Abstract
Abstract. The possibility of equipping UAVs with lightweight GNSS receivers in order to estimate the camera position within a photogrammetric block allows for a reduction of the number of Ground Control Points (GCP), saving time during the field work and decreasing operational costs. Additionally, this makes it possible to build photogrammetric models even in morphologically complex areas or in emergency situations. This work is proposing a non-intrusive and low-cost procedure to retrieve the coordinates of the camera projection centre with decimetric accuracy. The method was designed and tested with the quadcopter DJI Matrice 210 V2 drone equipped with a DJI ZENMUSE X5S camera and an Emlid reach M, a low-cost, single-frequency (L1) GNSS receiver. GNSS observations are post-processed in PPK in order to obtain the UAV trajectory. Synchronization between the camera and the GNSS receiver is achieved by looking at the camera triggering timestamps in flight telemetry data, without requiring an electronic connection between camera and the GNSS that may be troublesome with commercial UAVs. Two surveys were carried out, respectively to calibrate and validate the procedure. The validation test evidenced the possibility of obtaining the coordinates of the camera projection centres with decimetric accuracy. The centre of projections can then be employed for GNSS-assisted aerial triangulation as input of the bundle block adjustment. Provided that at least one GCP is used, it is possible to reach centimetric accuracy on the ground.
Highlights
The purpose of this work was to derive the trajectory of the DJI Matrice 210 V2 in order to use its position for assisted aerial triangulation
Having aimed to follow a procedure based on the use of the Unmanned Aerial Vehicles (UAVs) telemetry data, it was necessary to calculate the time offset that could correct the time obtained from the triggering time-stamps recorded in the telemetry in order to determine the effective half exposure time of the camera shutter release
Due to the errors cumulated during the workflow the position of the Camera Projection Center (CPC) at the shooting time is determined with an accuracy of about one decimetre
Summary
In the past few years Unmanned Aerial Vehicles (UAVs) have been widely considered as a valuable tool for applications, such as mapping (Zhang, 2008; Remondino et al, 2011; Nex et al, 2014; Neitzel et al, 2011, Taddia et al, 2020), inspections (Buschinelli et al, 2020; Pinto et al, 2020; Zhang et al, 2017), 3D modelling (Püschel et al, 2008; Remondino et al, 2011; Murtiyoso et al, 2017) and hazard monitoring (Chou et al, 2010; Gonçalves et al, 2015; Giordan et al, 2017). In order to solve the Bundle Block Adjustment (BBA) and achieve high geometrical accuracy of photogrammetric blocks, measuring a set of Ground Control Points (GCPs) is traditionally required. This operation is usually time-consuming and sometimes it may be dangerous. It is possible to acquire camera position during each shot by mounting small and lightweight GNSS receivers on board quadcopters or fixed-wing drones and it is a state-of-the-art issue. This allows photogrammetric blocks to be oriented by Assisted Aerial Triangulation (AAT) and, the number of GCPs to be reduced. The pros can be found in the lower cost of the surveys and in the possibility to survey interdicted areas; whereas the cons include the need of a very good calibration of the sensors and the necessity to detect at least some GCPs
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