Helsinki University of Technology
Department of Surveying
A method for measuring 3D objects, and checking and updating previously measured data by using digital aerial images is presented. The method is called monocular stereo measurement, which means that 3D objects are measured from two aerial images without physical stereo vision. In this method there is no need for transforming images. The orientations for the images must be known.
In this method, geometrical models of objects are used to help with the measurements. These object models can be moved, rotated and scaled in the 3D coordinate system. When the results of these operations are projected onto images, errors are easy to detect visually. Nadir lines can be used to help in adjusting models and detecting errors. Nadir lines reveal whether the error is only in the Z-coordinate or whether all the coordinates are incorrect. It is advantageous to use object models only when the 3D object is not a single point.
A program was made to test monocular stereo measurements. There were two aerial images for testing purposes, in the scale 1:16 000. The flying height was 3500 m and the camera constant was 210 mm. The analogous images were scanned at 15 micrometer accuracy. The accuracy of approximately 0.25 m for XY-plane and 0.5 m for Z-direction was achieved from the test data.
The method is proper for checking, updating and mapping, for example, large-scale maps and basic maps. To make this method suitable for practical working, the interface and the functions of the program need further investigations.
Keywords: Digital aerial images, object model, nadir line
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