When drone data is covering large wooded areas, surveyors want to create lightweight surface models of the ground elevation that falls through the vegetation (bare-earth level). Virtual Surveyor includes tools that will help you to efficiently do so.
In this use case we are going to create a lightweight CAD model that we will export to a 3rd party CAD software for further analysis. Hence there is no need to clean the terrain in Virtual Surveyor.
- Draw a boundary around the area of interest.
- Low-Pass point grid.
- Clean up the point grid.
- Add breaklines.
- Triangulate the survey.
- Clean up the created surface.
- Export to .dxf
First, define the area you want to clean. Use the Boundary tool to trace the limits of this zone. You don't need to pay attention to the vertices elevation.
Low pass point
Select the boundary and launch a Low-Pass point grid. Set appropriate grid size to make sure to have a dense grid with not too much points on trees.
What is the optimal grid size? Start with a larger grid size and make the grid size smaller step-by-step until points start to appear on surface objects. Read the full details here.
Point grid cleaning
Even if the Low-Pass algorithm is designed to place points on the ground, sometimes, in dense vegetation areas, points can land on trees. You have to manually remove these points by selecting them and pressing Delete.
In the case where there are a lot of these "floating" points, delete all the points by selecting the Multi-point item in the project view and run the Low-Pass point grid again with a higher grid size value.
If needed you can also add some breaklines to include some specific elevation terrain lines like road or bank edges. Use the Polyline tool available in the HOME tab to draw these breaklines. The Guided Breakline Tool may also be helpful.
When all the survey items are ready, disable the area boundary in the project view. Click Triangulate All in the HOME tab to generate the surface. Make sure you have selected the 3D triangulation method. Use the Styling options in the TOOLS tab to show the contour lines.
Some triangles from the surface may be meaningless, especially on the edges of the surface. Select the surface and enable the Remove Triangles tool in the TOOLS tab. Click on all the triangles you want to erase (multiple selection possible by holding Shift on the keyboard).
Finally we export the light-weight CAD model to .dxf for further use in a 3rd party CAD software.
Tips and tricks
- If your project contains only a few isolated trees, it's not necessary to use the Low-pass Point Grid. Those objects can be erased from the elevation terrain thanks to the Replace Terrain or Remove Object tools.
- To easily locate incorrect points generated by the Low-pass tool, triangulate all the points and enable surface contours with a small interval. Points dropped over the vegetation are surrounded by clustered concentric contour lines. You will better see those "pyramid" contours by activating the Transparent Terrain Lens.