Abstract: Background, aim and scope. The unmanned aerial vehicle (UAV) digital aerial photogrammetry (DAP) and aerial lidar scanning (ALS) have been widely applied in geomorphology research, ecosystem monitoring, engineering investigation, environmental planning, and forest inventory due to their advantages of high flexibility, high efficiency, and high resolution. Photogrammetry is the science of measuring ranges from photographs, especially for recovering the space positions of optical surface points. Photogrammetry can be dated back to mid-19th century when it was the beginning of modern photography. LiDAR (Light Detection and Ranging) is a ranging technique that measures distance to a target by illuminating the target with pulsed laser light and measuring the reflected pulse with a sensor. At the end of 20th century, photogrammetry was caught up by LiDAR because of its series of shortcomings, such as heavy equipment, low efficiency, and high expense. It did not take long before LiDAR went beyond photogrammetry in many applications. However, photogrammetry came back to the stage due to the fast development of small UAV, digital imaging devices, computational advance, photogrammetry algorithm, and related software development during the past decade. In this study, we aim to compare DAP and ALS and discuss their future trends. Materials and methods. This paper reviews current major advantages, applications and perspectives of UAV DAP and ALS. We briefly analyzed the fundamental techniques and principles of DAP and ALS through typical research and study cases. We focused on the differences between these two technologies in data acquisition, data processing, flight planning, cost, advantages, and applications through synthesizing published scientific results as well as practical operational considerations. Results. The UAV remote sensing technologies, including digital aerial photogrammetry, aerial lidar scanning, and high spectrum imaging, have provided a flexible platform for terrain- and vegetation-based surface observations. The resolution of DAP can be equal to that of ALS, and the former is much more flexible and economical. Discussion. ALS needs much more complex facilities than DAP to launch an aerial survey, which is difficult and expensive to operate and mostly contracted out to professional companies. In contrast, DAP is rather easy, for a small UAV with a digital camera can carry out an entire aerial survey in short time. The data processing of ALS is “direct” due to the raw outcome of ALS is point cloud. While DAP needs to extract point clouds from aligned images in the first place, the whole process is rather efficient owing to the SfM (Structure from Motion) algorithm based professional software, which mostly have applied the CUDA (Compute Unified Device Architecture) techniques to accelerate the whole processing. DAP and ALS have been applied to forest inventory, geomorphology evolution, glacier change, gully erosion, and many other fields, and the resolution of derived DSM (Digital Surface Model), DEM (Digital Terrain Model), CHM (Canopy Height Model) can be comparable between DAP and ALS, though ALS has more advantages in high resolution research. Conclusions. UAV-based DAP and ALS technologies have four features: quick response, quick deployment, quick result, and high resolution. DAP and ALS complement each other in topography study, landform research, gully erosion, glacier change, forest inventory, and ecosystem survey. These two technologies plus high spectrum imaging offer significant complement to earth surface observations in satellite-based remote sensing, which often has limitations in spatial and temporal resolutions. The resolution of SfM-based DAP results can be as high as 0.5m×0.5m and even 0.2m×0.2m, which makes DAP competitive to ALS. But DAP cannot take the place of ALS as lidar can penetrate tree canopy and retrieve point clouds of terrain surface and subjects above the surface, such as trees and buildings. DAP must be carried out in a bright environment, which means a sunny day, while ALS aerial survey can be conducted in a cloudy day. The SfM-based DAP requires photos having enough overlapping areas, 30% to 50%, to ensure right alignment. However, DAP has a vital advantage over ALS in terms of cost (the cost of the former can be one third of the latter). A small UAV (such as DJI phantom 4) with digital camera, which is only 1280 grams, can carry out a DAP aerial survey in a short time, whereas a lidar sensor can be twice the weight of a DJI phantom 4. It is quicker and easier for DAP to operate in scenario response, aerial deployment, and result presentation than those of ALS. DAP technology can complement ALS in geomorphology research in the Loess Plateau, and the former can take the place under certain circumstances. DAP can acquire rather “real” terrain surface data in the Loess Plateau during winter and early spring when slopes and gullies are covered with sparse trees. With the help of historical high-resolution terrain data (lidar-derived DEM), the DAP results can be more accurate when generating DSM, DEM, and CHM. Recommendations and perspectives. DAP and ALS have competed over years. These two technologies have contributed revolutionary changes in observation and quantification of the Earth’s surface study. Consequently, DAP and ALS offer tremendous opportunities in complementary ways in forest inventory, ecosystem survey, geomorphology invetigation, and land cover research.
Keywords: UAV; digital photogrammetry; LiDAR; geomorphology; land cover