# Improved Application of Hyperspectral Analysis to Rock Art Panels from El Castillo Cave (Spain)
## 2 Materials and Methods
### Georeferencing  
Georeferencing is the technique of assigning geographic coordinates to an object, which is used in any mapping procedure and in the development of digital cartographic  
Figure 4: Detailed workflow of the proposed method. Adapted from [3,4]. Figure 3: Overall workflow. databases. By georeferencing, the position of a given point on the Earth's surface is precisely located. Rock art needs absolute georeferencing to guarantee spatial coherence of the data in a time series, which might be acquired in the future to follow up its condition. Every measurement contains error: no measurement is ever exact and must be rigorously adjusted, with the results undergoing statistical analysis. In the field of geomatics, the integration of global navigation satellite systems (GNSS), topographic total station (TTS), 3D terrestrial laser scanner (3 DTLS), digital metric cameras, and hyperspectral imaging systems allow an accurate, reliable and rapid recording of information. This information can be integrated into a Geographic Information System (GIS) which can be used extensively for management, planning or decision making [22]. Accurate georeferencing allows layers of information to be superimposed and analyzed in conjunction over time [23] thanks to the creation of a common reference frame. A reference frame is the practical realization of a system; it is the materialization of a reference system, i.e., the set of points and their coordinates and the techniques applied in the measurements and methods used. To create an accurate reference frame within the cave, this work combines different geomatics methods [24]. Error propagation makes it possible to calculate the accuracy of the elements in the interior of a cave. The workflow followed is:  
1. **GNSS**: Outside the cave, the reference frame is created with visible basis among them which was measured with a Topcon Hiper SR GNSS (Topcon Corporation, Tokyo, Japan) connected to the active network of GNSS stations in Cantabria to receive corrections via NTRIP protocol (Networked Transport of RTCM via Internet Protocol). The result is a set of four coordinates, accurate to the last centimeter. 2. **Microgeodetic Network**: The GNSS coordinates are used to create a complex network [25; 26]. This is carried out to guarantee the quality of the different works carried out, as described by [PERSON] [27]. First, a free network is adjusted that is later attached to the reference frame, obtaining a set of coordinates that constitute the Reference Frame of the site. The coordinate system was ETS 1989 UTM Zone 30N--EPSG: 25830. A Topcon GPT-7503 (Topcon Corporation, Tokyo, Japan) total station was used. Its angular accuracy is 3\({}^{\prime\prime}\) and measurement accuracy is \(\pm\) (2 mm + 2 ppm \(\times\) Distance) mean squared error. 3. **Traverse/Radiation**: The microgeodetic network was adjusted outside the cave. To have accurate coordinates inside the cave, a closed traverse there and back was observed, and later adjusted and compensated (Figure 5). The bases were materialized by means of steel nails and from them a series of new bases and targets were radiated with a TTS. These were used as references for 3D laser scanning. 4. **3 DTLS**: A FARO system model X-130 (FARO Technologies, Florida, USA) [28] was used to scan the cave. The accuracy of a single point is 2 mm at 25 m with a reflectance of 85%. Around 87% of data was measured at less than 4.5 m. Once the data was adjusted, a subsampling method was used to obtain a homogeneous point cloud that will constitute the basis of the ground control/check points that will later be used in photogrammetry. Tensions regarding correspondences between scan points concluded that 81% of the points were below 3 mm. 5. **Photogrammetry**: The panels were digitized with a resolution of more than 50 microns. The ground control points were taken from the point cloud and a standard photogrammetric process as described in [29].