8.4 The effect of the built environment and the vegetation: elevation models

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:

Color and shaded part of a high-resolution lidar-based elevation model

Fig. 52. Artificial objects (roads, railroads, dykes) in a Hungarian flatland, shown in a lidar-based elevation model (Zlinszky et al., 2012).

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.