# A Survey of Low-Cost 3D Laser Scanning Technology
## 3 Classification of the Prototypes
### A Push-Broom 2D LiDAR  
Compared with a rotating 2D LiDAR and a pitching 2D LiDAR, the main feature of a push-broom 2D LiDAR is that there is no relative movement between the 2D LiDAR and the mobile platform on which it is carried, the 2D LiDAR is fixedly assembled on the mobile platform. The mobile platform mentioned here may be a vehicle [36; 37; 70; 71], a backpack [16; 72; 73], a handheld pole [74; 75], or a UAV (unmanned aerial vehicle) [76; 77], as shown in Figure 6.  
Figure 4: A rotating 2D LiDAR and a pitching 2D LiDAR in different attitudes. (**a**) A pitching 2D LiDAR whose rotation axis is horizontal; (**b**) a rotating 2D LiDAR whose rotation axis is horizontal; (**c**) a pitching 2D LiDAR whose rotation axis is vertical; (**d**) a rotating 2D LiDAR whose rotation axis is vertical [48].  
Figure 3: A pitching 2D LiDAR (left) and a rotating 2D LiDAR (right) [69].  
Figure 5: A rotating 2D LiDAR and a pitching 2D LiDAR. **(a)** A pitching 2D LiDAR mounted on a mobile platform. Left: the prototype in [68]; right: the pitching scanning example in [68]; **(b)** the prototype in [38], a rotating 2D LiDAR (circled in red) which is mounted on a skid-steer loader; **(c)** the prototype in [20], which is a rotating 2D LiDAR; **(d)** the prototype in [69], which is a pitching 2D LiDAR; **(e)** the prototypes in [48]. Left: the first-generation prototype, which is a pitching 2D LiDAR whose rotation axis is vertical; right: the second-generation prototype, which is a rotating 2D LiDAR. Compared with the first-generation prototype, in the second-generation prototype a slip ring has been used, so that the 2D LiDAR can be rotated endlessly, without being blocked by cables.  
Figure 6: Push-broom 2D LiDARs carried by different platforms. **(a)** A push-broom 2D LiDAR mounted on a vehicle. Left: the experimental platform in [36]; right: the push-broom scanning example in [71]; **(b)** push-broom 2D LiDARs mounted on a backpack. Left: the prototype in [72]; right: the 3D model of the prototype in [73]; **(c)** handheld push-broom 2D LiDARs. Left: the prototype in [74]; right: the prototype in [75]; **(d)** UAV-mounted push-broom 2D LiDARs. Left: the prototype in [76]; right: the prototype in [77].  
which the 2D LiDAR is mounted should move relative to the environment. The principle of a push-broom 2D LiDAR is to build a 3D point cloud by combining the data of the 2D LiDAR and the position and attitude of the mobile platform. Therefore, in order to build a 3D map accurately, the movement of the mobile platform should be accurately known. In Section 2, we came to the conclusion that, for a moving 2D LiDAR, the space rigid body transformation parameters between the 2D LiDAR coordinate frame and the world coordinate frame (that is, the rotation matrix \(R_{L}^{W}\) and the translation vector \(T_{L}^{W}\)) should be accurately known, so that the coordinates of the sampling points relative to the world coordinate frame can be accurately calculated by Equation (2). For a push-broom 2D LiDAR, \(R_{L}^{W}\) and \(T_{L}^{W}\) are determined by the movement of the mobile platform and the mechanical installation position of the 2D LiDAR on the mobile platform.