The  International  Archives  of  the  Photogrammetry,  Remote  Sensing  and  Spatial  Information  Sciences,  Volume  XLII-2,  2018
ISPRS  TC  II  Mid-term  Symposium  “Towards  Photogrammetry  2020”,  4-7  June  2018,  Riva  del  Garda,  Italy

This  contribution  has  been  peer-reviewed.
https://doi.org/10.5194/isprs-archives-XLII-2-961-2018  |  ©Authors  2018.  CC  BY4.0  License.

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5.3  Comparison
Overall,  the  effects  predicted  in  the  numerical  simulation  are  confirmed ­
  by  the  experimental  validation.  Compared  to  the  experimental ­
  results  the  simulation  tends  to  be  too  optimistic.  The  wave
induced  height  and  planimetry  coordinate  errors  obtained  from
the  experimental  validation  are  10  times  larger  than  the  values
predicted  in  the  simulation.  The  main  reason  is  the  limitation
of  the  simulations  to  identical  forward  and  backward  laser  pulse
paths  here.  Effects  of  dispersion  and  diffuse  reflections  at  the
water  bottom  are  neglected.
Another  reason  is  the  difference  between  real  and  simulated  water
surface.  The  simulated  wave  pattern  is  characterized  by  smoother
wave  crests  as  shown  in  figure  4.  Furthermore,  the  artificial  character ­
  of  the  machine-made  waves  is  not  optimally  reproduced
with  the  oceanographic  statistics  based  surface  wave  model  for
ocean  waves,  especially  reflections  of  waves  at  the  pool  wall.
Moreover,  the  distribution  of  the  water  surface  points  differs  in
simulation  and  experiment.  The  simulation  is  based  on  a  homogeneous ­
  distribution  with  1  respectively  10  points  per  square  meter. ­
  In  contrast,  the  water  surface  representation  used  for  the  experimental ­
  validation  is  characterized  by  an  inhomogeneous  distribution ­
  of  water  surface  points.  Figure  8  shows  an  example  for
a  typical  point  distribution  present  in  the  data  sets.  The  point  density ­
  varies  from  0  to  8  points  per  square  meter.  The  results  of  the
experimental  validation  remain  mean  values  for  the  whole  investigation ­
  area  averaging  zones  with  high  and  low  point  density.
Assessing  the  results  of  numerical  simulation  and  experimental
validation,  furthermore,  the  different  level  of  detail  used  for  the
ray  path  modeling  has  to  be  considered.  In  contrast  to  the  simulation, ­
  the  refraction  correction  of  the  measurement  data  does  not
take  into  account  the  beam  divergence,  instead  the  laser  pulse  is
treated  as  infinitesimal  small  line.
6.  CONCLUSION
This  contribution  investigates  the  effect  of  wave  patterns  on  coordinate ­
  accuracy  in  airborne  LiDAR  bathymetry.  For  this  purpose,
a  numerical  simulation  as  well  as  an  experimental  validation  was
carried  out  for  a  real  world  scenario.  The  comparison  of  the  errors ­
  predicted  in  the  simulation  and  the  errors  derived  from  real
measurement  data  show  a  consistent  tendency.  However,  the  experimental ­
  validation  reveals  significantly  larger  errors  than  predicted ­
  in  simulation.
Furthermore,  the  results  indicate  that  the  effect  of  wave  patterns
is  not  sufficiently  considered  in  common  refraction  correction
methods.  Therefore,  a  next  step  could  be  to  model  local  wave
patterns  on  the  basis  of  real  LiDAR  bathymetry  water  surface  reflections. ­
  As  a  final  result,  point-wise  strict  coordinate  correction
terms  can  then  be  applied  in  order  to  increase  the  accuracy  potential ­
  of  airborne  LiDAR  bathymetry.
ACKNOWLEDGEMENTS
The  research  work  presented  in  this  paper  has  been  funded  by
the  German  Research  Foundation  (DFG).  We  would  also  like  to
thank  Milan  Geoservices  for  acquiring  the  LiDAR  bathymetry
data  used  in  this  study.  Finally,  our  thanks  goes  to  the  operator  of
the  swimming  pool,  sbf  GmbH  Radebeul,  providing  access  to  the
swimming  pool  facility  and  controlling  the  wave  machine.

REFERENCES
Tessendorf,  J.,  2001.  Simulating  ocean  waters.  ACM  SIGGraph
Course  Notes.
Ullrich,  A.  and  Pfennigbauer,  M.,  2011.  Laser-Hydrographieverfahren.
  Patent  WO  2011137465  Al.  Riegl
Laser  Measurement  Systems  GmbH.
Westfeld,  P,  Maas,  H.-G.,  Richter,  K.  and  Weiß,  R„  2017.  Analysis ­
  and  correction  of  ocean  wave  pattem  induced  systematic  coordinate ­
  errors  in  airborne  lidar  bathymetry.  ISPRS  Journal  of
Photogrammetry  and  Remote  Sensing  (128),  pp.  314-325.
Westfeld,  P,  Richter,  K.,  Maas,  H.  and  Weiß,  R.,  2016.  Analysis ­
  of  the  effect  of  wave  patterns  on  refraction  in  airborne  lidar
bathymetry.  The  International  Archives  of  Photogrammetry,  Remote ­
  Sensing  and  Spatial  Information  Sciences  41,  pp.  133.