- freely available
2014, 6(6), 5671-5691; https://doi.org/10.3390/rs6065671
2.1. Data Collection Protocol
- Circular or “ring” network: used to obtain a 3D model of the whole building, which will then allow establishing the sections necessary to sketch the ground plan. The images axis must converge at the center of the object, and the minimum overlap between adjacent images must be about 80%. Moreover, the number of images in the corners should be higher, so that the user can “tie” the different façades of the object. The network of the shooting process should maintain an appropriate proportion between the base (distance between shots) and the distance to the object. As a general rule, in order to ensure image correspondence in the orientation process, the distance between two adjacent camera stations must be so that the camera axis forms an angle of intersection of approximately 15° with the object. The number of images necessary to obtain all of the measurements depends on the size, shape, morphology and location of the object (in relation to adjacent buildings) and the focal length (Figure 2, left).
- Planar or mosaic network: particularly recommended to document the façade of a building. It requires taking some frontal images of the façade, with an overlap higher than 80% between adjacent shots (Figure 2, center).
- Independent basic network: When documenting a small and accessible façade or any isolated architectonic element, the shooting network may consist of five images forming a cross with an overlap between adjacent shots higher than 90%. The main shot is a frontal image of the façade, which is then combined with four more images of the left, right, upper and lower part of the main shot, to conform a global, slight converging perspective (Figure 2, right).
2.2. Extraction and Matching of Features
2.3. Hierarchical Orientation of Images
2.4. Dense Model Generation
3. Results and Discussion
3.3. Laboratory Work
3.4. Typological Analysis
- The graphic quality of the models is supported basically by the point quantity, between 20 and 50 million points, which usually equals or surpasses the resolution provided by terrestrial laser systems (TLS).
- Even though the data volume may seem high, the working times must be taken into account: between 30 and 60 min of fieldwork and from 6 to 16 h of laboratory work, which, once again, can be compared with laser scanner performance. The image capture times are lower than those of a laser scanner (between 1/2 and 1/3 depending on the TLS performance), whereas the laboratory work times are similar. Therefore, compared to laser scanner technology in terms of devices availability and the difficulty of processes, photogrammetry stands out as a more advantageous solution.
- The high degree of accuracy (root mean square deviation of block adjustment; the results range from 1/4 to 1/3 of a pixel) is mainly due to the high level of redundancies (high number of tie points), which allows the user to adjust the images accurately
- The high level of redundancies (tie points), between 80,000 and 650,000 with an average of about 300,000, is due to the high number of images, between 28 and 177 with an average of about 90. Although the number of images may seem excessive, this issue must be contrasted to the time spent in the process.
- The results are highly consistent with each other: RMS are always between 0.18 and 0.33 pixels or between 0.0011 and 0.0037 mm, which guarantee the quality and reliability of the methodology chosen for the study.
- The different number of images needed for each church, which ranges from 28 to 177, as stated before, depends on the surroundings of the religious building, as well as on the possibility of fitting the whole object into the image. The size of the building is also important, although all the different aspects do not result in relevant differences, either in time or accuracy.
- With regard to the typological analysis, the approach developed improves the current techniques for the recording of architectural cultural heritage. Moreover, the method is suitable to carry out a typological classification study, where the reduction of image capture times (between 1/2 and 1/3, as stated above) allows the researcher to document a great number of buildings/architectural heritage in little time and by non-specialist staff, due to the simplicity of the process. Besides, the high resolution of the models, with GSDs between 0.006 and 0.008 mm, indicates that the method could be used for projects that require larger scales.
- These methods are appealing to architects due to their simplicity and speed. Better quality and more robust surveys are obtained, as not only the shape of the building was accessed, but also the information about its color and texture. The fieldwork hours are reduced with no negative effect on accuracy, and it is easier to systematize the process, by following the protocol set out in the present study.
Conflicts of Interest
- Author ContributionsAll authors contributed extensively to the work presented in this paper.
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|ALCAÑICES||Alcañices||FONFRIA||Fonfría||RABANO||Rábano de Aliste|
|Santa Ana||Bermillo de Alba||Sejas de Aliste|
|FIGUERUELA||Figueruela de Arriba||Castro de Alcañices||TRABAZOS||Trabazos|
|Figueruela de Abajo||Ceadea||Latedo|
|Gallegos del Campo||Fornillos||Nuez de Aliste|
|Moldones||Moveros||San Martín del Pedroso|
|Riomanzanas||Salto de Castro||Villarino tras la Sierra|
|Villarino de Manzanas||VINAS||Viñas de Aliste|
|Vega de Nuez|
|Fornillos||Vivinera||Rabano||Tola||Trabazos||Sejas||Santa Ana||Ribas||Ceadea||S. Martin Pedroso|
|Measurements (m)||25 × 16 × 20||18 × 16 × 12||21 × 13 × 12||22 × 16 × 14||27 × 16 × 23||32 × 17 × 22||20 × 15 × 10||23 × 16 × 11||19 × 10 × 11||22 × 15 × 13|
|No. of images||86||72||127||152||177||78||38||91||28||79|
|Distance max/min (m)||22.40/10.30||16.20/4.50||30.80/3.50||25.30/2.90||25.30/2.60||16.50/2.80||15.10/4.50||19.30/9.70||30.15/5.20||31.10/5.70|
|No. of 3D points||22,319,921||20,913,054||39,624,832||53,447,764||44,416,441||25,141,761||11,138,274||17,894,546||4,854,354||17,302,829|
|Laboratory work (hours)||14||10||15||15||17||16||7||12||6||12|
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