# 3D Documentation with TLS of Caliphal Gate (Centa, Spain)
## 1 Introduction  
Any project that implies the enhancement or development of a certain asset or space requires an adequate stage of documentation that assesses its initial state. The geometric definition that is obtained during these initial works must be applied to develop a well-founded analysis of the potential alternatives and uses for this enhancement project, so as to select the most suitable option. Geomatics plays a role of capital importance, as it links the conceptual dimension of the project, and the physical framework of reality. A simple analysis of this issue highlights the bi-directionality and the union between the works that must be held to be able to design the use for the asset (the surveying activities, which are aimed at capturing the geometry of the element of interest in an accurate way), and those to be developed to materialize the design itself (the stage of staking out). Considering the importance of geomatics in the field of the preservation and enhancement of cultural heritage, its vital contribution to the recovery or the German architectonic heritage after the Second World War can be presented. In 1885, the Royal Institute of Photogrammetry of Prussia wasestablished. The documentation of over 1000 monuments by means of photographic plates allowed for the restoration of those which were affected [1]. A more recent example of the application of geomatics to the preservation of cultural heritage is the surveying campaign that was developed by the Spanish National Geographic Institute (IGN) to obtain a reference framework, in terms of geometry and chromatism, for the reproduction of the Cave of Altamira [2]. These works provided an accurate definition of the volume of the cave and the polychromous ceiling by the application of classical surveying techniques and photogrammetry through a complex process, given the operative and spatial restrictions that were imposed by the environment. Three-dimensional models are common solutions to many problems in engineering and in other scientific disciplines, as they arrange the information in a more intuitive and visual way than other traditional approaches based in 2D documents. A terrestrial laser scanner (TLS) has implied a revolution in the field of surveying, both in the stages of fieldwork and data management. This is due to the nature of the equipment, which comprises a laser distance-meter and a deflector unit that allow for a sweeping of the surfaces or elements of interest with high rates of capture [3]. This high-speed observation provides an almost-continuous set of non-hierarchized data. When applying classical surveying methods, the operators must decide the relevant points or elements to measure [4], while in the case of TLS, this previous selection does not exist, as it is limited to the definition of the area to be scanned. This implies substantial benefits, such as the massive amount of data that is provided. However, disadvantages can be found: The presence of vegetation can hide the real surface of the asset, and the time required for the treatment of data is substantially increased. Files provided by TLS are high-density point-clouds, which is to say, matrices that comprise the coordinates of each scanned point (referred to the optic axis of the device), relative intensity, and three more columns which correspond to the red-green-blue (RGB) variables that define the color if the internal camera of the device is enabled while scanning. This technology has been proposed or evaluated for different applications, such as geotechnical studies [5; 6; 7; 8], evaluation of coastal geodynamics [9] or geological risks [10], pathological analyses of structures [11], urban management [12], and construction control [13]. Its use has also been proposed for zoological analysis of cavernicolous fauna [14], forestry evaluations [15; 16], and agriculture [17]. Several recent applications related to the enhancement, conservation, and broadcast of natural and/or cultural heritage have been mentioned [4; 18; 19; 20; 21; 22; 23; 24; 25; 26].