Abstract

Unmanned aerial photogrammetric surveys are increasingly being used for mapping and studying natural hazards, such as rockfalls. 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) with ground control points (GCPs), data acquisition in the field is not only time-consuming and labor-intensive, but also extremely dangerous. Therefore, the main goal of this study was to investigate how accurate photogrammetric products can be produced by using BBA 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 rockfall sites by using images acquired with a DJI Phantom 4 RTK and the two different BBA methods mentioned above (hereafter referred to as BBA_traditional and BBA_PPK). The accuracy of the products, in terms of the Root Mean Square Error (RMSE), was computed by using verification points (VPs). The accuracy of both BBA methods was also assessed. To test the differences between the georeferencing methods, two statistical test were used, namely a paired Student’s t-test, and a non-parametric Wilcoxon signed-rank. The results show that the accuracy of the BBA_PPK is inferior to that of BBA_traditional, with the total RMSE values for the three sites being 0.056, 0.066, and 0.305 m, respectively, compared to 0.019, 0.036 and 0.014 m obtained with BBA_traditional. The accuracies of the BBA methods are reflected in the accuracy of the orthomosaics, whose values for the BBA_PPK are 0.039, 0.043 and 0.157 m, respectively, against 0.029, 0.036 and 0.020 m obtained with the BBA_traditional. Concerning the DSM, those produced with the BBA_PPK method present accuracy values of 0.065, 0.072 and 0.261 m, respectively, against 0.038, 0.060 and 0.030 m obtained with the BBA_traditional. Even though that there are statistically significant differences between the georeferencing methods, one can state that the BBA_PPK presents a viable solution to map dangerous and exposed areas, such as rockfall transit and deposit areas, especially for applications at a regional level.

Highlights

  • The use of unmanned aerial vehicles (UAVs) platforms equipped with global navigation satellite system (GNSS) receivers that provide georeferencing without the use of ground control points (GCPs) is desirable when mapping large and dangerous areas, such as active rockfall areas, as there is no need to use GCPs, as with indirect georeferencing

  • We only tested the use of only the georeferencing of the bundle block adjustment (BBA) with PPK method, in comparison to the use of georeferencing of the BBA with

  • The results of georeferencing of the BBA with the PPK method were not comparable to those of georeferencing of the BBA with GCPs for all photogrammetric products, which was mostly due to the satellite configuration on the day of the UAV survey with the BBA_PPK method

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Summary

Introduction

The use of unmanned aerial vehicles (UAVs) for remote sensing has become more common in the study of natural hazards and geomorphological processes, especially due to the development of technologies and mapping systems at larger scales, leading to a more comprehensive understanding of natural processes. UAVs are especially suitable for studying and monitoring rockfalls [1,2,3,4], due to their smaller spatial extent and the fact that the potential rockfall areas are often located on steep slopes above infrastructure and settlements, where the terrain is hardly accessible and dangerous for classical field. The use of a UAV is suitable for the acquisition of remote sensing data at a local scale (a few square kilometers), to which rockfalls are mostly limited [11]. In differences the case ofinBBA_traditional, southeastern part of the rockfall area

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