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to align unpaired data (see |
the “Unpaired” column in Table 1), which allows the |
alignment of noncoregistered data (not even imaging the same location) or data with different spatial resolutions. |
◗The method should be able to align data of different dimen - |
sionality (see the “ D Dimensionality” column in Table 1) to |
allow multisource classification. |
◗The method should be able to align several domains at the same time (see the “Multisource” column in Table 1) |
to enhance multitemporal adaptation instead of pair - |
wise adaptation. |
◗The method should be able to align in a nonlinear |
way (see the “Nonlinear” column in Table 1), since the −0.65 −0.645 −0.64 −0.635 −0.630.090.0920.0940.0960.0980.1 |
−∆P |
−0.65 −0.645 −0.64 −0.635 −0.630.090.0920.0940.0960.0980.1 |
−∆P |
657075808590 |
556065707580 |
(a) (b) |
Figu Re 5. The Pareto front estimated using a multiobjective genetic algorithm for the selection of six features. Each dot corresponds to a |
feature set minimizing (1). The color indicates the OA on (a) the source test set Ts and (b) the target test set Tt, according to the reported |
color scale bar (adapted from [30]). |
tHe aim o F Da met HoDS |
iS to a Dapt a c LaSSiFieR |
tRaine D on t He SouRce |
Domain to ma Ke |
pReDiction S on tHe |
taRget Domain . |
Authorized licensed use limited to: ASU Library. Downloaded on March 08,2024 at 03:13:37 UTC from IEEE Xplore. Restrictions apply. |
ieee Geoscience and remote sensin G ma Gazine june 201648 |
transformation between domains can be nonlinear be - |
cause of atmospheric or illumination effects. |
◗The method should be able to use labeled information |
from the source domain when available (see the “Labels |
in s” column in Table 1). A discriminative transform tends |
to align the data sets better because it aligns the data ac - |
cording to the semantic classes required by the user. |
◗The method should avoid being forced to use labeled |
information in all domains (see the “No labels in t” |
column in Table 1), as labels might not be available in |
all domains or their acquisition might have a high cost, |
typically through terrestrial campaigns (see the “Adap - |
tation of the Classifier by Active Learning” section). |
Several methods have been proposed in recent remote sensing literature. We provide a brief review in this sec - |
tion and a summary in Table 1 . Depending on the specific |
situation, the analyst can use this table to select the most suitable approach. |
Most of the recent literature focuses on feature-extrac - |
tion strategies, where the extracted features align the data spaces with each other. In that space, the same classifier (or regressor) can be applied to all the domains. Beyond |
the works dealing with traditional or multidimensional |
histogram matching [ 33] or data alignment with principal |
component analysis (PCA) or kernel PCA (KPCA) [ 34], |
the authors of [35] propose to minimize the statistical distance between domains, which is assessed through a kernel-based dependence estimator, the maximum mean |
discrepancy (MMD) [ 18]. Other studies still focus on fea - |
ture extraction, but based on multiview models. In [ 36], |
Nielsen aligns domains with canonical correlation analy-sis (CCA) and performs change detection therein. The ap - |
proach is extended to a kernel and semisupervised version |
in [37], where the authors perform change detection with |
different sensors. In [ 38], the domains are matched in a la - |
tent space defined through an eigenproblem aiming at pre - |
serving label (dis)similarities and the geometric structure |
of the single manifolds. A nonlinear (kernelized) version of the algorithm is proposed in [ 39], where the approach |
is particularly appealing because it can align an arbitrary |
number of domains of different dimensionality, as do CCA and kernel CCA (KCCA), but without requiring paired |
examples. However, it has the disadvantage of requiring |
labeled samples in all domains. |
In [40], the authors relax this requirement by working |
on semantic ties, i.e., samples issued from the same object |
but whose class is unknown. This last method therefore requires at least a partial overlap between the images to |
find the ties, either manually or by stereo matching, as in |
[41]. The authors in [ 42] regularize the manifold alignment |
(MA) solution with spatial information, leading to a more |
stable feature representation transfer. In [ 43], they propose |
a multiscale approach, considering the preservation of |
both local and global geometric characteristics and relying |
on clustering pairs rather than labeled correspondences. |
Other recent methods rely on eigendecompositions, |
such as those proposed in [ 44] and [45 ]. In [ 44], two PCA |
eigensystems (i.e., one for the source domain and another |
for the target domain) are aligned by minimizing their di - |
vergence. In [ 45], the authors consider a sparse represen - |
tation approach where they reduce the difference between domains again by minimizing the MMD. In both [ 44] |
and [45 ], the authors aim to transfer category models that |
are learned on landscape views to aerial views from very high-resolution remote sensing images. In [ 46], the authors |
propose a set of techniques based on sample reweighing |
and transformation to address different DA situations. The |
study also offers a causal interpretation of the different forms of domain shift. The adaptation strategies are devel - |
oped on the basis of the embedding of sample distributions in the reproducing kernel Hilbert space. |
Beyond classical feature extraction, the authors in [ 47] |
align multitemporal sequences based on a measure of simi - |
larity between sequence barycenters, which corresponds to a global alignment of the spectra in a time series of images. |
In [48], the authors consider spatial shifts in large image TABLE 1. Th E REPRESENTATION ALIgN mENT m EThODS USED IN REmOTE SENSINg. |
mEThOD LABELS IN s NO LABELS IN t mULTISOURCE UNPAIRED D DImENSIONALITY NONLINEAR |
Pca # { # { # # |
KPca [34] # { # { # { |
(ss)tca [35] # { { # { # { |
cca [ 36] # { # { { # { # |
Kcca [37] # { # { { # { { |
ma [43] { { { { { # |
ssma [ 38] { # { { { # |
Kernel method for |
manifold ali Gnment [ 39] { # { { { { |
Gm [50] # { # { # # |
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