PE&RS January 2016 - page 39

expense during merging is reduced dramatically by resam-
pling individual DEMs onto coarser grids before merging. Only
minimal information will be lost. In our case, we compared 1
mm spacing for both merged and non-merged DEMs with 0·25
mm spacing of an individual DEM . In future, effective DEM
resampling will become more important for fit-for-purpose
analysis in order not to burden processing units with increas-
ingly detailed data.
Our merging methodology was tested on DEMs of various
geometric resolutions, in order to examine the effect of scale
variation, since research in a variety of fields now use DEM s
covering a range of spatial scales. The DEM sampling distance
was shown to exert significant control on the DEM co-regis-
tration, and thus merging performance. Merging coarse DEMs
improved by resampling DEMs onto finer grids, hence improv-
ing the DEM alignment precision, prior to DEM co-registration.
If done appropriately, it is shown that the merging method
presented is able to handle a range of spatial scales.
Although we only presented merging of DEM s along one
axis, our technique can be applied for DEM merging in both
the lateral and longitudinal directions. In field applications,
where the measurement area is generally larger than in the
laboratory, we expect DEM merging to soon become a common
application, allowing analyzing fluvial surfaces at different
scales, from the grain scale to larger bedforms and channel
shape. The merging process will not differ from the labora-
tory application presented herewith. However, extra difficulty
resides in ensuring adequate overlap and consistent viewing
geometry throughout the acquisition of the successive stereo
pairs. In the laboratory, this was achieved by moving the
setup along a straight rail over a distance set by the overlap
and camera-height (see Figure 2). For in-situ deployment, DEM
co-registration shall be improved by accounting for any roll
of the setup around the vertical axis. This can be efficiently
performed with MATLAB® by applying a solid rotation to
the DEM s prior to merging. The overlap distance can be noted
directly on the ground by using fixed markers .
We currently further develop our stereo-photogrammetric
solution by identifying the minimum area of DEM overlap,
allowing for accurate co-registration, and a more detailed
evaluation of suitable averaging methods. Residual maps
between overlapping DEMs show differences in the occluded
regions, the particles’ edges and gaps (Figure 7), and we ex-
pect that merged DEM s with weighted-averaging, for example
with weights depending on the DEM cells’ aspect angle, can
improve the DEM quality in those occluded regions, beyond
the quality of individual DEM s.
Acknowledgments
The authors would like to thank the Editor-in-Chief, Dr. Rus-
sell G. Congalton, and the anonymous reviewers for their
valuable comments, which helped to improve the paper.
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