IEEE Geoscience and Remote Sensing Magazine - December 2020 - 53
about certain objects may be of greater concern than accuracy about other objects. For instance, for urban monitoring, accuracy in representing buildings is crucial, while the
segmentation or the semantic segmentation of other objects
is less important. Thus, compared to computer vision, remote sensing needs a different evaluation system for selecting proper algorithms.
MULTISOURCE DATA
As discussed in the section " An Introduction to Point Clouds, "
point clouds in remote sensing appear different from point
clouds in computer vision. For example, airborne/spaceborne 2.5D and/or sparse point clouds are crucial components of remote sensing data, while computer vision focuses
on denser full 3D.
REMOTE SENSING ALGORITHMS
Published computer-vision algorithms are usually tested on
a small-area data set with limited categories of objects. However, remote sensing applications demand large-area data with
more complex and specific ground object categories. For example, in agricultural remote sensing, users expect vegetation
to be identified according to specific species, a task difficult for
current computer-vision algorithms to accomplish.
NOISE AND OUTLIERS
Current computer-vision algorithms do not pay much attention to noise; in remote sensing, sensor noise is unavoidable.
Currently, noise-adaptive algorithms are unavailable.
LIMITATION OF PUBLIC BENCHMARK DATA SETS
The section " Benchmark Data Sets " lists several popular benchmark data sets. Obviously, the number of large-scale data sets
with dense point clouds and rich information available to
researchers has increased considerably in recent years. Some
data sets, such as Semantic3D.net and S3DIS, have hundreds
of millions of points. However, those benchmark data sets are
still insufficient for PCSS tasks.
LIMITED DATA TYPES
Though several large data sets for PCSS are available, there is still
demand for more varied data. The real world has many more
object categories than those considered in current benchmark
data sets. For example, Semantic3D.net provides a large-scale
urban point cloud benchmark. However, it covers cities only of
a certain kind. If, for a PCSS task, researchers chose a different
city with different building styles, vegetation, and even ground
objects, algorithm results might in turn be different.
LIMITED DATA SOURCES
Most mainstream point cloud benchmark data sets are acquired from either lidar or RGB-D sensors. But, in practical
applications, image-derived point clouds cannot be ignored. As
previously mentioned, the airborne 2.5D point cloud is an important category in remote sensing, but for PCSS tasks only the
Vaihingen data set [31], [87] is published as a benchmark data set.
DECEMBER 2020
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE
New data types, such as satellite photogrammetric point clouds,
InSAR point clouds, and even multisource fusion data, are also
necessary to establish corresponding baselines and standards.
CONCLUSIONS
This article reviewed current PCSS and PCS techniques. This
review not only summarized the main categories of relevant
algorithms, but also introduced the acquisition methodology and evolution of point clouds. In addition, the advanced
deep-learning methods proposed in recent years were compared and discussed. Due to the complexity of point clouds,
PCSS is more challenging than 2D semantic segmentation.
Although many approaches are available, they have each
been tested on very limited and dissimilar data sets, so it is
difficult to select the optimal approach for practical applications. Deep-learning-based methods have ranked high for
most benchmark-based evaluations. Yet no standard neural
network is publicly available. In coming years, we can anticipate the appearance of improved neural networks for solving
PCSS problems.
Most current methods have considered only point features,
but in practical applications, such as remote sensing, noise and
outliers are still problems that cannot be avoided. Improving
the robustness of current approaches and combining initial
point-based algorithms with different sensor theories to denoise the data are two potential future fields of research for
semantic segmentation.
ACKNOWLEDGMENTS
We thank Dr. D. Cerra and P. Schwind for reviewing this article,
and we acknowledge the anonymous reviewers and the associate editor for their insightful comments.
The work of Yuxing Xie was supported by a DLR-DAAD
fellowship (57424731) funded by the German Academic Exchange Service and the German Aerospace Center.
The work of Xiao Xiang Zhu was jointly supported by
the European Research Council (ERC), under the European
Union's Horizon 2020 Research and Innovation Program
(grant agreement ERC-2016-StG-714087); the Helmholtz Association, under the framework of the Young Investigators
Group SiPEO (VH-NG-1018, www.sipeo.bgu.tum.de), and the
Bavarian Academy of Sciences and Humanities in the framework of Junges Kolleg.
AUTHOR INFORMATION
Yuxing Xie (Yuxing.Xie@dlr.de) received his B.Eng. degree in
remote sensing science and technology and his M.Eng. degree
in photogrammetry and remote sensing from Wuhan University, China, in 2015 and 2018, respectively. He is currently pursuing his Ph.D. degree with the Remote Sensing Technology
Institute, German Aerospace Center, Wessling, Germany, and
the Technical University of Munich, Germany. His research interests include point cloud processing and the application of
3D geographic data.
Jiaojiao Tian (jiaojiao.tian@dlr.de) received her B.S. degree in geoinformation systems from the China University of
53
http://www.Semantic3D.net
http://www.sipeo.bgu.tum.de
http://www.Semantic3D.net
IEEE Geoscience and Remote Sensing Magazine - December 2020
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