# A Survey of Low-Cost 3D Laser Scanning Technology
## 4 Problems of a Moving 2D LiDAR
### Discussion of the Problems of a Moving 2D LiDAR  
Until now, we have discussed the problems of a moving 2D LiDAR one-by-one. Among the problems mentioned above, the problem on accuracy and the problem on real-time performance are two core problems that should be urgently solved. When it comes to the accuracy of a moving 2D LiDAR, for each categories of the prototypes, there are two sources of error. One is the measurement error of the 2D LiDAR, see Problem 1.1 for more details. The other is the error of the space rigid body transformation between two coordinate frames. The coordinate frames involved here are the coordinate frame \(L\) of the 2D LiDAR, the coordinate frame \(P\) of the prototype, the coordinate frame \(M\) of the mobile platform, and the world coordinate frame \(W\), see Problems 1.2 to 1.6 for more details. Only when these two sources of error are effectively eliminated, the 3D map of the environment can be accurately built by a moving 2D LiDAR.  
When a moving 2D LiDAR is used in dynamic environments, the problem on its real-time performance should to be considered. Limited by the scanning frequency of the 2D LiDAR, the real-time performance of a moving 2D LiDAR is usually very poor. Its real-time performance is negatively correlated with the resolution of the 3D scanning, see Problem 2.1 for more details. Because of the poor real-time performance, distortions of the 3D point cloud collected by a moving 2D LiDAR can be caused by the movement of the mobile platform or the moving objects in the environment, corresponding to Problem 2.2 and Problem 2.3, respectively. A number of studies focus on the solving of the distortion caused by the movement of the mobile platform, while there are few studies on the solving of the distortion caused by the moving objects in the environment. Because of the low scanning frequency of a moving 2D LiDAR, the shapes of the moving objects cannot be captured accurately. In the 3D point clouds collected by a moving 2D LiDAR, the true shapes of the moving objects are not likely to be shown. Besides, due to the low scanning frequency, the 3D point clouds between adjacent frames may be very different, so it is not easy to find the corresponding relationship between them and identify the dynamic point clouds. In view of the poor real-time performance, a moving 2D LiDAR is more suitable for static environments. During a 3D scan, the mobile platform on which the prototype is carried should be stationary, and there should be no moving objects in the environment.  
For most applications, the environments are dynamic rather than static. Dynamic factors, such as the movement of the mobile platform or the moving objects in the environment require a moving 2D LiDAR to have a better real-time performance. However, since the scanning frequency and sampling speed of commercial 2D LiDAR are unlikely to be increased significantly in the near future, the problem on the real-time performance of a moving 2D LiDAR cannot be solved fundamentally. In order to build a global 3D map of the environment accurately, the movement of the mobile platform should be estimated and offset; the dynamic 3D point clouds corresponding to the moving objects should be recognized and eliminated. In comparison, if the cost is not considered, commercial 3D LiDARs are more suitable for dynamic environments, because their real-time performance is sufficiently high. They can collect 3D maps of the dynamic environments accurately.