2. Data and methods
The data used for near real-time monitoring of the Fagradalsfjall 2021 eruption consists mainly of aerial photographs and Pléiades stereoimages and by September 30, 2021, 32 surveys had been carried out (Supplement S1, Table S1). The processing of the Pléiades stereoimages is described in Gouhier et al. (submitted).
The airborne photogrammetric surveys of the eruption started on the morning of March 20, 2021, 11 hours after the eruption began. The bulk of surveys were done with the TF-BMW Partenavia P 68 Observer II survey aircraft operated by Garðaflug Corp (Table S1) with a Hasselblad A6D 100 MP medium-format camera with a 35 mm focal lens. Images were taken vertically at an altitude of 550–1800 m.a.s.l. with 75–90% overlap and image resolution of 7–30 cm. Up to 24 ground control points were placed around the lava flow-field and measured with a high-precision GNSS instrument (see Supplement S1).
The aerial photographs were processed in the software MicMac (Pierrot Deseilligny et al., 2011, Rupnik et al., 2017), following the semi-automated workflow of Belart et al. (2019), as well as in Agisoft Metashape (version 1.7.3) and Pix4D mapper (version 4.6.4) yielding the DEMs and orthomosaics. Each DEM was compared with a pre-eruption DEM and with the previous survey done, obtaining a thickness map and a thickness change map (Fig. 2 and 3).
Lava outlines were manually digitized from the orthomosaics. Volumes were calculated using the mean thickness of the erupted deposit multiplied by its area. The uncertainties of the volume were obtained using the Normalized Mean Absolute Deviation (Höhle and Höhle, 2009) of the stable areas surrounding the lavas, as proxy for the uncertainties of the thickness maps. The uncertainties of the TADR are described in Supplement S1.