9.6 The effect of the applied elevation model

In most cases, we have a terrain model for the surveyed region, which defines the terrain elevation with more or less accuracy. However, as it was discussed, the aerial photographs often show not the soil/terrain itself, but the top of the covering vegetation (field crops) or roofs of the buildings. If we omit this fact, e.g. because of missing data of the building heights, the fit of the image will be good at the terrain level. The top of the buildings will be shifted by several meters from the vertical axis from the camera (Fig. 61).

Google Earth image of part of Rome with overlapping cadastral map

Fig. 61. If the elevation model does not contain the building heights, the fit is valid at the terrain level only.

In case of accurate models, showing also the height settings of the buildings, all points of the resulted images will be in correct horizontal position. We will have data absences at the occultation pixels (e.g. the ones covered by buildings, higher towers). This is not an error but a consequence of the survey geometry: indeed, we don’t have any information about the covered terrain parts in the photo.