## 3 Full Description of the Approach
### Project Framework
#### 3.1.2 Input Data  
The developed approach considers as input data acquired by laser scanners namely registered point clouds. Our project focuses on how to process these data even if it must be noticed that the acquisition and the post-processing steps have obviously an impact on the quality of the data and consequently on the quality of models created from point clouds. It is essential to adopt the best possible acquisition protocol to avoid troubles during the modelling step. In order to ensure the quality of input data, the following recommendations have to be followed by the operator:  
* **Scan stations:*
* optimize the placement of scan stations in order to acquire the whole environment without causing redundant data by multiplying scan stations
* plan a consequent overlapping between point clouds in case of indirect georeferencing
* if possible perform loops to improve the quality of the registration of point clouds (global adjustment) and to limit the risks of deviation  
* use at least 4 targets in common between two scans: 3 targets are sufficient to register point clouds issued from 2 successive scans but it is recommended to take more than 3 and to compensate the whole network of targets at once
* prioritize targets placed in more than 2 scans. It should be ensured that targets are well-distributed in the space, that is to say placed at various ranges from the scanner, not in the same plane or along one line and located at different heights
* the use of natural targets such as edges, planes and cylinders can reduce and simplify data collection process; indeed, it can be difficult and time consuming to place targets throughout all the building
* **Point spacing:*
* consider upstream the desired point density according to project specifications
* adapt point spacing depending on space configuration and the positioning of targets in relation to the laser scanner
* **Georeferencing after registration:*
* georeferencing can be either direct when positions and orientations of the successive scanner stations are known within a geodetic network, or indirect when the georeferencing is performed after the registration
* in case of indirect georeferencing, the coordinates of targets or characteristic points well distributed in the scene are measured and are used to compute the transformation from the local system to the global system
* georeferencing can improve the quality of the registration notably for a linear project  
Considering the developed approach, several requirements have to be taken into account for the acquisition with the laser scanner. A minimum of one point per centimetre is required by the approach. Moreover, a scan has to be performed whenever the ceiling height changes. It shall also be ensured that the ceilings are not occluded at the scanner point of views since the approach largely exploits slices of point clouds at their levels.  
The data considered as input of the algorithms are supposed to be correctly registered indoor point clouds. In order to save time during the process and to gain more insight into the number of points of extracted elements, point clouds are spatially resampled at 1 cm. Point clouds are also rotated to follow the orientation of one of the major facade plane obtained using a Principal Component Analysis (PCA) to accelerate processing time. Finally, a global shift to a local coordinate system is temporarily performed so that the original precision is kept.