2.2.2. Changes in Relation to Indoor Space Subdivisions

The process of detection of permanent changes is continued by linking detected changes to the volumetric space or space subdivisions. Space subdivisions represent the semantic space in an indoor environment, such as offices, corridors, parking areas, staircases, and so forth. Each space subdivision is connected to space in a spatial unit in the 3D Cadastre model and all laser points in the space subdivisions carry the attributes of the corresponding Cadastre administration. In this step, we explain how these space subdivisions were extracted from the point clouds and linked to the previously detected changes. Note that an apartment may consist of one or more spatial units, a spatial unit may consist of one or more spaces, and a spatial unit may have invisible boundaries and needs to be checked by a Cadastre expert.

Following the method in [23], after the extraction of the permanent structures in each epoch, a voxel grid was reconstructed from the point clouds, including walls, floors, and ceilings. Using a 3D morphology operation on the voxel grid, space was then subdivided into rooms and corridors. Each space subdivision was represented with the center of voxels as a point cloud segment. To find out which changes occured in which space subdivisions, we intersected the space subdivisions of each epoch with the permanent changes detected earlier (see Figure 9). For example, in Figure 8, we can see that in the second epoch (Figure 8b) a wall was removed and two spaces were merged. Since this wall was detected as a change during the previous step (Figure 8c) by the intersection of changed objects with subdivisions, the changes in the two epochs were extracted (Figure 9). These changes were linked to a space subdivision, and each space subdivision or a group of them (e.g., a building level) may represent a spatial unit in the 3D Cadastre model.

**Figure 9.** The space subdivisions of PC2 (second epoch) after the change. The purple wall in the right image shows the intersection of a detected change with a space subdivision.
