As it was earlier discussed, some technologies of the elevation model creation cannot discriminate – or only with serious post-processing – the height of the soil, the vegetation and the buildings. And, as it is shown in the next chapter, for the ortho-rectification of the aerial photographs, these pieces of information are also needed to handle the effects of the partially oblique-photographed buildings. Therefore, besides the terrain models, showing the elevation of the terrain itself, elevation models that represent the real photographed surfaces, are also needed. Their construction can be made in two ways:
The terrain model can be over-written by the elevation of the estimated, modeled height of the vegetation and the buildings. The built objects can be modeled by some three-dimensional prism or a combination of prisms. The vegetation effect is represented by an added constant elevation, characteristic for the plant species (forest trees, agriculture crops). This method is somewhat similar to the ’railroad model’ toys: we add the extra elevation of the objects to the already existing terrain model.
The elevation model can be directly computed from laser scanned (lidar) data. The active reflecting surface can be any solid object (building roof or walls, foliage of forests). Using post-processing algorithms, the elevation model can be provided from the original three-dimensional point set that is the result of the laser scanning (Fig. 52).
It should be mentioned again here, that the above discussed SRTM elevation model contains height elements referring to the vegetation and the built environment. However, in this dataset, the systematic difference of the model height and the terrain height refers only to the extents of the towns and forests, and this vertical difference is far from the real surplus. Thus, the SRTM cannot be used as a certified elevation model.
In the practice of the geo-reference, the elevation models are raster-based datasets. This always causes some model errors, whose order of magnitude is depending on the horizontal resolution. The raster model cannot correctly describe the vertical walls and forest-boundaries in three dimensions. However, this ambiguity causes only subpixel registration errors at geo-reference of aerial photos and ultrahigh resolution satellite images. This small error is much more insignificant than the one occurs when no elevation model is used.