Coastal Mapping Using DJI Phantom 4 RTK in Post-Processing Kinematic Mode

Yuri Taddia,Francesco Stecchi,Alberto Pellegrinelli

doi:10.3390/drones4020009

Abstract

Topographic and geomorphological surveys of coastal areas usually require the aerial mapping of long and narrow sections of littoral. The georeferencing of photogrammetric models is generally based on the signalization and survey of Ground Control Points (GCPs), which are very time-consuming tasks. Direct georeferencing with high camera location accuracy due to on-board multi-frequency GNSS receivers can limit the need for GCPs. Recently, DJI has made available the Phantom 4 Real-Time Kinematic (RTK) (DJI-P4RTK), which combines the versatility and the ease of use of previous DJI Phantom models with the advantages of a multi-frequency on-board GNSS receiver. In this paper, we investigated the accuracy of both photogrammetric models and Digital Terrain Models (DTMs) generated in Agisoft Metashape from two different image datasets (nadiral and oblique) acquired by a DJI-P4RTK. Camera locations were computed with the Post-Processing Kinematic (PPK) of the Receiver Independent Exchange Format (RINEX) file recorded by the aircraft during flight missions. A Continuously Operating Reference Station (CORS) located at a 15 km distance from the site was used for this task. The results highlighted that the oblique dataset produced very similar results, with GCPs (3D RMSE = 0.025 m) and without (3D RMSE = 0.028 m), while the nadiral dataset was affected more by the position and number of the GCPs (3D RMSE from 0.034 to 0.075 m). The introduction of a few oblique images into the nadiral dataset without any GCP improved the vertical accuracy of the model (Up RMSE from 0.052 to 0.025 m) and can represent a solution to speed up the image acquisition of nadiral datasets for PPK with the DJI-P4RTK and no GCPs. Moreover, the results of this research are compared to those obtained in RTK mode for the same datasets. The novelty of this research is the combination of a multitude of aspects regarding the DJI Phantom 4 RTK aircraft and the subsequent data processing strategies for assessing the quality of photogrammetric models, DTMs, and cross-section profiles.

Highlights

Nowadays, the need for mapping long coastal sections up to the survey of the littoral of entire regions represents the starting point for coastal management and for the planning of restoration interventions to mitigate the impact of storm events by regional environmental protection agencies.the production of Digital Terrain Models (DTMs) is a primary tool for the monitoring of coastal erosion, as well as for developing detailed and high-resolution numerical simulation models able to predict the impact of storms, sea level rise, and flooding risk [1]
We investigated the Post-Processing Kinematic (PPK) approach for the reconstruction of photogrammetric models and DTMs of a coastal section located in the Northern Adriatic Sea (Italy) through the use of Receiver Independent Exchange Format (RINEX) files and the GNSS data of a Continuously Operating Reference Station (CORS) that is approximately 15 km away from our site (Figure 1)
The results show that DJI-P4RTK surveying methodologies exclusively based on the acquisition of aerial images are able to produce photogrammetric models with a centimeter-level accuracy, even if no Ground Control Points (GCPs) are placed or surveyed or if no local base receiver is used

Summary

Introduction

The production of Digital Terrain Models (DTMs) is a primary tool for the monitoring of coastal erosion, as well as for developing detailed and high-resolution numerical simulation models able to predict the impact of storms, sea level rise, and flooding risk [1]. Direct methods are very time-consuming, and this makes them hard to apply to the mapping of large coastal section extents. TLS is able to provide good spatial resolution, even if the duration of data acquisition is still high and the post-processing of scans generally requires an additional and considerable amount of time, while LiDAR by plane represents the most used technique for the mapping of Northern Adriatic.

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Discussion

Conclusion