In trying to explain the presence of the errors encountered, several
error sources could be traced, part of which were of such nature
that allowed corrections to be made by reprocessing the raw laser
data. This was true for errors related to the registrations by the
inertial navigation system (INS) and interaction between laser
beam and glass plate. Another systematic error source concerned
the transformation between the WGS'84 co-ordinate system and
the Netherlands National Reference System, a transformation that
was not defined with sufficient accuracy.
The terrain type and dense vegetation cover in some areas caused
the largest devaluation of accuracy and point density. In these
areas the laser was simply unable to hit the ground. As a
consequence, systematic offsets of several tens of centimetres
were found to be present between laser data and control points.
As these offsets varied for different control areas, even in the case
that areas had the same type of vegetation cover, no means were
at hand to apply a valid correction for them. The terrain relief
influences the degree in which the laser data can describe the
terrain surface. With a point density of 1 per 16 m”, small scale
variations may be missed. Furthermore, sudden changes in the
terrain surface, like steep dike slopes, were partly filtered out, as
the filtering program assumed these to be a terrain surface
anomalies. So here the filtering had been too severe, which once
again shows that no standard filtering algorithm can be used for
all parts and that additional terrain information must be added.
Based on the experienced gained during the 1995 test flights it
was still concluded that laser scanning is a powerful cost-effective
technique and that highly accurate data can be generated.
Therefore, in 1996 new test flights will be performed for the total
Dutch coast. The results will be compared with the traditionally
derived photogrammetric coastal profiles. If the accuracy and
reliability of the laser derived beach profiles are comparable with
the traditional measurements, laser scanning will be implemented
from 1997 onwards as an operational procedure to replace the
photogrammetric production process. This will lead to a cost
reduction of 1.2 million guilders a year.
Recently, other attemps at producing DEMs for coastal areas
using SAR data have been undertaken. Davenport reported, at one
of the recent EARSeL meetings, his work using multitemporal
data and tide-heigh data to plot contours in the intertidel region.
This method actually exploits the non-synchronicity of ERS
overflights with the tidal cycle. Work is also going on in Dundee
to examine the use of interferometric SAR (INSAR) to study
morphological changes in the castal regions and to map changes
to estuarine sandbanks and saltflats from processed SAR data.
4. ECOLOGY AND VEGETATION
4.1 Management problems
Vegetation information is used by policy makers and managers of
nature areas as an indicator of the ecological condition of the
terrain. The species composition and the vegetation structure are
determined by the site conditions. These conditions are the result
of action and interaction of all biotic and abiotic factors (viz.
climatic, geological, geomorphological, hydrological and soil
conditions and the effect of fauna and human influence) active at
a particular site. This is called the system (or holistic) approach to
vegetation. In time this relational complex may change due to
natural or human induced processes and the vegetation reacts
accordingly. Hence the role of vegetation maps in monitoring
programmes is to reveal which processes are active where and, in
the case of sequential mapping, what is the progression speed of
these processes. By evaluating the observed changes a manager
can decide whether it concerns a desirable or undesirable develop-
ment in relation to the management aims of an area and
management practices may be adapted accordingly. Remote
sensing (including aerial photographs) has since long proven to be
VEGETATION MAPPING
VEGETATION CLASSIFICATION
CLASSIFICATION SPATIAL PATTERNS
Ls HIGHER ORDER
H | VEGETATION TYPE
NATURE: | suB E
TARGET TYPE | <—| TARGET TYPE | =
NATURE MANAGEMENT UNITS
[
U y
VEGETATION
DEVELOPMENT SERIES
2
ECOLOGICAL
INTERPRETATION
Figure 4: The main components in mapping and monitoring of coastal ecosystems
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