## 2 Materials and Methods
### Remotely Acquired Images and Data  
#### 2.3.1 Instruments Calibration and Pre-Drone Survey  
The use of cameras for the Structure From Motion (SfM) and the creation of tri-dimensional virtual outcrops is now straightforward and fast because the instrumentation at our disposition is very light to transport during the field survey.  
The necessity of total stations GPS for the acquisition of more precise and more accurate data positioning is generally required, but in this work, we did not use it. The method used in this research is based mainly on the placement of markers Ground Control Points (mGCPs) at a well-defined known distance during the field survey. The objectives of this work do not take into account the accuracy in terms of positioning of the virtual outcrop in the space (that is in terms of absolute coordinates), but it aims at achieving high-precision in the measurement of the outcrops parameters, as f.i. height of slopes, width, and length of the virtual outcrop, amount of coseismic displacement associated with each part of the rupture. Therefore, this research proposes a method of utilizing photogrammetry for creating a georeferenced and scaled virtual outcrop without the use of total stations GPS. The method is based on the use of an mGCPs of known geometries located close to outcrop during the field survey. The markers located on outcrops are used as ground control points, and their distances are calculated.  
The acquisitions of mGCPs are essential in the pre and post-processing phase, to make scale changes, to position, orientate, and rescale the three-dimensional model in the chosen Geographic Reference System. In this work, \(25\times 25\) cm markers were used as mGCPs.  
During the subsequent analysis through the Agisoft Metashape software, the placement of markers were positioned at the center of each mGCPs surveyed, to optimize precision between the images, and for subsequent rescaling of the virtual outcrop. The contrasting black and yellow rectangles of mGCPs, allow easily recognizable common points on each acquired photo and also facilitate the automatic location of points belonging to different images.  
To improve the reliability of the measurement performed on a virtual outcrop, different calibers oriented in the third dimension (zy/zx plane = height, yx plane = width and length), perpendicular to each other were added.  
The caliber instruments as hand-ruler 25 m long, aluminum stadia graduated rod 5 m long, and markers positioned at predetermined distances were used.  
This method provided good precision in evaluating the virtual outcrop parameters, but it does not take into account the absolute accuracy in terms of geographical positioning; therefore, the whole model could also be shifted by some decimeters. In this work, all the markers were positioned at a distance of 10 m from each other and arranged in two preferential orthogonal directions North-South and West-East.  
In the sample area 1, they were positioned at the intersection points of a 10 m sided square grid oriented in N-S and W-E directions (Figures 1b and 2a,b). Overall, in this area, 15 mGCPs, 7 of whichin the fault footwall and 8 in the hanging wall, and different calibers were used for scaling the entire model proportionally in both the directions N-S and W-E of the obtained virtual outcrop model.  
The area 2, characterized by a narrow and elongated shape (1 \(\times\) 0.13 km), to reduce the 'doming' effect [eg 10, 12, 13], a total of 20 mGCPs were arranged in the fault footwall, in the hanging wall, and also at corners and central part of the investigated area (Figures 1b and 2c,d).