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water  depth  can  be  computed  as  the  difference  of  the  water  surface  peak  and  the  seabed  peak.  The  maximum  depth
which  can  be  reached  in  this  way  is  limited  by  the  water  turbidity,  and  is  measured  in  terms  of  the  so  called  Secchi  depth.
This  indicator  is  defined  as  the  maximum  depth  at  which  the  human  eye  is  able  to  detect  a  specific  circular  disk  with  a
diameter  of  30  cm  in  the  water.
ALB  was  developed  in  the  1960s  with  the  primary  goal  of  finding  submarines.  Hickman  and  Hogg  [3]  proposed  to  use
ALB  for  bathymetric  survey.  In  the  1970s  some  ALB  prototypes  were  successfully  developed  and  tested  [4].  In  recent
years  this  technique  has  become  increasingly  important  due  to  improved  hardware  and  better  processing  software.  This
development  has  also  made  possible  the  recording  of  the  waveform  of  the  backscattered  signal  [5].  In  [6],  a  seabed  map
of  sand  and  seagrass  was  produced  using  both  the  corrected  waveform  and  underwater  video  data  recorded  during  the
flight  campaign.  Moreover,  the  classification  of  the  ALB  data  was  studied  using  inherent  characteristics  of  the  recorded
waveforms  [7,  8].  Pe’eri  et  al.  [9]  investigated  ALB-based  land-water  interface  algorithms  with  green-,  red-,  and  infrared
channel  waveforms  to  detect  the  shoreline  of  rocks,  vegetation  and  man-made  features.  As  a  further  application  ALB  was
applied  to  map  submerged  archaeological  structures  over  large  areas  in  high  detail.  The  results  showed  that  the  technique
is  mature  enough  to  be  utilized  for  underwater  heritage  management  [10].  In  combination  with  side-scan  sonar,
shipwrecks  were  indentified  and  their  dimensions  could  be  measured  [11].  Recently,  full-waveform  processing
algorithms  for  single-wavelength  ALB  were  investigated  for  a  better  extraction  of  seabed  points  [12,  13].
3.  DATA  ACQUISITION
3.1  Study  area
To  investigate  the  applicability  of  ALB  monitoring  for  the  German  Baltic  Sea  Coast,  three  flight  campaigns  were
performed  close  to  the  island  of  Poel  in  the  German  Baltic  Sea.  The  test  areas  were  of  varying  size  with  slightly  different
locations.  In  order  to  carry  out  a  comparison  under  equal  conditions,  we  focus  only  on  the  overlapping  area  of  the  three
flight  campaigns,  which  is  95.8  km 2  in  size.  The  location  of  the  study  area  is  shown  in  Figure  1.  Most  parts  are  covered
by  water,  but  the  area  also  comprises  some  small  shore  regions.  Concerning  the  water  depth  a  variation  can  be  observed
from  0  m  to  the  maximum  depth  of  23.2  m.  This  value  is  deeper  than  the  expected  depth  which  is  currently  reached  by
ALB  sensors  under  the  available  turbidity  level;  in  fact  it  was  chosen  to  determine  the  limitation  of  the  penetration  depth.

Figure  1.  Overview  of  the  study  area,  which  is  highlighted  by  the  yellow  boundary.
3.2  Flight  campaigns
The  first  data  set  was  acquired  by  Milan  Geoservice  GmbH,  Germany,  with  a  Riegl  VQ-820-G  Sensor  in  the  beginning
of  November  2012.  This  sensor  works  with  a  green  laser  (/.  =  5.32  nm)  solely.  In  the  investigated  overlapping  area,
25  strips  with  an  overlap  of  70%  were  acquired.  For  this  survey  the  pulse  repetition  rate  was  set  to  149  kHz.  The  flying
height  was  500  m  above  ground  level.  A  first  quantitative  analysis  of  the  ALB  data  near  the  island  of  Poel  was  reported
in  [14,  15].  The  second  data  set  was  collected  by  TopScan  GmbH,  Germany,  in  the  end  of  September  2013  with

Proc.  of  SPIE  Voi.  9638  96380Z-2