Underwater photogrammetry 
28 
Hydrographische Nachrichten 
Fig. 7: Fused point clouds by photogrammetric analysis (RGB) and MBES soundings (coloured points) in side view (left) and isometric view (right). 
The dense point cloud filled areas lacking MBES soundings due to occlusion, such as on the bottom of the wreck. Colour scale depicts depth in metres 
code to the ground control points. After coarse 
alignment, fine registration was performed using 
the Iterative Closest Point (ICP) algorithm (Besl and 
McKay 1992). The result Is a georeferenced point 
cloud In ETRS89/UTM coordinates, which can be 
directly Integrated In the following data process 
ing chain of BSH hydrographic surveying. The hy 
brid origin of the data Includes furthermore RGB 
colour values and MBES backscatter values that 
can be attributed to the points. Additionally, the 
photogrammetric Imagery can be transformed 
to the relevant terrestrial reference system and 
stored, as well, providing a georeferenced Image 
database for further Information on the wreck. 
3 Results 
Fig.,.7 shows an overlayed photogrammetric col 
oured point cloud with the MBES data. The data 
depicts the starboard side of the wreck's bow. The 
area covered by the photogrammetric point cloud 
Is about 13 m x 2 m, which Is approximately half 
of the wreck's length and almost the entire height 
above the seafloor on the starboard side. 
It Is clearly visible that the photogrammetric 
point cloud has a significantly higher point den 
sity and provides colour Information. Furthermore, 
occluded areas near the bottom of the wreck can 
be observed with the photogrammetric data and 
thus provides completion of the data In these ar 
eas. This Is observable In Fi_g._7 (left), where areas 
of the wreck's bottom are filled with points In ar 
eas that were occluded to the MBES and thus not 
covered entirely. For accuracy evaluation, cloud- 
to-cloud distances were calculated, using least 
sguares planes on the MBES data (Low 2004). Dis 
tances from the photogrammetric point cloud to 
local planes of the MBES data are calculated and 
shown In Fig_._8.The mean cloud-to-cloud distance 
Is 0.053 m. Though not fully representative of the 
absolute accuracy achieved by this method, these 
numbers representa measure of Internal accuracy 
for the combination of both data sets. Apart from 
that, higher deviations occur In the lower part of 
the point cloud In areas that are not as densely 
covered as the rest of the point cloud. 
Comparing point densities, the MBES point 
cloud was reduced for the subset with the closest 
points to the photogrammetric point cloud. The 
0.200 rn 
0.150 
0.100 
Fig. 8: Cloud-to-cloud distances referenced to least squares planes on MBES point cloud (scale unit in metres)