# Optimised Extraction of Archaeological Features from Full 3-D GPR Data
## 5 Discussion  
Based on the geophysical survey, two zones were excavated in 2019 and 2020 by a team of archaeologists from the University of Verona: the first was placed between the two southern walls (200 sqm); the second (250 sqm) at the southwest corner of the western square (Figure 9(C1)). In 2020 the entire square was excavated (Figure 9A-B1). The important and well-preserved structure is paved by limestone stone slabs of various sizes. Some of them are reworked from earlier architectural elements and one of these was originally a drain cover. Since all the available data (geophysical, photogrammetry before and after excavation, archaeological excavations) are georeferenced and integrated within a dedicated GIS project, we can make quantitative ex post analyses on the accuracy of the geophysical data and the used parameters. The position and size of the excavated archaeological structures have good overall correlation with the geophysical isosurfaces, with the maximum vertical differences equal to 20 cm for test site 1 and 16 cm for test site 2. In particular, we considered the accuracy of the estimated EM velocity field by comparing geophysical data with the excavated structures. For instance, in the test site 2, the absolute elevation of the ground before excavations was 2.6-2.7 m above sea level, while the stones of the floor lie between 1.4 and 1.6 metres above sea level. So, the mean depth of the floor is close to 1 m, as estimated during GPR data analysis. With the aid of the photogrammetric surveys collected before and after excavation, all the topographic elements were integrated to complete the results and correlate the EM features with the absolute orthometric elevation, further aiding the end users in evaluating the meaning and correlation of the imaged features. Figure 8: **(A)** Exemplary 145 cm trace envelope depth-slice; **(B)** Results of isosurfaces extraction exploiting the envelope, within the 100–200 cm depth range. Figure 9: (**A**) Exemplary inline and crossline of a portion of the 3-D GPR volume (location in Figure 2) crossing, respectively, the stone floor of the new excavated square and the equally-spaced features around it (indicated by the yellow arrows); (**B**,**B1**) time-migrated slice where the edges of the new square are clearly depicted and the same picture with the photograph of the excavation superimposed, respectively; (**C**,**C1**) Envelope attribute calculated on a time-migrated slice and the same picture with the photograph of the excavation superimposed. The yellow arrows indicate some of the equally spaced elements while the red arrow indicates the foundation of a wall. Figure 9 shows the correlation between a portion of the 3-D GPR data volume and the totally excavated square. The inline (W-E direction) perfectly crosses the pavements and its edges are clear; we also note that the central bended break of the stones is imaged by the exemplary 2-D profile by a lowering of signal amplitude (Figure 9A). In addition, the surrounding elements, i.e., column pillars around the square, match with the local geophysical features of the crossline in the N-S direction, pointed to by the yellow arrows. The pillars are also depicted in B-B1 and C-C1 of the migrated slice, on which we applied the envelope attribute to better visualize their signature. We further noticed the foundation of a wall (Figure 9C) 73 cm wide, which runs north-south and shows truncations due to more recent activity. The foundations are in rough-hewn stones and mortar. The elevation was probably made with bricks. The wall would have closed off the eastern end of the arcade.