With the recent developments witnessed in technology, the traditional method of survey-ing and drawing has been replaced by digital surveying, the LIDAR technology, and computer modelling processes. However, these new methods offer new opportunities, in respect such as automatic orientation and measurement procedures, 3D data generation on point clouds, digital surface modelling and WEB representation in cultural heritage.
With regards to the heritage conservation, it is a continuous process, with lots of data to be integrated, acquired and analysed, which means that a lot of construction data has to be recorded on the site. Further, there is a series of workflows and operations that include surveying, drafting and engineering design, follow-up and analysis [11–15].
This paper explores the importance of an analytical methodology in the evaluation and assessment of architectural elements, especially vertical ones. The work was based firstly on the collection of data on the site by traditional measurement and by Terrestrial LIDAR using a Leica Scan Station P16 Laser Scanner. The choice of the object and its nature, its shape and its extraordinary longevity and resistance to natural phenomena (earthquake, rain, wear of materials...).
2.1 Materials
Indeed, the survey methodology proposes the use of simple procedures for assessment purpose of the surface alterations with regards to the minaret, based on an on-site TLS survey. It should be underlined that this study avoids studies on material properties and focuses essentially on architectural information and structural diagnosis. Above and beyond, the scanner used in this case is a Leica P30, a model that allows a very simple and fast survey process, main technical features deliver the highest quality 3D data and High-Dynamic Range (HDR) imaging at an extremely fast scan rate of 01 million points per second at ranges of up to 270 m.
The laser scanner uses the reflection and projection of the laser beam. Nonetheless, as objects are scanned by a laser beam, the 3D scanner calculates the distance of the point from the focal point and among other points on the object. Besides, the scanner thus records the actual space as cloud points. Additionally, our work uses a 3D laser scanner, but alike a high-quality digital camera. On the other hand, the uncertainty of the resulting model is less than 01 mm. For acquisition purpose of the 3D data relating to the building, we used an integrated and planned scanning job for the whole, originating from several different scanning stations that can combine a set of multiple view views (scanning mode). As shown in the plan above, twenty-two (22) scan positions have been indispensable to re-construct the body of the minaret from the outside and the traces of the Agadir Mosque.
2.2 Data acquisition
In the light of the above facts, this work for the Minaret totalled more than 22 survey stations on the site. The assembly of the different point cloud stations into a workstation and the merging into a single model of the data generated a digital model of more than 185 million points (Fig. 3). In Meshing's operation via the 3Dresharper software, more than 60 million triangles were generated in a mesh. However, the uncertainty of the mesh is less than 2 mm. We can improve the results by multiplying the number of stations.
The methodology refers to the application of digital techniques to analyse the data obtained from TLS surveys. The software used is Leica Cyclone, 3Dreshaper and Cloud-compare.
The objective is to identify a workflow to verify the initial hypotheses regarding structural and constructive deformations of the building that are not visible to the naked eye, and cannot be obtained with traditional measurement systems. This is achieved through exploratory data analysis techniques. Finally, the technique identified must be easy and re-producible on elements identical to our object of study.
This work was divided into three stages.
The first step in documenting-built heritage, the difficulties in terms of classical surveying of complex architectural forms and structures with difficult access. The advantages of TLS techniques in terms of surveying are no longer in question.
The second stage of documentation with a simplified workflow using specific software tends to render the object in 3D (digital restitution process).
The third and most important step is to carry out an evaluation study of the geometric and architectural components of the object. With an understanding of the constructive and structural problems, tools and processes.
2.3 Results
The workflow based on TLS generally consists of four phases to initiate the digitisation work [16, 17]. Hence, the process starts with the generation of a point cloud by terrestrial LIDAR technology, followed by a cleaning process on the workstation and the merging of the different stations so as to generate meshes (meshing process), in addition to a phase of reconstitution of the object’s surfaces by digital rendering. As for the other processing phases, they represent the model production. In closing, the diagnostic workflow (Fig. 2) is demonstrated with explanations hereunder:
• The first phase that uses TLS is the acquisition of raw point cloud data to reconstruct the surfaces of the case study across several stations. For acquisition purpose of the raw data via TLS, the integrated scan job must plan for the entire object originating from sever-al different scanner stations that can combine a set of multiple view orders (scan worlds). More to the point, the major steps in combining multiple views are positioning planning and accurate registration. Such a set of information is organised under the form of token ring or star topology.
• The second phase is performed by computer, through cleaning, merging and recording of the different workstations operated on the site so as to generate the minaret in a single unified point cloud in raw data.
• The third phase stands for the modelling of the point cloud (meshing) so as to generate surfaces by use of reverse engineering software to process these raw data (cloud points), such as Meshlab (open source), Cloud-Compare (open source) and 3DReshaper. Above and beyond, this consists of using a triangulation algorithm to transform the point clouds into a 3D mesh and then converting them into a 3D model. In light of which, this phase implies mainly the trans-formation of the data and the reconstruction of the 3D geometry of the object.
• In closing, the digital expertise’s work on the object (the minaret of Agadir) so as to analyse the architecture and dimensions thereof and alike to diagnose the deteriorations suffered by its structure.
Undoubtedly, the survey has tested the use of simple TLS-based procedures in order to analyse the state of conservation of masonry walls, focusing on the external surface. Some of the damage was clearly visible, but the technique used allows degradations not visible to the naked eye to be identified and quantified quickly, easily and without contact. Further, the adopted technique is based on the comparative study of a series of vertical and horizontal sections of the object on the mesh model generated by phase three of the work-flow. This technique has already been experimented by the laboratory team in previous works, particularly on a monument that has suffered serious alterations [18].
During this workflow phase, the interest consists of choosing the control sections of the 2D geometry in the three axes (X, Y, Z) by placing a section of the plan on the textured object (Fig. 4), the number of sections, their spacing depends, for sure, on the nature of the object treated, but alike on the precision required in particular for restoration or diagnostic operations. Likewise, it is possible to extract the geometry of a 2D section from a point cloud instead of the object in mesh, by a section of the plan, except that in this case, the precision of the edges and contours is not useful for a precise diagnostic work.
In the next step, once the 2D geometry of the sectional plan has been created on the object, it is mandatory to isolate them in order to use the same in a 2D CAD (Computer Aid De-sign) representation and then, after colouring the sections separately, to merge them together. Additionally, once a series of sections (the number and spacing of which depends on the precision of the work) has been obtained along the three axes (X, Y, Z), it is essential to merge the 2D representation. At this stage, we can distinguish the differences between situation 01 in 2D and situation 02 in the predefined XYZ axis.
As for the architectural analysis work, it has alike been based on ortho-photo plans generated by the object and the 3D dimensional calculation on the object produced by the work-flow. Thus, this work made it possible to check all dimensions with great accuracy and for inspection purpose of verticality and deformation of the monument.
The Mesh elevation allows the entire surface of the minaret to be represented in a single metric scale image. Thus, the inspection map complements this information and brings to light the different textures identifiable on the wall. On the basis of these two documents, it is easy to accurately identify and draw the main problems and degradations pertaining to structure, in respect such as differential settlements and torsion at the top, etc.
More to the point, further research in this direction will allow for further automation of this process, and the use of other survey data, as well; in respect of such as reflectance maps. With regards to the formal 3D assessment, the objectives are highlighted through verifying the verticality of the minaret walls.