Brief exposé of what is hidden under the concept of photogrammetry.
The creation of an orthophotomap starts with the planning of the aerial photography of a given area. For this process it is necessary to know the resulting scale of the orthophotomap (so-called ground element), flight heigth, respectively the scale of the photography and the lateral and longitutudinal overlap of the images. Simultaneously surveying works in the field take place, like the process of stabilisation, signalization and the measurement of natural or artificial photogrammetric points.
It is necessary to have good weather for aerial imaging, that is why the flight crew observes meteorological data and the weather development in any given area.
The photography is performed by camera on film or digital camera.
In the case of analog photography, the result of the survey flight are films that need further processing in photolabs. GEODIS BRNO has a modern lab equipped for these purposes.
The developed films are then scanned on very precise calibrated photogrammetric scanners with an internal accuracy of up to 2 µm. Three coloured scanners PhotoScan TD by Intergraph Z/I Imaging are being used in GEODIS BRNO and a fourth UltraScan 5000 by Vexcel.
The creation of an orthophotomap needs the locating of project centers of the images with the aid of analytic aerotriangulation (AAT). Planimetry and altimetry evaluation, edge measuring and DTM creation (digital terrain model) can be performed, if the customer requires it.
If the photography is performed by digital camera, the process of film development and the scanning of the images is eliminated.
Thus we go directly over to measuring and calculating the analytic aerotriangulation, i.e. to determine the spatial coordinates of the centres and to the calculation of the hypsography, planimetry, angle measurement and the creation of DTM.
On the basis of the calculated parameters of the spatial coordinates of the image centers and the DTM, the aerial images become orthogonalized (=transfer from central projection into orthogonal projection).
Out of these orthophotographs the so-called orthophoto-mosaic is created, where the images have to be colour-processed (radiometric and photometric – so called dodging) before being classified into the mosaic, so that the resulting product is colour-balanced.
In the upcoming phase the so-called seamlines are defined. They define the lines along which the various orthorectified images will be connected into the form of a mosaic.
The result of the processing are thus map sheets of individual orthophotomaps, usually in the form of big format maps.
The resulting seamless orthophotomap is handed over to the customers, in most cases in digital form in formats TIF, JPG or MrSID. The orthophotomap with the requested resolution and precise location is the basic information source for GIS systems.
