Orthophoto maps:

Orthophotos are perhaps the most significant results of digital photogrammetry. This sub-branch of photography does not include only the visual information but provides real survey images. The image (in analogous or digital form) may be used for measuring of the position and dimensions of objects.

Orthophotos become the material for vector and raster maps; they are included in state map work. The work with the final product does not require special SW or HW, a standard PC is sufficient. In a CAD program under any project the clearest accessible tools may be used. Vector interpreting of the elements of earth surface is complemented by their actual visualisation, the individual elements may be seen, we determine their position, and we measure their lengths or areas.

Besides the elements of interior and exterior orientation, the formation of orthophotos from aerial images requires a digital terrain model (DTM). The model is necessary because an orthophoto must be, as well as a map, a rectangular projection of terrain into a plane. However, the image is formed by means of an optical device; therefore it is a central projection, i.e., the position of each point in the image depends on its altitude. When creating orthophotos the position of each point of the image must therefore be corrected on the basis of its altitude. The quality of altitude data, i.e., the quality of DTM therefore significantly affects the positional precision of the resulting orthophoto.

A DTM in the form of an irregular triangular network

As we generate one orthophoto from each image (via the ORTHOMASTER from INPHO) we acquire a block in which the individual orthophotos overlap.

The resulting sheet of the final mosaic does not correspond to distribution of images and there are also radiometric (colour) deviations which must be removed to acquire a homogeneous, seamless mosaic. The formation of these anomalies is caused by the imperfection of photogrammetry cameras (objectives), image material, variable light in the course of the flight, time delays between taking images (especially in the case of larger projects) , etc.

Colour modification of orthophotos

Modified orthophotos prepared for aligning and cutting according to map sheets

Radiometry must be aligned in the image plane, in a block and then in the whole project. In situations where flights are performed in stages with greater time delays, the colour differences may cause problems. Formation of radiometrically homogeneous surfaces requires greater effort and it is usually not enough to just use automatic functions. Radiometric stabilisation and formation of mosaics is performed by means of the ORTHOVISTA from INPHO.

Resulting seamless mosaic

Positional accuracy of orthophotos should not be worse than 1.5 times the size of a pixel. In the case of medium – the most frequently used – scales of surveying we thus achieve the positional accuracy of terrain measuring achieved by geodetic methods. However, in most cases it is better to use vector plotting in stereo-models and to use the orthogonalised image only to complement the information. Measuring in model pairs is more accurate than measuring at the resulting orthophoto whose precision depends on the quality of the model used for orthogonalisation.

In general, the following chart may be applied.

Image scale
Final map scale
Attainable accuracy
Resolution of orthophoto
20 000
5 000
50 cm
20 cm
10 000
5 000
25 cm
20 cm
8 000
2 000
20 cm
10 cm
4 000
1 000
15 cm
10 cm
2 500
500
10 cm
10 cm

3D modelling

Automatic aerotriangulation

Orthophoto maps

Specific mapping

Surveying of facades

Contact

Adress:

SFP-carto
Radobyčická 10
301 00 Pilsen
tel.: +420 377 970 901
info@sfp-carto.cz

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SFP - Aerial & terrestrial photogrammetry, © 2007, výroba: ERB Production