Applications to Lunar Remote Sensing - 3D Computer Vision: Efficient Methods and Applications - page 315

Fig. 8.4 ( a ) Shadow-based three-dimensional reconstruction of the crater Reinhold B according
to Sect. 3.1.1 .( b ) Cross section through the crater floor, obtained based on shadow length mea-
surements in three images acquired at different illumination conditions
surface. Due to atmospheric seeing, however, the effective resolution (denoted by
the diameter of the point spread function rather than the spatial resolution per pixel)
of our best images approximately corresponds to 1 km on the lunar surface. All
other telescopic lunar images regarded in this section were acquired with the same
technique.
As an example of a complex crater shape not immediately revealed by the image
data, Hafezi and Wöhler ( 2004 ) examine the lunar crater Reinhold B (cf. Fig. 8.4 a).
Due to the fact that the time intervals between the images used for extraction of
shadow information typically amount to several weeks, the relative position of cam-
era and object cannot be kept constant during acquisition of all necessary images of
the scene; thus an image registration procedure (Gottesfeld Brown, 1992 ) becomes
inevitable to obtain pixel-synchronous images.
Although the floor of the crater Reinhold B looks flat, shadow analysis based on
two images according to Sect. 3.1.1 reveals that the elevation difference between the
ridge of the eastern (right) crater rim and the centre of the crater floor amounts to
1000 m and is thus 700 m larger than the elevation difference between the ridge of
the western (left) rim, while the ridges of both rims are 300-400 m above the level
of the environment. This means that the western part of the crater floor is on about
the same level as the environment, while its central part is 700 m deeper. Hence,
the western half of the crater floor is inclined by an angle of approximately 4 ◦ ,
given the crater diameter of 19 km. Figure 8.4 b shows a cross section of Reinhold B
which additionally contains the results of shadow analysis obtained with a third
image obtained at again different illumination conditions (marked by 'a'). Here, an
identical ground level east and west of the crater has been assumed (Hafezi and
Wöhler, 2004 ). For comparison, Fig. 8.5 shows the LOLA DEM of Reinhold B,
which also reveals the slight asymmetry of the crater floor in the east-west direction.
Wöhler and Hafezi ( 2005 ) construct high-resolution local DEMs of lunar im-
pact craters with the integrated method based on shadow and shading described
in Sect. 5.2 . It is not possible, however, to obtain the required images from exist-
ing image archives of lunar spacecraft missions, as these generally do not contain