# A Survey of Low-Cost 3D Laser Scanning Technology
## 5 Discussion
### Three-Dimensional LiDAR Is Still Indispensable  
When it comes to the measurement range, the density of the sampling points, or the frequency of 3D scanning, 3D LiDAR is generally far better than 2D LiDAR. Different from the 2D LiDARs mostly designed for 2D mapping and navigation in indoor environments [114, 115], 3D LiDARs are mostly used for surveying and mapping for large-size ground targets [82, 83, 116] or autonomous driving [84]. Therefore, for a 3D LiDAR, a sufficiently far measurement range is necessary. In addition, for a 3D LiDAR used for surveying and mapping, the density of sampling points needs to be dense enough to finely acquire the surface details. For a 3D LiDAR, used for autonomous driving, the frequency of its 3D scanning should be high enough to cope with the rapidly changing road conditions.  
Therefore, in some applications, 3D LiDAR is still necessary and indispensable and cannot be replaced by a moving 2D LiDAR, because the latter is not competent. In the field of autonomous driving, a moving 2D LiDAR with limited measurement range and 3D scanning frequency cannot be used for the environment detecting of the car, this is the job of the 3D LiDAR. In the field of surveying and mapping of civil infrastructure, as a state-of-the-art instrument and a right alternative for traditional ways of inspection and surveying, 3D laser scanner is wildly used. In [117], a terrestrial laser scanner (TLS) is used to scan taxiways in international airports and perform geometric analysis of the pavements; it is helpful for the evaluation of the performance of the pavement, which is very important for the maintenance design of the pavement. A TLS can survey roads non-destructively, compared with traditional ways of inspection and surveying, its excellent efficiency can shorten the time required and minimize the interference to traffic as much as possible. Similarly, in [118] a TLS is also used for faulting detection of rigid airport pavements. Given that traditional fault detection methods are time-consuming and laborious, which seriously hinders the passage of airplane, in [118], a TLS is used to obtain 3D point clouds of the pavements of airport, and a data processing method is used to detect faulting. Different from the above-mentioned research using a TLS, in order to further improve the efficiency and shorten the working time, in [119], a mobile laser scanner (MLS) is used to assess the flatness of the road surface, for the MLS system is significantly more efficient than static systems, that is, TLS.  
Similar remote detection technology has also been used to classify the dune vegetation in the coastal areas [120]. Coastal sand dunes provide protection for the inland areas from  
Figure 14: The prototype built by us, which is a low-cost 3D laser scanner.  
the impact of waves. The plant species that make up the sand dune vegetation community describe the evolution of the sand dunes and reveal the ongoing coastal dynamics. In [120], the UAV system is used to remotely monitor the vegetation classification on the sand dunes. Besides, in [121], a TLS is used to scan the landslide to monitor the morphological changes of the landslide surface between different time points, to effectively warn the landslide disaster.  
From the above applications, it can be seen that 3D LiDAR is still indispensable. Although in some applications with low accuracy and range requirement, and high cost-control requirements (such as the 3D mapping and navigation of home service robots), a moving 2D LiDAR may be competent, but in areas such as autonomous driving, pavement detection, and disaster prevention, 3D LiDAR still plays a significant role. Moreover, 3D LiDAR has excellent measurement performance that a moving 2D LiDAR does not have.