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.