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Co-ordinate reference systems
Published in Martin Vermeer, Antti Rasila, Map of the World, 2019
In Europe today, the system ETRS89 is in general use, even as a reference system standard accepted at the EU level. The system was initiated by the Reference Frame Subcommission for Europe (EUREF subcommission) of the IAG. This is also the origin of the name “EUREF system.”
Pre-modelling as a tool for optimizing morphodynamical numerical simulations
Published in International Journal of River Basin Management, 2020
In accordance with the criteria previously established, the selection of the best pair of hypothesis and scenarios for pre-modelling was based on the changes in bed level introduced by the pre-modelling itself. The changes introduced by each pair of hypothesis j and scenario i (respectively numbered as j-i) in the bed level were analysed using maps of the morphodynamical evolution of the channel. The maps representing dH along the study segment are presented in Figure 8. All of these numerical simulations were performed in the standard PT-TM06/ETRS89 coordinate system and are presented as such.
Combination of nadiral and oblique UAV photogrammetry and HBIM for the virtual reconstruction of cultural heritage. Case study of Cortijo del Fraile in Níjar, Almería (Spain)
Published in Building Research & Information, 2020
Patricio Martínez-Carricondo, Fernando Carvajal-Ramírez, Lourdes Yero-Paneque, Francisco Agüera-Vega
The photogrammetric process was carried out using the software package Agisoft PhotoScan Professional © version 1.2.4.2399. This kind of photogrammetric software based on the SfM algorithm was used because it has been proven to outperform other software applications in terms of accuracy (Sona, Pinto, Pagliari, Passoni, & Gini, 2014). The workflow is a three-step process (Verhoeven, 2011). The first step is the alignment of the images by feature identification and feature matching. While carrying out the image alignment, this software estimates both the internal and external camera orientation parameters, including nonlinear radial distortion. Only an approximate value of the focal length is required, which is extracted automatically from the EXIF metadata. This task was carried out with the PhotoScan accuracy set to high. The results of this step are the camera position corresponding to each picture, the internal calibration parameters, and the 3D coordinates of a sparse point cloud of the terrain. In the second step, the sparse point cloud is referenced to an absolute coordinate system (ETRS89 and frames in the UTM, in the case of this study) and densification of the point cloud is achieved with the quality set to medium, which is based on a pairwise depth map computation. This point cloud needs to be ‘cleaned up’ to eliminate all of the wild points that do not belong to the model. This process is performed manually. This resulted in a more detailed 3D model. Using the height field method, the mesh is obtained from the dense point cloud. The third step applies a texture to the mesh obtained in the previous step. Finally, the orthophoto is exported and a grid DSM can be generated from the point cloud. The dense point cloud can also be exported in *.las format, as well as the mesh in *.obj, *.3ds or *.dxf format. The bundle adjustment can be carried out using at least three GCPs, but more accurate results are obtained if more GCPs are used, and it is recommended that more of them be used to obtain optimal accuracy (Agüera-Vega, Carvajal-Ramírez, & Martínez-Carricondo, 2017; Rosnell & Honkavaara, 2012). In this study, four targets placed in the field around the building were used as GCPs for the georeferencing of the project. The rest of the targets not used in the block adjustment were used as CPs to evaluate the accuracy of the photogrammetric project, according to the formulation of the root mean square error, described in (Agüera-Vega, Carvajal-Ramírez, & Martínez-Carricondo, 2016). The process was completed with about 15 h of cabinet work by a technician with high knowledge of UAV-SfM photogrammetry.