## 3 Methodology
### Dataset Preparation  
Twenty aerial images covering a period of 93 years were used, with images in the following years: 1930, 1946, 1956, 1977, 1985, 1989, 1997, 1999, 2003, 2005, 2007, 2009, 2012, 2014, 2017, 2018, 2019, 2020, 2021 and 2022 (see Table 1 for details). They were obtained from different sources: National Geographic Institute (Instituto Geografico Nacional (IGN) [32]) and Valencian Cartographic Institute (Institut Cartografic Valencia (ICV) [33]) photo libraries. The used reference system is ETS89, UTM projection zone 30. In all images, the sea is calm, and therefore, all available data can be used for a comparative study. \begin{table}
\begin{tabular}{c c c c c} \hline Year, Source & Flight Type & Date & GSD (m) &
\begin{tabular}{c} **Photogrammetric** \\ **Processing** \\ \end{tabular} \\
1930 Ruiz de Alda & Analogical B/W & 1929/1930 & 0.75 & Orthomosaic \\
1946 American Flight Series A & Analogical B/W & 1946 & 0.43 & Rectification \\
1956 American Flight Series B & Analogical B/W & 1956 & 0.63 & Rectification \\
1977 Interministerial Flight & Analogical B/W & February 1977 & 0.45 & Rectification \\
1985 National Flight & Analogical B/W & March 1985 & 0.75 & Rectification \\
1989 Coastal flights & Analogical Color & March 1989 & 0.12 & Rectification \\
1997 Flight OLISTAT & Analogical B/W & October 1997 & 1 & Orthomosaic \\
1999 Five-Year Flight & Analogical Color & 27 August 1999 & 1 & Rectification \\
2002 Institut Cartografic Valencia & Analogical Color & 12 May 2002 & 0.50 & Orthomosaic \\
2005 Institut Cartografic Valencia & Analogical Color & 30 September 2005 & 0.50 & Orthomosaic \\
2007 Institut Cartografic Valencia & Digital Color & 23 August 2007 & 0.50 & Orthomosaic \\
2009 Institut Cartografic Valencia & Digital Color & 5 August 2009 & 0.25 & Orthomosaic \\
2012 Institut Cartografic Valencia & Digital Color & 16 June 2012 & 0.50 & Orthomosaic \\
2014 Institut Cartografic Valencia & Digital Color & 21 June 2014 & 0.22 & Rectification \\
2017 Institut Cartografic Valencia & Digital Color & 18 August 2017 & 0.25 & Orthomosaic \\
2018 Institut Cartografic Valencia & Digital Color & 13 June 2018 & 0.25 & Orthomosaic \\
2019 Institut Cartografic Valencia & Digital Color & 14 May 2019 & 0.25 & Orthomosaic \\
2020 Institut Cartografic Valencia & Digital Color & 2 May 2020 & 0.25 & Orthomosaic \\
2021 Institut Cartografic Valencia & Digital Color & 13 May 2021 & 0.25 & Orthomosaic \\
2022 Institut Cartografic Valencia & Digital Color & 8 May 2022 & 0.25 & Orthomosaic \\ \hline \end{tabular}
\end{table} 
Table 1: Main information from the aerial imagery used in this research. Note that GSD is the ground sampling distance: the distance between two consecutive pixel centers measured on the ground. This concept is equivalent to spatial resolution in remote sensing. The photographs of the first flight used (Ruiz de Alda 1929-1930) were treated with AgiSoft Metashape to obtain an orthomosaic of the study area since the photos were not taken systematically [34]. Control points identified on the 2022 orthomosaic were used to make the 1929-1930 orthomosaic. It seems clear that, although the 1930 orthomosaic has an error (Table 1), it can be considered within acceptable limits when working with data 50 years old or more [35], especially when you consider that it provides an important piece of information that is worth considering. The 1946, 1956, 1977, 1985, 1989, 1999 and 2017 aerial photographs were rectified using easily identified points on the 2022 orthomosaic, which was taken as a reference for the entire work. The remaining used images were orthomosaics produced by the official cartographic agencies and downloaded from their data servers. Once the 20 mosaics with the orthophotos of all the years of study were uploaded to GIS, the next step was the vectorization of the shorelines, the cliff top and the houses of Babilonia Beach.