這讓我想起在2008年交通大學史天元教授在東沙群島執行水透光達實驗，利用機載雷達探測海底地形的實驗。
pulses of light travel to the ground. They return and are detected by the sensor giving therange (a variable distance) to the Earth. This is how LiDAR earned its name – Light Detection and Ranging.
看似簡單原理，卻有許多技術必須突破，包含多次的反射、雷達光速強度、反射點的分類等等。
LiDAR data is a rare, precious GIS resource
From ground to air, explore the types of LiDAR systems
1. Profiling LiDAR was the first type of Light Detection and Ranging used in the 1980s for single line features such as power lines. Profiling LiDAR sends out an individual pulse in one line. It measures height along a single transect with a fixed Nadir angle.
2. Small Footprint LiDAR is what we use today. Small-footprint LiDAR scans at about 20 degrees moving backwards and forwards (scan angle). If it goes beyond 20 degrees, the LiDAR instrument may start seeing the sides of trees instead of straight down.
Two types of LIDAR are topographic and bathymetric:
i. Topographic LIDAR maps the land typically using near-infrared light.
ii. Bathymetric LiDAR uses water-penetrating green light to measure seafloor and riverbed elevations.
3. Large Footprint LiDAR uses full waveforms and averages LiDAR returns in 20m footprints. But it’s very difficult to get terrain from large footprint LiDAR because you get a pulse return based on a larger area which could be sloping. There are generally less applications for large footprint LiDAR. Only SLICER (Scanning Lidar Imager of Canopies by Echo Recovery) and LVIS (Laser Vegetation Imaging Sensor), both built by NASA and are experimental.
4. Ground-based LiDAR sits on a tripod and scans the hemisphere. Ground-based LiDAR is good for scanning buildings. It’s used in geology, forestry, heritage preservation and construction applications.
