When you create a mesh, Mesh Buildup Wizard is typically used. If scan data consists of line type point clouds, then it is not easy to maintain the original shape of its boundaries while generating the mesh.

If you use the manual way of using 3D triangulation command in the Addins menu, you will get better result than the automatic way of using Mesh Buildup Wizard as shown in the image below.

Results from Mesh Buildup Wizard (image above)

Results from 3D triangulation in Add-ins (image above)

Method
1. When you apply triangulation with line type scan data which is already aligned by a scan device, accurately align scan sets again in the software. Select the Align button in the bottom pallet menu, or Click Tools > Scan Tools > Align in the menu.

Use the Global And Fine method with Apply Only Small Transform option as shown in the below image. This option considers that initial alignment is already done and accurately aligns scan shots removing gaps among scan shots by small transformation. And also if overlapped regions have planar shape, its alignment may slide along the planar area. This option prevents featureless scan shots from sliding during alignment.

2. Apply Addins > Combine command to make a single scan data.

3. Click the Addins > 3D Triangulate in the menu and check Assume Vertices Are Densely Placed Along A Direction option. Especially, when you try to triangulate line type point cloud, this option is appropriate for keeping edges or boundaries as it is as shown in the below image.

4. If necessary, execute the Enhance, Global Remesh and Fix Normal command in Mesh mode to get a mesh which is well optimized and featured.