Coastal zone mapping 
28 
Hydrographische Nachrichten 
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November 
does not produce resultsthat signi cantly deviate 
from the resultsof Chiropteraand Hawteye II. 
2.3 The investigation of Secchi depths 
In order to extrapolate the ndings to the whole 
German coast of the Baltic Sea, it is necessary to in 
vestigate the Secchi depths of the whole German 
waters (Hubner ). These depths vary much 
during the year. There are two time windows of 
spedal interest: 
• Spring time hasthe best Secchi depths, but the 
time window issmallerand dependsmuch on 
the weather conditions of the months before. 
• Fall time has slightly worse Secchi values, but 
thetimeislongerand better to predict. 
The spatial variation isvery large ( . to m). This 
leadsto a consideration of the relationship (depth/ 
Secchi depth) instead of the Secchi depth itself. 
The result of this investigation is a chart (Rg. ) 
showing areasof: 
• 0.7 x Secchi depth for Regl V3-820-Gor similar; 
• 1.3 x Secchi depth for Chiroptera or similar; 
• 2.0 x Secchi depth for Hawkeye III or similar. 
3 Conclusion 
3.1 The ndings of the project 
The main conclusion is that bathymetric airborne 
laser scanning will not replace traditional hydro- 
graphic surveys as such, but will be an important 
supplement in shallow areas with minor impor 
tance for mariners It is a strong improvement 
against single-beam hydrographic survey, since 
the survey iscompliant with & order b instead 
of order (IHO ). Within the German part of 
the Baltic Sea, , km 2 may be surveyed with 
ALBwith full coverage up to a relation depth/Sec- 
chi depth of . and with minor resolution up to a 
relation of (Rg. ). 
These areas are mostly to be resurveyed every 
years It makes sense to do the survey in parts 
of this area in order to limit the size of data setsto 
be processed. In order to improve the e ciency it 
is necessary to coordinate a ight campaign with 
all stakeholders of the area to be surveyed, espe 
cially between the hydrographic o ce, the coastal 
engineers, the coastal protection authorities, and 
the ordnance survey. 
There are still some questions remaining, espe- 
dally: 
• The detection of obstructions, 
• Data gaps due to vegetation or other material 
with bad backscattering. 
Sblving these questions would signi cantly im 
prove the use of ALB for hydrographic purposes 
Fbrticularly at the German Baltic Sea coast, lots of 
stones, dangerousfor shipping, could be detected. 
3.2 The next step 
Following this project it becomes necessary to up 
date the general plan of surveys in the Baltic Sea. 
An area of , km 2 may be described asALEarea. 
Thisgivesthe possibility to concentrate the multi 
beam capacity to other areas where the frequency 
of surveys must be increased in order to improve 
the safety of shipping (Helsinki Gemmission ). 
The next stepswill be to arrange the nancingand 
the coordination of the projects 
The ALB campaign will be organised with a 
public call for tenders The project has shown that 
the area should be described between some dis 
tance landwards from the coastline up to a line 
described by the relation depth/Sbcchi depth. 
Thus, it is necessary to organise shipborne Secchi 
measurements together with shipborne control 
check lines very close to the ALB measurements 
The time of survey should be open, but with good 
communication in order to coordinate the ights 
with the shipborne measurements "