Drone surveys often have vegetation, man-made features, or other kinds of surface "noise" that you don't want to include when creating your topographic surveys. Using Low-pass Points allows you to avoid unwanted surface features and automate your topographic ground shot placement by automatically placing points at areas of lower elevation over a defined grid area. Typically this results in points that are placed on the ground where you need them, rather than on top of vegetation, buildings, vehicles, or other items. Knowing how and when to use the Low-pass Point tool can save you a lot of time.
The Low-pass Point tool works similarly to the Rectangular Point Grid, but has some extra intelligence built into it. Low-pass Points uses the parameter Grid Size, which sets the relative spacing of your points. Rather than placing the points at specific intervals like the Rectangular Grid would, the Low-pass Points tool places points at the lowest surface elevation within your set Grid Size parameter.
The final output of Low-Pass Points is a Multi-point item containing all of your points. You can find the Low-pass Points function as part of the Point Grid tools within the HOME tab of Virtual Surveyor.
Quick Usage Guide
- Create a Boundary around the area you wish to run Low-pass Points and select it.
- Within the HOME tab, navigate to the Point Grid section and choose Low-pass in Type.
- Set a Grid Size parameter based on your typical surface items and desired spacing.
- Click on the Point Grid button to start generating Low-pass Points.
- A progress bar in the bottom left of the software displays and points are created in the Viewport.
- Your Low-pass Points display in the Viewport and are contained within a Multi-point item.
How Low-Pass Points Work
- Select the boundary you wish to run Low-pass Points in. Your boundary(s) should cover the desired area for the survey and can be simple, complex, or broken into multiple areas to only run Low-pass where appropriate. In this case, we are running Low-pass Points on a suburban property and on the vegetated hills between the golf greens. These areas both offer surface features that have significant elevation differences suited to the Low-pass Points tool; the golf greens and the road are less suited as they do not have the highs and lows and would be better surveyed with another method.
- Select the Low-pass type in the Point Grid tools area and set the Grid Size. It is important to set your Grid Size depending on the context of your survey area. Your ideal grid size is just a bit larger than your typical surface feature. This allows the points to be placed on the ground around your features. For this example, I measured the diameter of the trees—the most common features—to determine an appropriate Grid Size for the Low-pass tool.
- It is not necessary to measure your features to find the appropriate Grid Size. Most of the time, it is actually faster to iteratively test results directly. E.g. start at a commonly used spacing such as 30ft (9.14m), and adjust downward or upward depending on the results of using that spacing. If you see multiple points on top of your features, increase the Grid Size.
- A 30ft (9.14m) Grid Size provides results where most points are located on the terrain. A few areas where the surface features had larger areas than 30 x 30ft (9.14 x 9.14m)—such as the house or tree groupings in the North Eastern corner—have a few points on them due to their size.
- Keep in mind, if you create a Grid Size significantly smaller than your typical surface feature, you end up with points on the lowest areas of the surface feature, instead of the ground. In the image above, we ran a grid size of 10ft (3.048m), about 2x smaller than our typical feature. You can see this resulted in multiple points being placed on top of the surface features.
Most Common Uses
- Topographical (TIN / Contour) modeling over environments with surface features such as vegetation, urban buildings, construction yards, etc.
- The Low-pass tool is not suitable for fully closed canopy environments.
Tips and Tricks
- The Low-Pass Points work best for environments containing multiple surface features. Using it on bare earth environments may not give you the results you expect as only the lowest areas are captured.
- You need to capture breaklines and important elevation changes with separate tools.
- When using Low-pass over areas with a significant slope, you need to supplement your survey as the Low-Pass Points will group at the bottom of the slope.
- If you are creating a topographical surface as your final export, display your surface as Contours and enable the Transparent Terrain Lens to quickly identify high points in your model.
- Remember, you can always use any Point Grid tool over small subsets of your whole project. If you have areas where Low-pass Points would work best, use them there, and other Point Grid types in the other areas where it may not be suitable.