XXII ISPRS Congress 2012: Technical Commission VII

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Multivolume work

Persistent identifier:: 1663813779

Title:: XXII ISPRS Congress 2012

Sub title:: Melbourne, Australia, 25 August-1 September 2012

Year of publication:: 2013

Place of publication:: Red Hook, NY

Publisher of the original:: Curran Associates, Inc.

Identifier (digital):: 1663813779

Language:: English

Additional Notes:: Kongress-Thema: Imaging a sustainable future

Corporations:: International Society for Photogrammetry and Remote Sensing, Congress, 22., 2012, Melbourne; International Society for Photogrammetry and Remote Sensing

Adapter:: International Society for Photogrammetry and Remote Sensing, Congress, 22., 2012, Melbourne; International Society for Photogrammetry and Remote Sensing

Founder of work:: International Society for Photogrammetry and Remote Sensing, Congress, 22., 2012, Melbourne; International Society for Photogrammetry and Remote Sensing

Other corporate:: International Society for Photogrammetry and Remote Sensing, Congress, 22., 2012, Melbourne; International Society for Photogrammetry and Remote Sensing

Document type:: Multivolume work

Volume

Persistent identifier:: 1663821976

Title:: Technical Commission VII

Scope:: 546 Seiten

Year of publication:: 2013

Place of publication:: Red Hook, NY

Publisher of the original:: Curran Associates, Inc.

Identifier (digital):: 1663821976

Illustration:: Illustrationen, Diagramme

Signature of the source:: ZS 312(39,B7)

Language:: English

Additional Notes:: Erscheinungsdatum des Originals ist ermittelt.; Literaturangaben

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Corporations:: International Society for Photogrammetry and Remote Sensing, Congress, 22., 2012, Melbourne; International Society for Photogrammetry and Remote Sensing

Adapter:: International Society for Photogrammetry and Remote Sensing, Congress, 22., 2012, Melbourne; International Society for Photogrammetry and Remote Sensing

Founder of work:: International Society for Photogrammetry and Remote Sensing, Congress, 22., 2012, Melbourne; International Society for Photogrammetry and Remote Sensing

Other corporate:: International Society for Photogrammetry and Remote Sensing, Congress, 22., 2012, Melbourne; International Society for Photogrammetry and Remote Sensing

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2019

Document type:: Volume

Collection:: Earth sciences

Chapter

Title:: [VII/7, III/2, V/3: WAVEFORM LIDAR FOR REMOTE SENSING]

Document type:: Multivolume work

Structure type:: Chapter

Chapter

Title:: EXPLORING WEAK AND OVERLAPPED RETURNS OF A LIDAR WAVEFORM WITH A WAVELET-BASED ECHO DETECTOR C. K. Wang

Document type:: Multivolume work

Structure type:: Chapter

International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia EXPLORING WEAK AND OVERLAPPED RETURNS OF A LIDAR WAVEFORM WITH A WAVELET-BASED ECHO DETECTOR C. K. Wang Dept. of Geomatics, National Cheng Kung University, No. 1, University Road, Tainan, 701, Taiwan - p6896102@mail.ncku.edu.tw Commission VII, WG VII/7 KEY WORDS: LIDAR, Detection, Algorithm, Simulation, Accuracy ABSTRACT: Full waveform data recording the reflected laser signal from ground objects have been provided by some commercial airborne LIDAR systems in the last few years. Waveform data enable users to explore more information and characteristics of the earth surface than conventional LIDAR point cloud. An important application is to extract extra point clouds from waveform data in addition to the point cloud generated by the online process of echo detection. Some difficult-to-detect points, which may be important to topographic mapping, can be rediscovered from waveform data. The motivation of this study is to explore weak and overlapped returns of a waveform. This paper presents a wavelet-based echo detection algorithm, which is compared with the zero- crossing detection method for evaluation. Some simulated waveforms deteriorated with different noises are made to test the limitations of the detector. The experimental results show that the wavelet-based detector outperformed the zero-crossing detector in both difficult-to-detect cases. The detector is also applied to a real waveform dataset. In addition to the total number of echoes provided by the instrument, the detector found 18% more of echoes. The proposed detector is significant in finding weak and overlapped returns from waveforms. 1. INTRODUCTION Studies have demonstrated that an airborne LiDAR system (ALS) can provide accurate estimation of topographic surfaces. An important application is for exploring some key characteristics of a forest such as canopy height, vertical distribution, above-ground biomass, leaf area indices and terrain models (Pirotti, 2011). Most of papers, however, have shown that LIDAR data underestimates the canopy height (Chauve et al., 2009). The reason is explained that only a small proportion of laser pulses interacting with the tree apices. Low point density of tree tops results in underestimation of tree heights. Another tricky problem is to classify the ground points in a complex forest area. Usually when a laser emits into a forest area, only some of the laser energy is scattered back by the tree tops. The other energy penetrates through and is reflected by branches, shrubs and the ground. The received echo scattered by the ground could be too weak to being detected (Figure 1 (a)). Additionally if scatters are separated around the range resolution of the instrument, for example, the terrain and shrubs, an ALS will produce a waveform composed of a superposition of echoes. The overlapped echoes are difficult to resolve and usually only an uncertainty point located between the terrain and shrubs is detected (Mallet and Bretar, 2009) (Figure 1(b)). The terrain height represented by this point will be a slight higher than the true terrain. Fortunately, thanks to the development of LiDAR technology, the laser scanner systems with full waveform digitizing capabilities have become available. Compared with the conventional LiDAR system, waveform LiDAR further encodes the intensity of the reflected energy along the laser lighting path. Users now can utilize the waveforms for the interpretation of ground objects and develop their methods to detect the effective return echoes. This gives us a great opportunity to improve the canopy height estimation and terrain model if an approach which can deal with the weak and overlapped echoes is obtained. 529 UTIO]SA8M ULIOJO9AE AA (b) Figure 1. Interpretation of difficult-to detect echoes, (a) weak echo due to the loss of laser energy, (b) overlapped echoes due to the scatters separated around the range resolution of systems. The purpose of this study is to explore extra return echoes in waveform data, especially weak and overlapped echoes which are not detected in the online process of an ALS. However, weak echoes are difficult to detect due to the low signal-to-noise (SNR), and overlapped echoes are also hard to resolve because they are synthesized signal. To deal with this problem, one has to extract signals from a waveform that is composed of echo signals and data noises. This paper proposes a wavelet-based detector for the solution. The detector first applies a wavelet transform (WT) to decompose a LiDAR waveform in terms of elementary contributions over dilated and translated wavelets, and then searches possible echoes in the wavelet coefficients (WC) at a certain scale. Using the WT of a LiDAR waveform enables us to detect the echoes at various scales and to suppress the noises simultaneously(Shao et al, 1998). The detector detects all possible return echoes from a waveform signal. Focused on the detection of weak and overlapped echoes, our analysis aims at how much signal strength relative to noise and

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