## 1 Introduction
### Multispectral LiDAR Systems  
Recently, several attempts have been conducted to use multispectral LiDAR data collected from either laboratory-based multispectral LiDAR systems or terrestrial laser scanning (TLS) for vegetation applications. [PERSON] et al. [24] used laboratory-based measurements from a tunable laser operating at four wavelengths of 531, 550, 660, and 780 nm. The normalized difference vegetation index and photochemical reflectance index [25] were then used to measure plant structure and its interaction with the environment. [PERSON] et al. [26] used a laboratory-based prototype of multispectral LiDAR, designed by [27], to record the reflectance values at four different wavelengths (556, 670, 700 and 780 nm). The physiology of the canopy was then studied using three vegetation indexes, and the reported overall classification accuracy yielded above 90%. Regarding the TLS platforms, [PERSON] et al. [28] used a TLS developed by the Finnish Geodetic Institute, called the Hyperspectral LiDAR system. The system was tested in an outdoor experiment using seven wavelength bands ranging from 500 to 980 nm to discriminate man-made targets from vegetation based on their spectral response. Four vegetation indexes were used in the classification process and achieved an overall accuracy of up to 91%. Other studies used the TLS system for measuring the three-dimensional structure of forest canopies by two lasers with wavelengths of 1063 nm and 1545 nm [29] and discriminating leaves from woody material in forestry areas by using data collected at dual-wavelengths (i.e., 1064 and 1548 nm) [30]. A few attempts have been conducted to combine different flight missions from airborne LiDAR systems of the same study area [31; 32]. However, it was difficult to use the intensity data from different missions because those flight missions were conducted at different times. Thus, the surface conditions were not identical and, hence, intensity values were affected. Teledyne Optech has recently launched the first commercial multispectral airborne LiDAR sensor, the Optech Titan. Optech Titan's typical configuration includes three active laser beams operating simultaneously at three wavelengths, collecting data in three channels. The order of the channels according to their wavelengths are as follows: C1 with wavelength of 1550 nm at 3.5\({}^{\circ}\) forward looking, C2 with wavelength of 1064 nm at 0\({}^{\circ}\)adir looking, and C3 with wavelength of 532 nm at 7\({}^{\circ}\) forward looking. The beam divergence of C1 and C2 is 0.35 mrad, while the beam divergence of C3 is 0.7 mrad. The preferable flying height for topographic applications is between 300 and 2000 m above ground level (AGL), while the flying height is between 300 and 600 m AGL for bathymetric applications. The pulse repetition frequency of each laser beam varies from 50 to 300 kHz. The scan frequency can be programmed to take values between 0-210 Hz. The scan angle (field of view) is programmable in the range from 0\({}^{\circ}\) to 60\({}^{\circ}\)[33; 34]. A few studies have been conducted to explore multispectral LiDAR data collected by the Optech Titan in land cover classification. Raster images were created from the LiDAR intensity and height data, and image classification techniques were then applied [35; 36; 37; 38]. As well, the multispectral LiDAR data were explored for 3D point classification in urban areas [39; 40; 41; 42]. [PERSON] et al. [34] presented an overview on the use of the multispectral Titan data in different applications, such as land cover classification, measuring water depths in shallow water areas, and forestry mapping, however, nothing was reported on land/water discrimination.