camera tilt, ensuring that the scale is uniform throughout (see image above). After these corrections aerial photographs accurately represent the Earth’s surface and measurements on such maps reflect true distances. Using the Sfm technique, relying on stereographic vision and photogrammetry, we can reconstruct a 3D surface and relative camera positions in order to correct arial photograhs. An oblique view of this 3D reconstructed surface of aerial images by a drone is provided below. Notice that at a very high resolution you pick up the relative height of the canopy as well.

A first quick pass using PhotoScan and a swath of survey images (path 9) shows promissing results. Although artifacts remain (as this was a test), the stitching of all images occured smoothly and provides a continuous view of the Yangambi region. At the confluence of the Congo river running from East to West and the Lomami (left side, left bank of the Congo river) is the the village of Isangi, while Yangambi and surrounding plantations is located in the middle of the image (right bank of the Congo river). As a reference, this view is approximately 70km long and 15km wide.

Fig 1 - Ortho mosaic of aerial photos taken over the Congo Basin in 1958.

A high resolution version can be found here.