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Remote sensing for resources development and environmental management
Damen, M. C. J.

Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
Detection of subpixel woody features in simulated SPOT imagery
Patricia G.Foschi
School of Geography, Oxford, UK
ABSTRACT: A method for detecting small woody features in digital imagery was developed. Woody features, ranging
in size from hedgerows to strips or patches several trees wide, were tested. The classification method correct
ly detected all subpixel woody features larger than hedgerows and identified about 20 percent of the hedges.
In Britain, changes in farming practices since the
1940s have resulted in tree, pond, and hedgerow remov
als, larger field sizes, and less frequent crop rota
tions (Body 1982, Sturrock & Cathie 1980). Although
tree and hedge removals are small land cover changes,
they are significant landscape and habitat changes.
These changes which decrease habitat diversity affect
animal and bird populations. Studies have shown that
birds and mammals utilize diverse landscape elements
(Pollard & Relton 1970, Wegner & Merriam 1979) and
that their distributions are related to the shape,
size, and spatial arrangement of these landscape ele
ments (Forman & Godron 1981, Helliwell 1976).
Monitoring and quantifying agricultural change is
necessary for effective land use planning and wildlife
habitat management. Computer-assisted methods using
remotely sensed data could provide timely monitoring
of changes in woody vegetation which affect scenic
beauty and wildlife.
Since linear woody features are often subpixel tar
gets or features smaller than the pixel size of the
image, pixels containing these features are usually
mixed pixels or pixels containing two or more land
cover classes. Because the spectral values of mixed
pixels containing woody features frequently do not
correspond to the spectral values of woody vegetation,
conventional multispectral classification techniques
which operate on single pixels are problematic and re
peatedly result in mixed pixels being placed in con
stituent or extraneous classes. A method for detecting
subpixel woody features in digital imagery was devel
oped. Unlike conventional classification techniques,
this method incorporates information about adjacent
classes and mixture phenomena at the individual pixel
level. 2
As the method developed is concerned with detecting
small landscape features, it was appropriate to use
digital imagery of a high spatial resolution. Since
SPOT data was not available when the project was be
gin, simulated SPOT data was acquired for the project.
In 1984, the National Remote Sensing Centre (NRSC)
in Famborough, England organized a campaign to in
vestigate the usefulness of SPOT imagery prior to its
availability. Simulated SPOT data was collected over
a wide variety of sites in the United Kingdom in order
to test a number of applications (NRSC 1985)■ This im
agery was also sold to the public and, subsequently,
a scene was acquired for this project. Of the 39 test
sites imaged, the Winchester data flown on 6 July 1984
was selected because it is representative of agricul
tural lands in lowland Britain and because it contains
numerous linear woody features.
A subscene of the Winchester image was then selected
for use in developing and testing algorithms. This
subscene, approximately 17 sq km on the ground, is lo
cated southeast of the city of Winchester in a gently
rolling area of mixed farmland and woodland.
Panchromatic photography, commissioned by the Plan
ning Department of the Hampshire County Council, was
used to locate and map woody vegetation within the
subscene. This photography was flown by Meridian Air-
maps Limited on the evening of 28 July 1984 at 1:10000
scale. Four categories of woody vegetation were
mapped: hedgerows, single trees, single rows of trees,
and denser woody features. Specific species were not
identified. The airphoto interpretation was checked by
surveying parts of the study site on the ground.
2.1 Comparison of real and simulated SPOT imagery
The simulated imagery was flown by Hunting Geology and
Geophysics Limited with a Daedalus DS-1268 multispec
tral scanner. Daedalus channels 3 through 7 were used
singly or in combination to simulate the SPOT channels
(Hunting Geology and Geophysics Ltd. 1984). The wave
lengths of these simulations do not exactly match
those of the real SPOT bands. The effect of these dif
ferences is not known.
Table 1. Comparison of SPOT and simulated SPOT chan
nels .
in microns
Simulated SPOT
in microns
O.5O - O.59
O.52 - O.60
O.6I5 - 0.68
O.605 - O.69
0.79 - 0.89
O.76 - O.9O
O.5I - 0.73
O.52 - O.75
The spatial resolutions of real and simulated SPOT
data are the same: 20m in the three multispectral
bands and 10m in the panchromatic band at nadir view
Since the simulated SPOT imagery was flown by high-
altitude aircraft, it is less map-accurate than sat
ellite imagery. Distortions in aircraft imagery are
caused by changes in aircraft altitude and angular
orientation during scanning. Since spatial fidelity is
not important in this project and since techniques for
geometric correction inevitably involve interpolation
which further "mixes" the information in the pixels,
there has been no attempt to geometrically correct the
Another difference between real and simulated SPOT
data is the sun angle. The simulated imagery was col
lected at about mid-day and, consequently, shadows are