# WELDMAP: A Photogrammetric Suite Applied to the Inspection of Welds
## 1 Introduction  
Welding is a very important task in the field of engineering and construction, since welds that are not properly carried out reduce the mechanical properties of joints, minimising their effective service life, and possibly promoting their collapse, which could entail drastic consequences, being that welding joints are the weakest links in assemblies [1]. For this reason, quality requirements are highly standardised (e.g., [2, 3, 4] and also highly specified for specific cases such as pressure vessels [5]). Therefore, welding is a joining process that requires inspection and monitoring tasks. The visual inspection is a simple and widely extended non-destructive test modality [6], which is applied for the assessment of weld bead geometry and the detection of surface imperfections and flaws. Visual inspection tests are regulated by the standard [7]. The application of visual inspection allows for detecting defects and imperfections that have a manifestation on the surface of the weld. The taxonomy of defectology in welds is widely established by the international quality standards (e.g., [3, 4]). So, inspection techniques are aimed at detecting and evaluating defects established in these norms. Visual inspection, as a non-destructive testing method, should be implemented by inspectors with training and expertise who have acquired a level of competence also established by the international standards (e.g., [8]). These inspectors are often engineers with extensive knowledge of the welding process and who have passed several exams to demonstrate that they have the right skills. The inspector usually has the obligation to visit the welds on-site to evaluate the quality of each weld performed according to the international standards, which entails a significant economic investment [6]. In addition, the inspector must evaluate the weld at the site, often in a very short time and record the information of each weld in an analogicalway or in traditional databases. To do this, it is common to use different auxiliary tools to evaluate the quality of welds such as tweezers or weld gadget kits [9]. These tools, being for manual use, generate measures with significant errors and especially result in visual fatigue for the inspector. Currently, some authors are investigating other, more sophisticated techniques for the detection and identification of external pathologies in welds: laser systems [10; 11], scanning cameras [12], hybrid laser-based systems and cameras [13; 14; 15; 16], stereo imagery [12; 17], close-range photogrammetry [9; 18], and, finally, active thermography systems as supports for visual inspection, which are applied directly to the study of cracks in welds [19]. Among these techniques, close-range photogrammetry and laser scanning allow the accurate three-dimensional reconstruction of weld bead geometries in a non-invasive way. Such 3D models can be used for the inspection of external welding imperfections and defects. However, there are several differences in terms of operating principle (i.e., active versus passive sensors), working range, and cost-efficiency ratio. The reader can find more information about this comparison in [20; 21]. Close-range photogrammetry has been used for the generation of 3D models that allow for the analysis of defects such as overlap and lack of fusion, among others [9]. It has also been used for the three-dimensional assessment of plaque misalignment [22]. In [9], a pioneering method of inspection based on close-range photogrammetry was established. For its part, active systems such as laser scanning have been also used for the evaluation of the external quality of welds. In [22], authors used a structured light system, which allows the accurate assessment of the weld bead according to the quality criteria. The procedure developed enables the extraction of metric information about the thickness and the deformation angle of the welds.