applied to images with a configuration similar to an aerial block. In some cases, the DSM can be
approximated by a plane.
2.2.3 LSM refinement. As shown in Remondino 2006,
image coordinates of homologous points extracted using feature-based matching FBM methods can be refined with
area-based matching ABM approaches, in particular with the Least Squares matching LSM - Grün, 1985. This ensures a
sub-pixel accuracy of the locations and therefore a higher accuracy of the bundle solution.
2.2.4
Corner detection. It also noteworthy that a FBM
matching with or without LSM normally provides results worse than a traditional manual orientation with interactive
measurements. This is mainly due to the tie point redundancy and distribution. In fact, with manual measurements the same
point can be easily identified in several convergent images, while a feature-based operator has less repeatability. Moreover,
the larger is the number of images in which the same point appears, the better is the precision of the global adjustment.
According to this consideration, the FAST interest operator Rosten and Drummond, 2006 is included in the pipeline to
assure a large number of corners under a higher repeatability and also with a better distribution in the images. The corners are
automatically extracted in the images, but the user has fixed the operator threshold by visually checking at least one image in
order to verify the point distribution. Using the exterior orientation parameters computed with a precedent FBM
approach, homologues rays are compared in order to find corresponding points in the object space. It is also possible to
specify the minimum number of images in which a point to be used during the orientation step must appear a good choice is
4. Then, image point locations are improved via LSM by fixing the position of a feature template and searching for the
remaining points slaves. 2.2.5
Image enhancement. In some cases the radiometric
content of the images is not sufficient to achieve a good number of well distributed image correspondences extracted with the
FAST operator. To overcome this drawback a pre-processing procedure can be applied in order to stretch the radiometric
information of the images by locally forcing the grey value mean and contrast dynamic range to fit certain target values
Wallis, 1976.
3. EXAMPLES
