Figure 4: Dependency of the accuracy from the ground resolution
(Ground sampling distance) of the original images for various
datasets without using GCPs for the reconstruction (“geotags
only“).
In order to assess the quality and accuracy of this automated process,
we consider here several projects that differ with respect to
the coverage area, ground resolution, overlap between original
images and the number of images. For all datasets we measured
GCPs, which we then used to evaluate the precision of the automated
reconstruction. Thereby we evaluated two different methods.
One is purely based on the geotags of each original image
as provided by the UAV autopilot and one which in addition also
takes manually designated GCPs into account. For both we measure
the accuracy of the result as the mean distance between the
triangulated GCPs xj , as optimized by Eq. 1 and the GCP positions
Xj as obtained by a high-precision GPS receiver on the
ground. Let pij be the position of GCP j in image i. The collection
of all pij for a given GCP j give rise to a 3D point xj that
minimize the projection error: