In late 2018, the Bududa region in Eastern Uganda experienced torrential rain for a number of days. The area falls just outside of Mount Elgon National Park on the Kenyan border and is characterized by its extinct volcanic crater, dense tropical rain forest and very intensive agricultural activities on steep slopes. Given the high saturation of the soil and large mass of water, landslides where experienced in the area, causing major damage to property.
Already saturated mud flowed into rivers along with other debris blocking normal discharge. This lead to localized flooding. Unfortunately, many villagers are settled close to the river banks and numerous buildings where either washed away or badly damaged. A number of lives were sadly lost.
In order to assess the damage and to place preventive measures in place, the Office of the Prime Minister of Uganda along with the World Bank approached 3DroneMapping in providing detailed orthophotos of 2 major regions identified to be the worst affected by the flooding / land slides. The request was also for ground level filtered DTM at a very high resolution. Since 3DroneMapping is one of the very few authorized legal drone operators in Uganda and we have the requisite equipment / experience for high accuracy and efficient aerial surveys, we were the best choice for the project.
The Bududa valley is very steep, with valleys ranging from 1300m - 2800m AMSL. This tight space within the valley is densely vegetated with tall trees, houses, agricultural activities, etc. Since the areas of interest varied 1000m in altitude, flight-planning for photogrammtric purposes was required to be parallel to the ground. Land areas where identified and used in planning. Control points where also placed over the areas for survey with a survey grade GPS. These where used to ensure that ac curacies required by the client where adhered to.
Our 2m wingspan, dual engine airframe was used for the flight operations. This has the advantage of being able to carry a high resolution full frame camera as well as PPK equipment for well over 2 hours. However it requires a large area for landing which was in short supply in valley. A smaller PPK equipped wing was used for the harder to reach areas that uses a parachute for landing, enabling it to be deployed in the higher locations where communications with the larger aircraft where limited.
Processing of data required a high level of skill and experience. Images where photogrammetrically rendered to produce high resolution orthophotos and pointclouds. These pointclouds where then classified to extract ground levels. Due to the steepness of the terrain and dense vegetation, most automatic algorithms used to classify data could not be used and manual editing was required. This time consuming process took many man (and woman!) hours to complete. The end pointcloud, stripped of all vegetation, buildings, rivers and other non-ground points was triangulated and gridded into a Digital Terrain Model (DTM) for the client to use to slope analysis, etc.
Mapping teams in Uganda are already working on the data to calculate densities of settlements, digitizing areas of concern and possible short term, high risk locations. We are confident that this data will be put to good use to both mitigate future disasters and also enable good spatial planning.
