# 3D Documentation with TLS of Caliphal Gate (Centa, Spain)
## Materials and Methods
### Methods
#### 2.2.2 TLS Surveying  
TLS can be considered as a non-invasive technique, given the fact that the surveyor does not interact with the elements that are scanned. The strategy to capture data in an optimal way must consider three main aspects. Firstly, it is necessary to define the number of scans that are required to obtain a complete definition of the heritage element itself, according to its size or geometrical complexity. Secondly, the best location for the set of references (standard spheres or targets that will be applied to join adjacent point-clouds) with respect to the TLS must be decided, as a proper intervisibility between them is necessary. It is worth mentioning that a quasi-optimal location must be selected for all the involved elements. That is to say, those that provide the best possible quality for the final product, taking into account spatial constrictions, and guaranteeing the integrity of all the elements of value within the archaeological site. Thirdly, an adequate relative location of these reference elements must be also taken into account: A linear arrangement of the spheres or targetswould imply a mathematic indetermination and, therefore, the definition of a rotation axis instead of a joining plane for an adequate alignment of neighboring scans.  
After setting the TLS and the references in the selected locations, several data, such as the resolution, accuracy, record number, or destiny and format of the file, must be selected or defined to start the scanning process. A previous low-resolution scan can help to define both the area to be captured by a more detailed subsequent scan, and to check if the location given to the different references with respect to the TLS is adequate. In this regard, it is advisable that the capture has a suitable resolution for the element that is modeled. Some devices even allow for naming of these references, which eases the management of the point-clouds in later stages. It is worth noting that, in addition to this, the development of sketches in field notebooks that include the location of the device and the references, and any other helpful detail of the lay out, can be very interesting to simplify these tasks, especially if data were surveyed and managed by different staff members.  
Any registered point belonging to a certain cloud will be defined by its spherical or Cartesian coordinates, with respect to the optical center of the device. It is possible to relate both types of coordinates with Equations (1) to (3), where d is the range or distance between the TLS and the point itself, \(\theta\) is the horizontal angle, \(\alpha\) is the elevation angle, and \(x\), \(y\), \(z\) are the Cartesian coordinates. These coordinates can be corrected by calibration [38].  
\[\mathrm{d}\ =\ \sqrt{x^{2}+y^{2}+z^{2}} \tag{1}\]  
\[\theta\ =\ \arctan\left(\frac{y}{x}\right) \tag{2}\]  
\[\alpha\ =\ \arctan\left(\frac{z}{\sqrt{x^{2}+y^{2}}}\right) \tag{3}\]  
The integrity of each raw point-cloud should be assessed on the screen of the device after the scan has been finished to confirm that it has been fully acquired. It is important to check the continuity of raw scans, since vibrations or movements of the device can break that coherence, it being necessary to repeat certain sections.  
The adaptation of this strategy for the particular case that is considered in this research, which is characterized by spatial constraints (such as slopes, hollows related to the excavations, heritage elements, etc.), is one of the innovations of this work.