As-built graphic documentation of the Monumento a la Tolerancia. Validation of low-cost survey techniques

EGE - Revista de Expresión Gráfica en la Edificación, Nº 13, 2020. ISSN: 2605-082X https://doi.org/10.4995/ege.2020.14766 APEGA, UPV *Correspondence author: Sara Morena, [email protected] Received: 11 December 2020, Accepted: 15 December 2020, Published: 31 December 2020 AS-BUILT GRAPHIC DOCUMENTATION OF THE MONUMENTO A LA TOLERANCIA. VALIDATION OF LOW-COST SURVEY TECHNIQUES LEVANTAMIENTO GRÁFICO DEL MONUMENTO A LA TOLERANCIA. VALIDACIÓN DE TECNICAS DE LEVANTAMIENTO DE BAJO COSTE Sara Morena a , Borja Molero Alonso b, José Antonio Barrera-Vera c , Salvatore Barba d a Department of Civil Engineering, University of Salerno, Italy. [email protected] b Mave Aeronáutica, Trigo Group, Spain. [email protected] c Departamento de Ingeniería Gráfica, University of Sevilla, Spain. [email protected] d Department of Civil Engineering, University of Salerno, Italy. [email protected] Abstract The Monumento a la Tolerancia is an emblematic sculpture realized by Eduardo Chillida, a perfect example for the documentation of sculptures with uniform textures, non-reflective colours and with poorly elaborated shapes through the implementation of various photogrammetric tools, as well as using different applications for processing phase. The basic data are photos taken quickly and without an accurate previous study, which is why the implementation of any target was not foreseen. In order to prove the results different kind of analysis were conducted. The first type was carried out analysing the results obtained from different software, with the use of the same instrument. The second one was a comparison of the models obtained with different tools, elaborated with the same software and the last one a validation of photogrammetric model with the laser scanner one. The complete results will be presented and critically discussed to validate these tools and applications. Keywords: Architecture survey; 3D Photogrammetry, Point cloud; Comparison; Accuracy. Resumen El Monumento a la Tolerancia es una emblemática escultura realizada por Eduardo Chillida, un perfecto ejemplo para la documentación gráfica de objetos con texturas uniformes, colores mate y con formas simples mediante diversas herramientas fotogramétricas, así como mediante el uso de diferentes aplicaciones en la fase de postproceso. Los datos de partida proceden de fotos tomadas sin una planificación previa, por lo que no se ha contemplado la colocación de ningún tipo de dianas. Con el objeto de poner a prueba los resultados se han llevado a cabo diferentes tipos de análisis. El primero se ha realizado analizando los resultados obtenidos con diferente software pero usando la misma instrumentación. El segundo ha sido la comparación del los modelos obtenidos con diferentes herramientas pero elaborados con el mismo software. Finalmente se ha llevado a cabo una validación del modelo fotogramétrico con el modelo obtenido con escáner láser. Todos los resultados se presentan y discuten para validar las herramientas y aplicaciones empleadas. Palabras clave: Levantamiento arquitectónico; Nubes de puntos; Fotogrametría; Comparación; Exactitud. 1. INTRODUCTION Architectural survey with the implementation of digital technologies represents today one of the main methods for as-built graphic reproduction, both in the architectural (Molero et al. 2013: 673), archaeological (Fioretti et al. 2020: 2) and sculptural field (Loaiza Carvajal et al. 2020: 234). The possibility of reaching rigorous measurements and the massive nature of the technology, in fact, allow interrogating repeatedly the documentation and to obtain technical elaborations with different degrees of resolution. Based on previous studies (Morena et al. 2019: 135) and SfM comparative analysis (Rodríguez-Navarro 2012: 100; Gil- Piqueras et al. 2019: 645), this contribution aims to analyse not only the opportunities that the new low-cost technologies guarantee in the field of sculptural heritage (with homogeneous textures, simple geometric shapes and non-reflective materials), but also comparative evaluations of surveys of the same object when the variables are not only the instruments but also the times and above all the operators. The progress achieved, both hardware and software, in the field of surveying makes it increasingly easy to generate models even from acquisitions that are not perfectly pondered. The possibility of implementing photos acquired through technologies that are now widely used, compact cameras and smartphones (Shults 2017: 481), as well as the availability of lowcost and user-friendly software, in fact, allows pursuing sufficiently accurate results. This ensures greater digitization of the heritage (Tabone 2019: 48), even if, only for divulgation purposes: web visualization, virtual tours, as well as innovative ways to catalogue and store documents in eminently graphic, technical and architectural history databases. The aim of this paper is the analysis and comparison of threedimensional models of the Monumento a la Tolerancia (Figure 1). The attention is focused on this monument following the collection of many survey experiences conducted by different people (even non-experts) obtained during the years with various instruments (FARO focus3D X330, Nikon COOLPIX L11 and Apple iPhone 11) and processed with some software (SCENE, PhotoModeller, Metashape, eyescloud3D, 3DF Zephyr and ReCap Photo). To prove the accuracy of these data, several analyses have been carried Fig. 1. Monumento a la Tolerancia and Puente de Isabel II. Source: own elaboration 2008. MORENA, S.; MOLERO ALONSO, B.; BARRERA-VERA, J.A.; BARBA, S. EGE - Revista de Expresión Gráfica en la Edificación, Nº 13, 2020. ISSN: 2605-082X | 99 out, the first one to evaluate the difference pursued by the various software in the processing of data coming from the same instrumentation. A second analysis was carried out, instead, comparing the best results obtained with the two different instruments processed with the same software. Finally, the need to practically prove commercial and usual technologies for the restitution of models inevitably involved the comparison of the data acquired with a laser scanner characterized by high precision and accuracy (Figure 2). Although the results of this investigation are achieved with scientific rigor, the data of this research are presented here with a predominantly informative intent. 1.1 HISTORICAL INTRODUCTION The Monumento a la Tolerancia is an emblematic sculpture located in Seville and signed by the author Eduardo Chillida (San Sebastián, 1924- 2002). It was inaugurated in 1992 at the Universal Exhibition in Seville in memory of the Edict of Granada, with which the Jews were expelled from the Catholic monarchs. The work, financed by the Fundación Amigos de Sefarad, is located on the “Muelle de la Sal”, Paseo del Alcalde Marqués del Contadero, next to the Puente de Isabel II, popularly known as Puente de Triana (Rabe 2014: 137). Made entirely of reinforced concrete, it rises to a total height of about 4 m and extends 12 m wide. Opposing the violent and suffering past, it presents itself as a great embrace between different cultures, to encourage understanding and tolerance (Matos 2015: 99). Despite its apparent simplicity, seen from different perspectives, it always assumes different configurations, once the left arm protrudes, another one prevails the right one, seemingly every portion of the work is “always different but never the same” (Chillida 2016: 103). The absence of visual uniformity of the work is the very emblem of tolerance, there is no single point of view, only respect for diversity that can coexist together. Despite its artistic and symbolic importance, following several acts of vandalism, the sculpture had an extremely degraded appearance, which is why it was necessary to clean and consolidate it. This, however, was not enough to limit further acts of vandalism that replicated themselves on the structure, underlining even more the need to document, detect and digitize Fig. 2. Correspondence view based on point clouds laser acquired with FARO Focus 3D X330. Source: own elaboration 2020. EGE - Revista de Expresión Gráfica en la Edificación, Nº 13, 2020. ISSN: 2605-082X | 100 AS-BUILT GRAPHIC DOCUMENTATION OF THE MONUMENTO A LA TOLERANCIA. VALIDATION OF LOW-COST SURVEY TECHNIQUES our heritage to preserve, divulge and catalogue it by generating an archive of data available for any eventuality. 2. DATA ACQUISITION AND PROCESSING The data acquired by different operators and in different time were reached by three different instruments and processed with various software, described below (the information contained is mainly taken from the respective manuals). It is also essential to underline that some acquisitions were carried out by inexperienced personnel and only for documentary purposes. The choice of photogrammetric programs for data processing fell on both professional and user-friendly applications where the interaction with the user during the process is strongly limited. In particular, the software implemented are Metashape by Agisoft and 3DF Zephyr by the Italian 3DFLOW. They are applications that operate in Structure for Motion (SfM) for the pseudo-automatic processing of three-dimensional photo-realistic models. In the second case, the choice fell on eyescloud3D and ReCap Photo. The first consists of an app connected to the cloud, accessible via both mobile phone and computer, of the Spanish eCapture3D. It automatically proceeds at the generation of point cloud and mesh, without the need of high-performance computers, and, at the end, it provides to display the model on a networked platform. With a very simple interface, it also allows different iterations of measurement and editing, as well as the possibility to geo-reference and download the model. ReCap Photo is a service included with Autodesk’s ReCap Pro, which allows processing close-range or drone photos for the generation of point cloud, mesh and orthophoto. With a simple graphical interface, also in this case the process of the data is performed on the internet, allowing data editing once the model has been processed. However, given the characteristics of the monument and the close-up shots, the best results were obtained by operating in object mode, which automatically generates a textured mesh. The workflow with the use of Metashape and 3DF Zephyr software is mainly based on five consecutive steps: cameras orientation and images alignment with the generation of a sparse cloud, detection of control points (extracted from the laser scanner model) to locate and scale the model, dense cloud construction, mesh generation and model texturization. Differently to the previous cases, with the eyescloud3D and ReCap Photo applications, we proceeded to select the photos and start the process and, only after the generation of the model, we could scale and export it. At general level, the main difficulties encountered were: the alignment of the cameras, despite a sufficient overlap of about 70% with both COOLPIX L11 and iPhone 11, and . Fig. 3. 7-points identified on the laser scanner point cloud. Source: own elaboration 2020. MORENA, S.; MOLERO ALONSO, B.; BARRERA-VERA, J.A.; BARBA, S. EGE - Revista de Expresión Gráfica en la Edificación, Nº 13, 2020. ISSN: 2605-082X | 101 the absence of targets related to a monument characterized by homogeneous texture and simple geometric shape. The selected Control Points were extracted from the laser scanner model. The coordinates of 7 points were identified to locate and scale the photogrammetric models (Figure 3). Finally, the absence of a photographic pole or a UAV flight made impossible to return the upper part of the model, which is why all the processed data have gaps in the upper part. Tables visible in the figures 4 and 5, schematize the main data processing obtained during the alignment phase, dense cloud and mesh generation respectively. 2.1 FARO FOCUS3D X330 Phase difference laser produced by the CAM2 company, is characterized by its weight (only 5 kg) and compactness, properties that guarantee an easy management of the device facilitating its movement between the various positions. The field of view covers an opening of 360° horizontally and 300° vertically, with an acquisition depth from 0.6 m to 330 m (Gollob et al. 2019: 4). The photographic sensor, which allows colour acquisition during scanning, is integrated in the system and allows further limitation of errors during data processing. To facilitate and speed up the acquisition of scans, realized in 2015, a first 360° pre-acquisition has been carried out for each station, with a distance between points of 15.35 mm at 10 m (1/10 resolution and 4× quality). Only afterwards, a higher resolution acquisition (1/4 with the same 4× quality), with a points distance of 6.14 mm at 10 m and with the addition of colorimetric information, was carried out. Data management and processing were conduct in 2020 with the proprietary software of the scanner, SCENE (version 2019.0.0.1457). The information collected directly from the instrument during acquisition (altimeter, compass, inclinometer, and GPS) guaranteed easier management of the 7 scans used to generate the reference point cloud. At a first manual recording, through the identification of homologous elements in the scans, an automatic cloud-to-cloud recording followed with the aim of making improvements in the final global spatial registration of the model. The generated cloud has about 58,000,000 points, with an average point error Fig. 4. Main data processing in the align phase. Source: own elaboration, 2020. Fig. 5. Main data processing in the built phase of dense cloud and mesh. Source: own elaboration 2020. EGE - Revista de Expresión Gráfica en la Edificación, Nº 13, 2020. ISSN: 2605-082X | 102 AS-BUILT GRAPHIC DOCUMENTATION OF THE MONUMENTO A LA TOLERANCIA. VALIDATION OF LOW-COST SURVEY TECHNIQUES of about 8 mm and a 20% overlap of scans. Finally, through a Clippinbox , the area of interest has been selected to export it for comparison on third-party software. 2.2 COOLPIX L11 Compact Nikon camera featuring 1/2.5” CCD sensor of 6.2 MP, a variable focal length between 6.2-18.6 mm and a focal aperture of f/2.8-5.2. Its simplicity of operation and its particular sensitivity to light with an ISO 800 make it easy to use even for non-professional people. The photographic acquisitions made on site in the 2008 were made only for documentation purposes and, therefore, without the use of targets. The inspection concluded with the acquisition of 77 photographs of the sculpture from different angles. The Metashape processing, carried out by setting the Highest accuracy (maximum resolution of the photos) and a key point limit of 40,000, ended with the alignment of 74/77 photos. A dense cloud, consisting of about 42,000,000 points and a mesh model of 8,000,000 polygons, was then created. In the case of the 3DF Zephyr application the best results were achieved by setting, during the orientation phase of the cameras, key point limits of 7,500 and a 100% resolution of the images guaranteeing an alignment of 70/77. The results obtained are a dense cloud of about 2,000,000 points and a polygonal mesh of as many faces, characterized by several gaps especially near the left arm. The data processing through the eyescloud3D application gave better results with the selection of only 50/77 photos, generating a cloud of about 1,500,000 and a mesh of 170,000 polygons, with the presence of various gaps scattered in the model. Finally, the data was also processed with ReCap Photo, which allowed the generation of a mesh model of about 870,000 faces but with various deformations especially near the arms. A further software, implemented in 2008, to manage this data was PhotoModeler v.6. In this case, the calibration of the camera was carried out successively to the survey: it consists in a process where it is necessary to take photos (maintaining fixed the parameters) of a grid with a series of points with theoretical coordinates provided by the software. The comparison of these points with the reality captured by our camera allows us to define the degree to which the field images are deformed by the lens, for subsequent correction. Successively, we proceed in the generation of the three-dimensional scene with the detection of homologous points between the images, by which the positions of the cameras in the space are calculated now of making the shot. This is an arduous process which Structure from Motion systems (also available in newer Photomodeler releases) have improved. Once all the necessary images have been oriented, the model is constructed on which, subsequently, the surfaces representing the monument are defined. Finally, the textures are projected on the surfaces, ensuring that they come from the images in a more orthogonal situation, and without obstacles, for a better photorealistic quality. 2.3 IPHONE 11 Smartphone designed and manufactured by Apple characterized by a 1/2.55” CMOS sensor and a double camera of 12 MP, one wide angle and one ultra-wide-angle. Very popular in recent years, it guarantees the production of good quality photos even by inexperienced users. The inspection, once again carried out without a properly significant purpose and in the absence of targets, it was conducted in the 2020 and ended with the realization of 83 photos. The parameters used in Metashape, very similar to the previous ones, with maximum accuracy and resolution of the photos during the orientation phase, allowed to obtain an alignment of 82/83 photos. Despite the higher number of acquisitions, the generated cloud has a number of points equal to 38,000,000 and a mesh of about 8,300,000 faces similar to the model with the Nikon. In the same way of 3DF Zephyr, the best results were obtained by setting key point limit values at 7,500 and maximum photo resolution, ensuring an alignment of 82/83 photos, a cloud model with 1,050,000 and a mesh of about 12,000,000 faces. Using 83/83 photos it was possible to proceed with the creation of the model also in eyescloud3D, a cloud of about 2,000,000 and a mesh of 405,000 polygons was generated. Finally, we use ReCap Photo with the generation of a mesh consisting of about 1,000,000 polygons and deformations evident in various parts of the model. MORENA, S.; MOLERO ALONSO, B.; BARRERA-VERA, J.A.; BARBA, S. EGE - Revista de Expresión Gráfica en la Edificación, Nº 13, 2020. ISSN: 2605-082X | 103 3. COMPARISON The processed data were compared to validate what was attained. A first analysis was carried out by verifying the best results achieved using the same tool and processing the data with different software. Subsequently, based on what had been obtained, the main differences between two of the best models realized with COOLPIX L11 and iPhone 11 were evaluated and finally everything was validated by comparing the preferable model with the one detected by laser scanner. However, while in the case of professional applications such as Metashape and 3DF Zephyr the comparison can be based on statistical data extracted and processed directly by the software during the elaboration time, the same cannot be conducted in the case of the remaining two applications. To make further comparisons, the models were exported in universal format and compared in a third-party software, CloudCompare V.2.10.2. For a comparison between dense clouds, “Cloud–to-Cloud Distance computation” (C2C) was used. It is the default way to compute distance between dense clouds in the software: for each point of the compared cloud, it searches the nearest point in the reference cloud and computes their (Euclidean) distance. In the case of point cloud and mesh comparison, necessarily we should have proceeded with “Cloud- to-Mesh Distance computation” (C2M) in which for each point of the compared cloud, the software should identify the closest triangle in the reference mesh. However, operating in this way, the software will automatically identify the mesh as the reference element. To invert the models and consider the cloud generated by Metashape as a reference, it was necessary to first sample the mesh and, only then, compare it. 3.1 SOFTWARE COMPARISON Tables (Figure 6 and 7) show, for both COOLPIX L11 and iPhone 11, the re-projection errors of the x, y, z coordinates for each Control Point used in Fig. 6. Re-projection errors for each Control Point with the Nikon COOLPIX L11 instrument. Source: own elaboration 2020. Fig. 7. Re-projection errors for each Control Point with the iPhone 11 instrument. Source: own elaboration 2020. EGE - Revista de Expresión Gráfica en la Edificación, Nº 13, 2020. ISSN: 2605-082X | 104 AS-BUILT GRAPHIC DOCUMENTATION OF THE MONUMENTO A LA TOLERANCIA. VALIDATION OF LOW-COST SURVEY TECHNIQUES the first two software (the position of the points are visible in the previous figure 3). As can be seen, in the case of Nikon tool, the 3DF Zephyr software has slightly higher deviation values than Metashape, reaching a medium value of error of about 1.50 cm respect to 1.38 cm of Metashape, with maximum value of 3.0 cm for the point 3. More difference, instead, are visible in the case of iPhone 11 instrument with the point 2 that reach an error of about 5.00 cm in the case of 3DF Zephyr software. A further comparison, as previously specified, has been carried out within the CloudCompare software to evaluate the existing deviations between the model surfaces (Barba et al. 2019: 149). The Metashape point cloud achieved the best results and was taken as a reference for comparison with the remaining models extracted from the software. In the figures 8, 9 and 10 it is possible to visualize the result of the above analysis. The column on the right of the image represents the range of the existing deviation between points belonging to two different models which are, in fact, represented in false colours. The maximum deviations (in red) reach values equal to 10 cm and are visible in the first comparison only in a portion of the left arm. Satisfactory, are also those obtained with eyescloud3D software, instead, obvious Fig. 8. The comparison between the photogrammetric point clouds of Metashape and 3DF Zephyr (Nikon). Source: own elaboration 2020. Fig. 9. The comparison between the photogrammetric point clouds of Metashape and eyescloud3D (Nikon). Source: own elaboration 2020. Fig. 10. The comparison between the photogrammetric point clouds of Metashape and ReCap Photo (Nikon). Source: own elaboration 2020. Fig. 11. The graphic distribution of the comparison Metashape with 3DF Zephyr (Nikon). Source: own elaboration, 2020. Fig. 12. The graphic distribution of the comparison Metashape with eyescloud3D (Nikon). Source: own elaboration, 2020. MORENA, S.; MOLERO ALONSO, B.; BARRERA-VERA, J.A.; BARBA, S. EGE - Revista de Expresión Gráfica en la Edificación, Nº 13, 2020. ISSN: 2605-082X | 105 deviations, are mainly shown in the model returned with ReCap Photo. The results of the point-to- point comparison of the models generated by the different software used to process the COOLPIX L11 data have been schematically shown in the graphs where on the x-axis are the deviation values in m and on the y-axis the number of points to which this value corresponds (Figure 11 and 12). From what we can see, the best results were achieved in comparison with 3DF Zephyr which has the lowest standard deviation value (1.3 cm), however, good results were also achieved in the case of eyescloud3D with standard deviation values close to 1.8 cm as well as the return of a complete model without significant gaps. Less satisfactory are those generated by ReCap Photo with a standard deviation reaching a value of about 3 cm. The peak of the curve is close to 0.15 cm for comparison with 3DF Zepyr and the analysis of the graphic show as the 90% of the point have a deviation less than 2 cm. In the case of eyescloud3D, most points have values of deviation of 0.80 cm with around 70% of with deviation less than 2 cm. Further comparisons have been made with the iPhone 11. From the analysis of the figure 13, we can be seen that the comparison of the Metashape and 3DF Zephyr models has more deviations than the previous comparison with the Nikon. Values of about 10 cm occur mainly near the left arm, they are confirmed by an incorrect alignment in 3DF Zephyr model. Better results, in this second case, came from the eyescloud3D application, which allowed to generate a dense cloud with slight deviations from Metashape, reaching a standard deviation value of about 1.4 cm (Figure 14). Once again, the distribution is schematically shown in the graphic visible in the figure 15, with the highest frequencies near the value of 0.15 cm and around 80% of points with deviation less than 2 cm. Otherwise, ReCap Photo has returned a model with a strong deviation from the reference one, not only on the arm, but in the general model, reaching in some points significant differences more than 10 cm (Figure 16). 3.2 INSTRUMENTS COMPARISON As previously mentioned, a further comparison was made between the results obtained Fig. 13. The comparison between the photogrammetric point clouds of Metashape and 3DF Zephyr (Apple). Source: own elaboration 2020 Fig. 14. The comparison between the photogrammetric point clouds of Metashape and eyescloud3D (Apple). Source: own elaboration 2020 Fig. 15. The graphic distribution of the comparison Metashape with eyescloud3D (Apple). Source: own elaboration 2020. Fig. 16. The comparison between the photogrammetric point clouds of Metashape and ReCap Photo (Apple). Source: own elaboration 2020. EGE - Revista de Expresión Gráfica en la Edificación, Nº 13, 2020. ISSN: 2605-082X | 106 AS-BUILT GRAPHIC DOCUMENTATION OF THE MONUMENTO A LA TOLERANCIA. VALIDATION OF LOW-COST SURVEY TECHNIQUES MORENA, S.; MOLERO ALONSO, B.; BARRERA-VERA, J.A.; BARBA, S. EGE - Revista de Expresión Gráfica en la Edificación, Nº 13, 2020. ISSN: 2605-082X | 113 3.1 COMPARACIÓN DE SOFTWARE Las tablas (Figura 6 y 7) muestran, tanto para COOLPIX L11 como para iPhone 11, los errores de reproyección de las coordenadas x, y, z para cada punto de control utilizado en los dos primeros programas (la posición de los puntos es visible en la figura 3). Como se puede observar, en el caso de la cámara Nikon, el software 3DF Zephyr tiene valores de desviación ligeramente superiores a Metashape, alcanzando un valor medio de error de unos 1,50 cm respecto a los 1,38 cm de Metashape, con valor máximo de 3,0 cm para el punto 3. Más diferencia, en cambio, se observa, en el caso de la cámara del iPhone 11, con el punto 2 que alcanza un error de unos 5,00 cm en el caso del software 3DF Zephyr. Se ha realizado una comparación adicional, como se especificó anteriormente, utilizando CloudCompare, para evaluar las desviaciones existentes entre las superficies del modelo (Barba et al. 2019: 149). La nube de puntos Metashape obtuvo los mejores resultados y se tomó como referencia para comparar con el resto de modelos obtenidos mediante software. En las figuras 8, 9 y 10 es posible visualizar el resultado del análisis anterior. La columna de la derecha de la imagen representa el rango de desviación existente entre puntos pertenecientes a dos modelos diferentes que está representado en falso color. Las desviaciones máximas (en rojo) alcanzan valores iguales a 10 cm y son visibles en la primera comparación solo en una porción del brazo izquierdo. Satisfactorios son también los resultados obtenidos con el software eyescloud3D. En cambio, desviaciones obvias se muestran principalmente en el modelo conseguido con ReCap Photo. Los resultados de la comparación punto a punto de los modelos generados por los diferentes programas utilizados para procesar los datos de la COOLPIX L11 se han mostrado esquemáticamente en los gráficos donde en el eje X están los valores de desviación en m, y en el eje Y, el número de puntos a los que corresponde este valor (Figura 11 y 12). A la vista de lo expuesto, los mejores resultados se obtuvieron con 3DF Zephyr que tiene el valor de desviación estándar más bajo (1,3 cm), sin embargo, también se lograron buenos resultados en el caso de eyescloud3D con valores de desviación estándar cercanos a 1,8 cm, así como la obtención de un modelo completo sin huecos importantes. Menos satisfactorios son los generados por ReCap Photo con una desviación estándar que alcanza un valor de unos 3 cm. El pico de la curva se acerca a 0,15 cm para la comparación con 3DF Zepyr y el análisis del gráfico muestra como el 90% de los puntos tiene una desviación menor a 2 cm. En el caso de eyescloud3D, la mayoría de los puntos tienen valores de desviación de 0,80 cm con alrededor del 70% de desviación menor a 2 cm. Se han realizado más comparaciones con el iPhone 11. Del análisis de la figura 13, se puede ver que la comparación de los modelos Metashape y 3DF Zephyr tiene más desviaciones que la comparación anterior con Nikon. Los valores de aproximadamente 10 cm ocurren principalmente cerca del brazo izquierdo, se confirman por una alineación incorrecta en el modelo 3DF Zephyr. Mejores resultados, en este segundo caso, vinieron de la aplicación eyescloud3D, que permitió generar una nube densa con ligeras desviaciones respecto del modelo de Metashape, alcanzando un valor de desviación estándar de aproximadamente 1,4 cm (Figura 14). Una vez más, la distribución se muestra esquemáticamente en el gráfico visible en la figura 15, con las frecuencias más altas cercanas al valor de 0,15 cm y alrededor del 80% de puntos con desviación menor a 2 cm. Por lo demás, ReCap Photo ha devuelto un modelo con una fuerte desviación respecto del modelo de referencia, no solo en el brazo, sino en el modelo general, alcanzando en algunos puntos diferencias significativas de más de 10 cm (Figura 16). 3.2 COMPARACIÓN DE INSTRUMENTOS Como se mencionó anteriormente, se realizó una comparación adicional entre los resultados obtenidos con Metashape a partir de los datos adquiridos con dos instrumentos diferentes COOLPIX L11 y iPhone 11. El análisis del error de reproyección para las coordenadas de los puntos de control, visible en las tablas anteriores en la figura 6 y 7, muestra un valor superior para los datos procesados con el smartphone, así como una nube de puntos y un modelo de malla, con algunas lagunas. El estudio de la desviación entre las dos superficies se realizó con CloudCompare, operando con el algoritmo C2C. Por lo que podemos ver en la figura 17, los dos modelos tienen una distribución bastante similar. La desviación estándar que se puede extraer de la comparación es igual a un valor de 1,7 cm y el gráfico de distribución alcanza el punto más alto cerca del valor de 0,40 cm. Estos resultados, por tanto, permiten confirmar la similitud existente entre los dos modelos, de hecho, más del 90% de los puntos tienen una distancia menor a 3 cm salvo algunos presentes principalmente en los brazos y en los bordes del monumento. 3.3 VALIDACIÓN DEL MODELO Para probar los resultados obtenidos, se realizó una comparación adicional comparando la nube extraída de Metashape, realizada a partir de los datos adquiridos con la COOLPIX L11, y los obtenidos con el láser FARO Focus3D X330. La elección recayó en el modelo Nikon por arrojar los mejores resultados, tanto métricos como geométricos, presentando una nube completa sin lagunas significativas. El análisis se ha vuelto a realizar con el algoritmo C2C de CloudCompare. Los resultados perseguidos se pueden ver en la figura 18. Las desviaciones superficiales con valores cercanos EGE - Revista de Expresión Gráfica en la Edificación, Nº 13, 2020. ISSN: 2605-082X | 114 DOCUMENTACIÓN GRÁFICA DEL MONUMENTO A LA TOLERANCIA. VALIDACIÓN DE TÉCNICAS DE LEVANTAMIENTO DE BAJO COSTE a los 10 cm son casi nulas salvo en algunos puntos cercanos a los bordes, principalmente debido al ruido de las nubes de puntos. La desviación estándar que se desprende de esta comparación alcanza valores cercanos a 1,3 cm, teniendo el 90% de los puntos un valor inferior a 3 cm. El gráfico muestra la mayor intensidad de puntos con una desviación de aproximadamente 0,35 cm. 4. CONCLUSIÓN Antes de proceder con las conclusiones, es necesario resaltar y precisar las diferencias más importantes entre las tecnologías utilizadas. Primero, las diferentes herramientas en la fase de adquisición de datos. En el caso de los levantamientos fotogramétricos, optamos por dos instrumentos económicos, fáciles de usar y ampliamente disponibles en el mercado. Estas características permiten a los operadores no especialistas acceder a equipos que ofrecen resultados satisfactorios para las actividades fotogramétricas, sin apenas inversión, obviamente, considerando los objetivos de cada proyecto. El escáner láser, en cambio, es una herramienta tecnológica y especializada con un precio muy superior a los anteriores. Puede adquirir densas nubes de puntos (976.000 puntos por segundo, según especificaciones de FARO) que dificultan su manipulación, interpretación y gestión sin un equipo y un software profesional. Otra aclaración a considerar es la característica de la escultura, que presenta una textura homogénea y una forma geométrica simple. Tales peculiaridades, de hecho, dificultan que el software fotogramétrico alinee y oriente las imágenes, más aún si la toma de datos está realizada por un usuario sin experiencia y sin la colocación de dianas. Finalmente, en el análisis realizado decidimos implementar dos programas que operan en la nube donde la gestión de parámetros es casi nula y, consecuentemente, manejables por un público cada vez más amplio. Analizando los datos obtenidos es posible ver cómo, con la llegada de numerosas tecnologías comerciales de bajo costo, la comparación de algunos modelos (Metashape obtenido con fotos Nikon vs. Scene FARO) ha generado diferencias satisfactorias en la nube de puntos. Lo mismo puede decirse de la nube de puntos generada por eyescloud3D para los datos del iPhone 11. Los dos modelos, no solo se ven completos y sin espacios, sino que las desviaciones de las nubes de puntos respecto a las de referencia son aceptables. En el caso del resto de modelos, se puede destacar la presencia de errores mayores, pero aún aceptables como datos a utilizar para la generación de modelos para visualización vía web o para impresiones 3D (Figura 19), sobre todo dada su mayor ligereza y manejabilidad. Tras los grandes avances conseguidos en el campo fotogramétrico, ha quedado probado que es posible conseguir resultados cada vez más satisfactorios, compensando a menudo los déficits instrumentales o la inexperiencia de los operadores. A nivel general, es posible apreciar un alto nivel de desfase entre los modelos en las áreas de los brazos del monumento debido ciertamente a la ausencia de puntos de control con estación total en el levantamiento fotogramétrico realizado. Por tanto, es un aspecto de fácil solución. Sin embargo, esta decisión se ha tomado en función del objetivo inicial de comparar herramientas de amplia difusión baratas y fáciles de usar por personas no expertas. El modelo tridimensional del escáner láser, como se esperaba, proporciona información métrica precisa y un alto grado de detalle de las superficies representadas. Este modelo se puede conservar como una verdadera copia digital del monumento real. En definitiva, será una valiosa base de datos desde la que analizar o realizar más estudios en gabinete. Sin embargo, de lo expuesto anteriormente se puede inferir cómo el potencial de estas tecnologías, aunque sean de bajo costo, muestra un presente y un futuro prometedor para la conservación de la cultura y el patrimonio. En los últimos años, el potencial de la fotogrametría unido a los avances en visualización computacional SfM y a la accesibilidad de la tecnología para el gran público, nos brindan la oportunidad de generar resultados satisfactorios en esta área, siempre en consonancia con los objetivos perseguidos, teniendo en cuenta que sin uso de puntos de control, equipos y procedimientos profesionales los resultados no son aptos para fines métricos o científicos. Como se sabe la fase más compleja de la fotogrametría SfM es la toma de las fotografías, sin embargo, en el trabajo se quiso expresamente evaluar fotografías tomadas por no profesionales y así constatar las presencias de huecos, aberraciones, etc. Así mismo, para aprovechar la posible economía y sencillez de la fotogrametría SfM, se trabajó con softwares de consumo (un desarrollo posible sería utilizar otros softwares libres también) y cámaras con objetivos de distintas calidades. En este sentido, hemos de hacer algunas consideraciones al respecto: por ejemplo, las desviaciones que se muestran en el artículo – muchas no admisibles – se refieren a un elemento arquitectónico pequeño y no permiten generalizar los resultados en el caso de una gran construcción (donde, por otro lado, el levantamiento debería llevarse a cabo sólo por profesionales). Como conclusión, se observa que el uso por no profesionales puede traer consigo resultados de baja calidad que, aunque puedan ser usados para el consumo, nada a que ver con los levantamientos profesionales, científicos, donde el operador maneja muchos más factores, todos ellos complejos, pero que garantizan la calidad, rigor y fidelidad de los modelos obtenidos. Todo esto nos obliga a enfatizar el aporte de ingenieros y/o arquitectos, también con recursos automatizados. Para finalizar podemos argumentar que hacemos el levantamiento no con la fotogrametría SfM, que es un instrumento, sino con la cabeza, con el cerebro, con la mente.