True Ortho
The Ortho photo, but truly vertical: no building lean guaranteed.
The True Ortho photo is recorded with a high forward and sideways overlap. This extra recorded data is then processed by specific camera software in such a way that an end product is created with a 0% building lean. In short, this works as follows: if a building is placed upright, a gap is created behind the building, where, if it is done with 1 photo, no data is available. After all, this information was hidden behind the building. This is a large gap for a high building, but filling is also necessary for small buildings. That is the reason that we collect with a higher overlap; this allows the software to use the information from adjacent photos to fill in everything. This only works if the aerotriangulation is very accurate.
An accurate fit is made possible by an intensive aerotrangulation combined with fitting points and INS and GPS data. The result consists of a high absolute and relative accuracy, which allows the aerial photo to be used with high precision for many purposes and processes. The accuracy is at least 2 pixels. At a 5 cm resolution this means a maximum inaccuracy of 10cm.
For the reasons mentioned above, the True Ortho ECW-product is suitable for and complies with the demands for mapping. Mapping from an ECW is now very achievable: with no building lean in the ECW, there is an optimal overview for processes such as mapping.
With the True Ortho photo, a height model is generated from the aerial photographs themselves, moreover with a higher density than the AHN height model. This so-called “dense point cloud” is then used to correct the Ortho photo. In this way, not only is correction made at ground level, but also objects above ground level such as buildings are corrected so that every pixel is in the correct location. Vertically, the gutter of a building is perfectly straight above the foot of the building and that is also what will be the case in the True Ortho photo. The lean in the ECW products is 0%.
An additional advantage of using the height model generated from the photos is that it matches the exact moment of the recording. With the AHN it is possible that there have been changes in the landscape that are on the new aerial photo but not in the AHN because it is several years old. Think for example of changes to infrastructure or a new housing estate etc. Wherever heights have been adjusted after the acquisition date of the AHN, that file is not correct and if the AHN is used for correction then the Orthophoto will also not be correct. This is where the difference between the Orthophoto and Stereophoto is important, for mapping the Stereophotos are of course used and no height model has been applied to them and the mapping is therefore independent. The mapper also makes a height correction from the photos themselves before the mapping work is started.
Thee True Ortho photo, just like the stadard Ortho photo, is delivered as Stereo (.jpg), Geotiff (.tiff) and ECW (.ecw). wordt net als de standaard Orthofoto geleverd als Stereofoto (.jpg) Geotiff (.tiff) en ECW (.ecw). Here too, various variations are possible, such as supplying .jp2000 instead of .ecw and supplying Near InfraRed (NIR) images in various outputs.
Slagboom en Peeters makes aerial photographs for various clients and applications. Are you interested in a True Ortho photo? We are happy to inform you about the possibilities. Feel free to contact us; we are ready to answer your questions.