Landslide mapping in the Himalayan region using multiple reference station for PPK-based indirect georeferencing of UAV Images.

Unmanned aerial photogrammetric surveys are increasingly used to map and study natural hazards, such as landslides. Surveys using unmanned aerial vehicles (UAVs) can be performed in remote, hardly accessible, and dangerous areas, while the photogrammetric derived products, with high spatial and temporal accuracy, can provide us with detailed information about phenomena under consideration. However, as photogrammetry commonly uses indirect georeferencing through bundle block adjustment (BBA), data acquisition in the field is time-consuming, labor-intensive, and extremely dangerous. Therefore, the main goal of this study was to investigate how accurate photogrammetric products can be produced by using PPK without GCPs and auxiliary data, namely using the coordinates X0, Y0, and Z0 of the camera perspective centers computed with PPK (Post-Processing Kinematic). To this end, orthomosaics and digital surface models (DSMs) were produced for three landslide sites by using images acquired with a DJI Phantom 4 RTK and the three different reference station data . The accuracy of the products, in terms of the Root Mean Square Error (RMSE), was computed by using verification points (VPs). To test the differences between the georeferencing methods we compared the results with Checkpoint values collected from the landslide sites. The cumulative RMSE values were 0.751409 m,0.685491 m, and 0.917512 m in the X, Y, and Z directions respectively.It is possible to say that the PPK gives a workable approach to map hazardous and exposed locations, such as landslide transit and deposit zones.