Forestry: An Example in Ivory Coast
- Sassandra Forest, Ivory Coast.
(Original ERS images © ESA 1993;
SPOT image © Spotimage 1991; processed images © PRIVATEERS N.V. 1996)
Forest Fragmentation
This example site is located in the
west of the Ivory Coast within the natural zone of transition between
evergreen forest and savannah.
The remaining forest blocks in the area
are generally considered relics of the natural high semi-deciduous forest.
Historic management practices of selective wood cutting however, have
led to a general depletion of the forest canopy, locally accentuated
in a fragmented canopy featuring insular gaps.
These gaps are generally
smaller than one hectare and are occupied by a low vegetation cover
consisting of mixed shrub and young secondary tree species.
Perception of this kind of subtle and small scale disturbances in forest
canopies is seriously hampered by the interfering effect of speckle noise.
Both visual interpretation and digital classification require optimal
restoration of the underlying scene radiometry with maximal preservation
of structural elements.
Within this site, a representative area of about 6x5 km featuring the
typical forms of canopy fragmentation as well as some small rock outcrops
(Inselbergen) was selected. Field work carried out in 1994 confirmed the
position and extend of the gaps observed on the SPOT panchromatic reference
image (Bottom Right).
The ERS-1 images used (Upper Left) were acquired during the dry
season (23 Feb. 1993, in green) and at the end of the rainy season (30
Nov. 1993, in red).
In the latter image, relatively high backscatter levels
are observed in less densely vegetated areas.
In these images (C-VV band,
3-look, resolution: 22x25 m; pixel size: 12.5x12.5m), the differences
between the forest cover and shrub vegetation in the gaps can only be
vaguely observed.>BR>Some zones of brighter return can be marked out, but
their perception is obscured by the interference of speckle noise and
scene texture.
After speckle filtering using the
Improved Gamma-Gamma MAP speckle filter (Upper Right), the
insular gaps can be discriminated more clearly based on both their radiometric
and textural appearance.
The edges between the brighter and more homogeneous
shrub vegetation and the forest respectively, as well as structural elements
(inselbergen, and exploitation tracks) are well preserved or enhanced
at the original pixel spacing.
The remaining distributed strong scattering
effects detected throughout the forest area, are associated with the specific
coarse texture of a forest canopy with an incomplete crown closure in
the upper layer.
At the given incidence angle of the ERS-1 SAR sensor,
even small canopy openings allow for the appearance of local strong scattering
effects at the level of trunk and branch reflection.
A comparable textural
appearance is observed in the SPOT image, while in this case the spatial
variability is induced by the high impact of shadowing of isolated dominant
tree crowns.
Detection of Very Thin Structures
A texture index based on the use of the spatial autocorrelation functions
is illustrated in the Bottom Left image.
In this representation,
the most homogeneous areas (depleted areas or very fragmented forest)
appear in darker tones.
On the other hand, the textured areas mainly corresponding
to densely forested areas are brighter.
The black lines running across
the forest correspond mainly to the detected structures (tracks inside
the forest, etc.).
These structures, which are very thin (about 10m width
on the ground), are hidden by the canopies of the trees when imaged from
the side as it is the case with a SAR.
They are almost indiscernible in
the original ERS SAR image.
Nevertheless, they are detected locally by their
second order statistical properties.
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