2.3.1. Image Preparation

The images were assembled using Agisoft Metashape to obtain orthomosaics for each acquisition year. These were georeferenced with ArcGIS 10.8.1 using ground control points (geospatial reference: tefenua.gov.pf (accessed on 15 January 2021)) with either adjust or 2nd polynomial transformations. Fixed geomorphological features such as lagoon pinnacles were used along with anthropogenic features as ground control points (similarly to [17]).

#### 2.3.2. Shoreline Tracing

The shoreline was manually traced based on the sea-side edge of vegetation (including the sea-side edge of mangroves and grass/reed areas), as per previous publications (similarly to the stability line used in [17]), with the Editor tool on ArcGIS. When sparse coastal vegetation (i.e., spaced-out trees) had dense canopies, the middle of the canopy, in the alignment of non-vegetated areas with a clear shoreline position, was used to position the shoreline. For sandy beaches, the line of first vegetation was used and not the sea-sand boundary for two reasons:


Clouds were present over a few tens of meters of the coastline in 2019 in the urban area of Vaitape, in a zone with embankments and no natural shoreline. A ground survey confirmed the lack of further constructions or changes compared to the previous image of 2016. The shoreline was, hence, traced in the same position as for 2016. No other clouds were present over the coastline at any date.

The total error (*Etot*) of the shoreline positioning is taken as the square root of the sum of the squares (cf. [18]) of the three sources of error identified for this study: the spatial resolution (*Eres*), the georeferencing error (provided by the forward error of the ground control points on ArcGIS, *Egeo*) and the shoreline tracing inaccuracies (*Etra*) (Equation (1)).

$$E\_{\rm tot} \ (\dot{m} \ m) = \sqrt{E\_{\rm rcs}^2 + E\_{\rm \%}^2 + E\_{\rm tra}^2} \tag{1}$$

The shoreline error (total and yearly) is provided in Table 1 for each date.
