## 2 Materials and Methods
### DEM Validation  
In order to check the accuracy of the resulting DEM, and use it to extend the morphometric data collected in the field for a limited number of sinkholes with our elevation model, two separated methodologies were applied to extract the horizontal and vertical accuracies: (a) a qualitative evaluation of the horizontal accuracy was performed using the Pleiades orthoimages acquired in 2019, where the shapes of the sinkholes were overlaid (Figure 5); (b) the depth of the dolines was measured in the field using a Lietz Sokkisha (Now Sokkia, Tokyo) rangefinder with 50 cm base and an operating range of 10-250 m and an accuracy of around 5%. The data collected in the field were compared with the same parameters extracted from the DEM using statistical tests. The position of the dolines was acquired using a handheld rugged GPS (Trimble Juno equipped with the extension ESRI's ArcPad), allowing us to identify the previously numbered dolines (from Landsat  
Figure 5: Orthoimage of a sector of the study area showing the good horizontal accuracy of the DEM extracted from Pleiades tri-stereo images. The elevation profiles, obtained from this DEM, demonstrate the correct position of the knickpoint along the border of the depressions, as mapped from photointerpretation. The contours confirm the results. A total of 357 dolines were remotely recognized in this study. Of these, 89 were field-checked and validated (Table 3). Dolines range in shape from symmetric bowl-shaped depressions (\(n=18\)), dolines with flat bottom (\(n=50\)), funnel-shaped dolines (\(n=12\)), and asymmetric dolines with mixed shapes (\(n=9\)) (Figure 7). \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**TYPE** & **Doline** & **Depth\_Field** & **Z\_Min** & **Z\_Max** & **Z\_Mean** & **2D\_Surf.** & **Perimeter** & **Depth\_DEM** & **AD** & **l\_AD** \\
**ID** & **(m)** & **(m)** & **(m)** & **(m)** & **(m)** & **(m\({}^{2}\))** & **(m)** & **(m)** & **(m)** & **(m)** \\ \hline bowl & 251 & 17 & 1881 & 1892 & 1890 & 7733 & 264 & 11 & \(-6\) & 6 \\ bowl & 252 & 9 & 1883 & 1894 & 1890 & 12,457 & 350 & 11 & 2 & 2 \\ bowl & 1 & 28 & 1899 & 1920 & 1912 & 9203 & 263 & 21 & \(-7\) & 7 \\ bowl & 5 & 11 & 1916 & 1928 & 1924 & 5529 & 199 & 12 & 1 & 1 \\ bowl & 30 & 9 & 1912 & 1917 & 1915 & 2068 & 99 & 5 & \(-4\) & 4 \\ bowl & 34 & 10 & 1870 & 1883 & 1878 & 16,217 & 392 & 12 & 2 & 2 \\ bowl & 37 & 6 & 1883 & 1888 & 1887 & 4197 & 168 & 5 & 0 & 0 \\ bowl & 104 & 4 & 1961 & 1966 & 1964 & 7986 & 263 & 5 & 1 & 1 \\ bowl & 105 & 10 & 1948 & 1957 & 1953 & 3863 & 157 & 9 & \(-2\) & 2 \\ bowl & 174 & 26 & 1862 & 1892 & 1881 & 13,461 & 332 & 29 & 3 & 3 \\ bowl & 219 & 11 & 1910 & 1919 & 1915 & 3896 & 162 & 9 & \(-2\) & 2 \\ bowl & 237 & 27 & 1924 & 1948 & 1939 & 10,926 & 291 & 24 & \(-4\) & 4 \\ bowl & 283 & 7 & 1890 & 1896 & 1893 & 5338 & 200 & 7 & 0 & 0 \\ bowl & 297 & 3 & 1886 & 1889 & 1887 & 11,237 & 319 & 3 & 0 & 0 \\ bowl & 300 & 5 & 1884 & 1887 & 1886 & 2466 & 114 & 3 & \(-2\) & 2 \\ bowl & 316 & 17 & 1949 & 1970 & 1960 & 12,961 & 331 & 21 & 4 & 4 \\ bowl & 257 & 67 & 1851 & 1911 & 1890 & 48,579 & 662 & 60 & \(-7\) & 7 \\ bowl & 35 & 15 & 1871 & 1892 & 1884 & 35,182 & 601 & 21 & 6 & 6 \\ flat & 12 & 22 & 1883 & 1907 & 1898 & 13,071 & 330 & 24 & 2 & 2 \\ flat & 19 & 8 & 1863 & 1868 & 1866 & 1852 & 89 & 4 & \(-3\) & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary table with the morphometric measurements of the 89 dolines investigated in this study. Figure 7: **(A)** Boxplots of the depth (D) of the dolines as calculated in the field (green boxplot) and using the DEM (orange boxplot). The limits of the boxes and whiskers represent respectively the first and third quartiles. The black lines inside the boxes represent the median values. **(B)** Frequency histogram of the doline shapes classified by type.