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Fusion of sensor data, knowledge sources and algorithms for extraction and classification of topographic objects
Baltsavias, Emmanuel P.

International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 7-4-3 W6, Valladolid, Spain, 3-4 June, 1999
Bruno Aiazzi 1 , Luciano Alparone 2 , Stefano Baronti 1 , Ivan Pippi 1
^‘Nello Carrara” Institute on Electromagnetic Waves IROE-CNR, Via Panciatichi, 64, 50127 Firenze, Italy, baronti@iroe.fi.cnr.it
“Department of Electronic Engineering, University of Firenze, Via S. Marta, 3, 50139 Firenze, Italy, alparone@lci.die.unifi.it
KEYWORDS: Data Fusion, Multispectral Images, Landsat TM, MOMS-2P, Multiresolution Analysis, Generalized Laplacian Pyramid
Goal of this work is to present a general and formal solution to the problem of synergetic integration of multi-sensor image data,
which may be collected with practically whatsoever spectral and ground resolution, although scale ratios larger than, say, 10, may
be questionable in certain applicative contexts. The proposed data fusion methodology is applied to a specific example concerning
observations from two different multi-spectral satellite sensors: Landsat TM (30 m resolution) and MOMS-2P (18 m ). The fusion
procedure relies on the generalized Laplacian pyramid (GLP), which is a non-octave band-pass analysis structure unconstrained from
the ground scales of the imaged data. The advantage is that the base-band extracted from the finer image exactly matches both in
displaying scale and in resolution, i.e. content of spatial frequencies, the coarser image. For any rational scale ratio, a unique low-pass
filter is needed. The filter design, however, is easy and noncritical for performances. The GLP scheme is superior in enhancing spatial
features while retaining multi-spectral signatures, according to both objective and subjective quality criteria. The experiment reported
highlights the assets of the pyramid approach: 5:3 fusion is designed for spectral enhancement of MOMS-2P (three 18 m bands)
through Landsat TM (six 30 m bands). Since the three available multi-spectral bands of MOMS-2P overlap with Bands 2, 3, and 4 of
TM, the test consists of synthesizing the three missing MOMS-2P bands -one in the blue and two in the infrared wave-lengths- from
the TM data, in order to obtain 18m R-G-B true colour and three 18 m bands in near-medium infrared.
The ever increasing availability of space-borne sensors imag
ing in a variety of ground scales and spectral bands makes multi
sensor fusion of multi-spectral data a discipline to which more
and more general formal solutions to a number of applications
are demanded. Since physical constraints impose a trade-off
between spatial and spectral resolution, spatial enhancement of
poor-resolution multi-spectral data is desirable, as well as spec
tral enhancement of data collected with adequate ground resolu
tion but poor spectral selection.
After data registration and resampling to obtain a homoge
neous spatial scale, image data fusion may be approached at three
levels of increasing hierarchy (Pohl, 1998):
• Pixel-level combination:
- deterministic: Intensity-Hue-Saturation (IHS) transform;
- statistical: Principal Components Substitution (PCS).
• Feature-level combination:
- High-Pass Filtering (HPF) method (Chavez, 1991);
- ARSIS method based on wavelet transform (Ranchin, 1996);
- Generalized Laplacian pyramid (GLP) (Aiazzi, 1998).
• Decision-level combination of pixel values and/or features
based on previously extracted information.
In applications of classification, however, merging algorithms
are often requested to maintain the spectral characteristics of the
original data (Wald, 1997). In this light, the HPF method is more
efficient than pixel-level algorithms (IHS and PCS) in preserv
ing spectral features of spatially enhanced bands (Chavez, 1991).
HPF consists of an injection of high-frequency components into
resampled versions of the multi-spectral data. Such components
are extracted by taking the difference between a higher resolution
observation and its low-pass version originally obtained through
a box filter, i.e. a square digital filter with equal coefficients.
The idea of HPF was developed in a formal multi-resolution
framework by employing the wavelet transform (WT) as
analysis-synthesis tool (Garguet-Duport, 1996; Li, 1995; Nunez,
1999; Yocky, 1996; Zivco, 1998). Perhaps, the rationale ex
pressed in HPF was best embodied by the ARSIS method
(Ranchin, 1996), which gives excellent results when the scale
ratio of the data is a power of 2 (Wald, 1997), but experiences
some troubles otherwise (Blanc, 1998), because of non-flexible
structure of the WT, which requires a filter-bank, i.e. a couple of
filters, one low-pass and another high-pass, having complemen
tary symmetry of responses, whose design is non-trivial.
A different multi-resolution approach was pursued in the con
text of Laplacian pyramids (LP) (Burt, 1983, 1984; Baronti,
1994; Aiazzi, 1997b) by other researchers, including the authors
(Toet, 1989; Wilson, 1997; Aiazzi, 1997a, 1997c, 1998). Fol
lowing the LP structure, an image is decomposed into nearly dis
jointed band-pass channels in the spatial frequency domain, with
out losing the spatial connectivity of its edges. The LP can be
easily extended to deal with scales whose ratios are rational num
bers, not only powers of two, as for the WT. Specifically, for a
rational scale ratio (> 1) a unique low-pass filter is needed. Fur
thermore, filter design is easy and non-critical for performances
(Aiazzi, 1997a). Thus, data collected with ground resolutions of,
say, 3 m, 5 m and 7 m can be easily compared and merged.
When spatial enhancement was concerned, the GLP scheme
has been shown to be superior in retaining multi-spectral sig
natures (Aiazzi, 1998) also according to objective quality crite
ria (Wald, 1997). However, fusion of two sets of multi-spectral
data having different, and complementary, number of bands and
ground resolution, may be also approached as a problem of spec
tral enhancement of the higher-resolution data. In this paper,
we wish to describe an example highlighting how the method is
simple to be designed and generalized to images having a vari
ety of ground resolutions, as future applications concerning new-
generation sensors will demand in a close future.