What is TrueOrtho?
The new technology of the digital processing of aerial photography developed by the GEODIS GROUP, together with the use of complex digital technology for acquiring aerial images and their processing is a guarantee of the significant improvement of the geometric quality of this kind of products. A relatively new product, which is being introduced on the market by our company, is TrueOrtho.
The current trends in digital aerial photography allows for the use of new technology for processing images to create colour orthophoto-maps. It increases processing accuracy and the resolution of the new products is much higher than it was a few years ago.
The use of orthophotomaps for the purposes of GIS applications is now completely standard, and orthophotomaps are also often used to determine how up-to-date a variety of map data is, including cadastral maps.
Solving the perspective problem
The problem in using aerial orthophotomaps in connection with their application in GIS is that high objects in the terrain are not displayed correctly. With these buildings the perspective projection means that the bottom edge is displayed correctly, while the upper edge of the building is projected in perspective into the plane of projection, which is shifted away from the centre of projection.
The perspective distortion of buildings is a serious obstacle in using orthophotos in precise GIS systems, particularly in town management.
The perspective distortion of buildings is a serious obstacle in using orthophotos in precise GIS systems, particularly in town management. Correcting perspective distortion involves creating a surface model of all terrain objects, including vegetation, that is as precise as possible. If we know this model or measure it using other technology, it is possible to correct the perspective distortion of buildings and convert it to the correct orthogonal form. After the surface model has been computed, it is then possible to determine the “concealed” areas caused by the perspective shadow of the buildings.
In more detail on the perspective distortion
Picture no. 1 Perspective distortion of the upper edges of the Hilton hotel in Prague in a selected photograph.
Picture no. 2 Perspective shadow of buildings in one of the shots (yellow areas).
If we compare pictures 1 and 2, it is clear that the top edges of the buildings have shifted to the correct perspective position when a digital surface model is applied. The areas of perspective shadow are marked in yellow in the picture. One must remember that on each image which has a different centre of projection, such as when photographing blocks, completely different areas are concealed.
The analytical task of resolving this problem involves seeking all the perspectively concealed points on the photographs.
The situation of creating perspective shadows can be easily shown with the help of pictures no. 3 and 4.
Picture 3 shows a situation where a building or part of the terrain is only caught in one image, while in picture 4 the building is in the same situation, but is photographed from two points.
Obviously, the photographs can be combined so as to select and complete any missing shaded areas.
From the left of the plane as it flies towards the building in picture 4, it is possible to fill in the concealed areas caused by the lens as it shoots from the plane as it flies to the right away from the building.
From the left of the plane as it flies towards the building in picture 4, it is possible to fill in the concealed areas caused by the lens as it shoots from the plane as it flies to the right away from the building.
This data can be used without the risk of error to perform a variety of analyses, measurements, and similarly to automatically detect changes, while the results are not dependent on the direction the shots are taken from.
This means that repeated shooting gives the same positional results, regardless of which actual centre the parent aerial shot was taken from.
Picture no. 5 TrueOrthophoto register with a vector design.
A digital or analogue register of a TrueOrtho map with a cadastral or other vector design is a logical means of checking the procedure, as can be seen in picture 5.
