# 3D Documentation with TLS of Caliphal Gate (Centa, Spain)
## Materials and Methods
### Methods
#### 2.2.3 Data Management According to the Desired Outputs  
Opposed to the simplicity of the observations, data management is time consuming and requires highly specialized personnel. The presence of the operator in charge of their treatment during the surveying activities, or at least the generation of detailed sketches and schemes by surveyors will ease this stage. Considering the experience gathered through the development of this and other works related to TLS application and modeling, the relation between the times required to survey and manage data is estimated to range between 1 and 5 for the cases of clean point-clouds or simple meshes. The characteristics of the final outputs can condition the procedures to be followed, but stages of alignment/registration, georeferencing (application of the absolute coordinates of the micro-geodetic network vertices to calculate those of the point-clouds), unifying (merging the complete set of point-clouds not only in a single file, but also in a single point-cloud), cleaning (removal of odd points, duplicated data, etc. ), and exportation are common. An adequate combination of unifying and cleaning can help to reduce the total size of the files, although they imply an important consumption of time. Special attention should be paid to the process of georeferencing, as it can contribute to guarantee the accuracy of the final model. Taking into account the main aspects considered in Sections 2.2.1 and 2.2.2, every single point-cloud will have its own coordinate system. Hence, it will be necessary to consider a common reference system in order to have them joined in a single file. This process is known as \"registering\" or \"alignment\". Two options can be adopted for the definition of that common reference system. The relative coordinate system of one of the scans could be adopted as general system. In that case, the remaining scans should be referred with respect to the one selected as basis. The second alternative implies the adoption of an external and absolute coordinate system. In this case, the process is known as \"georeferencing\". The application of UTM as absolute reference system has two main advantages: A wide-context definition is possible, and the outputs generated from 3D surveying with TLS can be integrated in other preexisting digital models, cartographies, or even in online repositories (such as Google Earth). An intermediate alternative is possible, as the different point-clouds can be referred to the local system of one of them, and the total set can be transformed to the absolute system in a second step. The selection of the most suitable option will depend on the size of the project itself, and the number of available control points. In the case of archaeological sites, such as the one that is proposed in this research, both the registering/alignment and the georeferencing present complications, such as the fact of being commonly developed in confined spaces or with poor lighting conditions. A combination of direct and indirect georeferencing can be required, which guarantees an effective management of field data. The first option requires setting the TLS in points whose coordinates are known in advance, or defining them during the campaign with surveying techniques. A proper leveling, the selection of an external reference of known coordinates to orientate the device, and the height of the latter are also required to apply the direct referencing. Indirect georeferencing is based on a 3D transformation. To apply this alternative, subsequent point-clouds must be pairwise superposed, considering at least three evenly distributed points that appear in both scans and are easy to identify. If their coordinates with respect to the external reference system are known, a rapid and direct transformation will be possible. The Helmert transformation [39], which is applied for this process, is expressed in Equations (4) and (5). \[(\mathrm{X_{E}})\ =\ (\Delta\mathrm{X})+\mu\cdot(\mathrm{R})\cdot(\mathrm{X_{i}}) \tag{4}\]  
\[\left(