PE&RS January 2016 - page 39

expense during merging is reduced dramatically by resam-
pling individual
DEM
s 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
DEM
s 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
DEM
s 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
DEM
s
improved by resampling
DEM
s 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
DEM
s 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.
References
Barazzetti, L., M.Previtali, and M. Scaioni, 2013. Stitching
and processing gnomonic projections
for close-range
photogrammetry,
Photogrammetric Engineering & Remote
Sensing
, 79(6):573–582.
Bertin, S., and H. Friedrich, 2014. Measurement of gravel-bed
topography: Evaluation study
applying statistical roughness
analysis,
Journal of Hydraulic Engineering
, 140(3):269–279.
Bertin, S., H. Friedrich, P. Delmas, and E. Chan, 2013. The use of
close-range digital stereo-photogrammetry to measure gravel-bed
topography in a laboratory environment,
Proceedings of the 35
th
IAHR Congress
, Chengdu, China, unpaginated CD-ROM.
Bertin, S., H. Friedrich, P. Delmas, E. Chan, and G. Gimel’farb, 2014.
DEM quality assessment with a 3D printed gravel bed applied
to stereo photogrammetry,
The Photogrammetric Record
,
29(146):241–264.
Bertin, S., H. Friedrich, P. Delmas, E. Chan, and G. Gimel’farb, 2015.
Digital stereo photogrammetry for grain-scale monitoring of
fluvial surfaces: Error evaluation and workflow optimisation,
ISPRS Journal of Photogrammetry and Remote Sensing
,
101(0):193–208.
Bouguet, J.-Y., 2010. Camera Calibration Toolbox for Matlab
®
, URL:
/
, (last date
accessed, 16 November 2015).
Bouratsis, P., P. Diplas, C.L. Dancey, and N. Apsilidis, 2013. High-
resolution 3D monitoring of
evolving sediment beds,
Water
Resources Research
, 49(2):977–992.
Bruno, F., G. Bianco, M. Muzzupappa, S. Barone, and A.V. Razionale,
2011. Experimentation of
structured light and stereo vision for
underwater 3D reconstruction,
ISPRS Journal of Photogrammetry
and Remote Sensing
, 66(4):508–518.
Butler, J.B., S.N. Lane, and J.H. Chandler, 2001. Characterization of
the structure of river-bed
gravels using two-dimensional fractal
analysis,
Mathematical Geology
, 33(3):301–330.
Chandler, J., P. Ashmore, C. Paola, M. Gooch, and F. Varkaris, 2002.
Monitoring river-channel
change using terrestrial oblique digital
imagery and automated digital photogrammetry,
Annals of the
Association of American Geographers
, 92(4):631–644.
Chandler, J., K. Shiono, P. Rameshwaran, and S. Lane, 2001.
Measuring flume surfaces for hydraulics research using a Kodak
DCS460,
The Photogrammetric Record
, 17(97):39–61.
Costantini, M., F. Malvarosa, F. Minati, E. Zappitelli, and F.M. Seifert,
2006. A data fusion
algorithm for DEM mosaicking: Building
a global DEM with SRTM-X and ERS data,
Proceedings of the
IEEE International Geoscience and Remote Sensing Symposium
,
Denver, Colorado, pp. 3861–3864.
Dowling, T.I., A. Read, and J.C. Gallant, 2009. Very high resolution
DEM acquisition at low
cost using a digital camera and free
software,
Proceedings of the 18
th
World IMACS Congress
and MODSIM09 International Congress on Modelling and
Simulation
, Cairns, Australia, pp. 2479–2485.
Fonstad, M.A., J.T. Dietrich, B.C. Courville, J.L. Jensen, and P.E.
Carbonneau, 2013
.
Topographic structure from motion: A new
development in photogrammetric measurement,
Earth Surface
Processes and Landforms
, 38(4):421–430.
Fusiello, A., E. Trucco, and A.Verri, 2000. A compact algorithm for
rectification of stereo pairs,
Machine Vision and Applications
,
12(1):16–22.
Gallant, J. C., and J.M. Austin, 2009. Stitching fine resolution
DEMs,
Proceedings of the the 18
th
World IMACS Congress
and MODSIM09 International Congress on Modelling and
Simulation
, Cairns, Australia, Modelling and Simulation Society
of Australia and New Zealand and International Association for
Mathematics and Computers in Simulation, pp. 2486–2492.
Gesch, D., and R. Wilson, 2002. Development of a seamless
multisource topographic/ bathymetric elevation model of Tampa
Bay,
Marine Technology Society Journal
, 35(4):58–64.
Gimel’farb, G., 2002. Probabilistic regularisation and symmetry in
binocular dynamic programming stereo,
Pattern Recognition
Letters
, 23(4):431–442.
Heays, K.G., H. Friedrich, and B.W. Melville, 2014. Laboratory
study of gravel-bed cluster
formation and disintegration,
Water
Resources Research
, 50(3):2227–2241.
Hodge, R., J. Brasington, and K. Richards, 2009. In situ
characterization of grain-scale fluvial morphology using
Terrestrial Laser Scanning,
Earth Surface Processes and
Landforms
, 34(7):954–968.
James, M.R., and S. Robson, 2014. Sequential digital elevation
models of active lava flows from
ground-based stereo time-lapse
imagery,
ISPRS Journal of Photogrammetry and Remote Sensing
,
97(0):160–170.
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