Many drone surveys have vegetation, man-made features, or other "noise" that you don't want to include when creating your topographic surveys. Low-pass Points allow you to avoid these 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, rather than on the tops of vegetation, buildings, vehicles, or other surface items.
Low-Pass Points works similarly to the Rectangular type of 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 as Rectangular would, the Low-Pass Points tool will place a point on the location of the lowest elevation within your set Grid Size parameter, whereas the Rectangular type would have placed the point in the exact middle of the Grid Size.
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 then 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
- Click on the Point Grid button to start generating Low-pass Points
- A progress bar in the bottom left of the software will display and points will be created in the Viewport
- Your Low-pass Points will display in the Viewport and will be contained within a Multi-point item
How it works
- 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 Points 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, while 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 surrounding your features, as the Grid Size also covers the area surrounding features. For this example I measured the diameter of the most common features, the trees to determine an appropriate Grid Size.
- However, it is not necessary to measure your features to find the appropriate Grid Size. Most times, it is actually faster to iteratively test results directly. I.e. 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 many points on your features, increase the Grid Size.
- 30ft (9.14m) Grid Size provided results where most points are located on the terrain. A few areas where the surface features had larger areas than 30x30ft (9.14m 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 will end up with points on the lowest areas of the surface feature, not on the ground. Here we ran a grid size of 10ft (3.048m), about 2x smaller than our typical feature. You can see this resulted in many points ending up on top of the surface features.
Most common uses
- Topographical (TIN / Contour) modelling over environments with many surface features such as vegetated, urban, construction yards, etc. (Not suitable for fully closed canopy environments).
- Low point extraction.
Tips and Tricks
- Low-Pass Points work best for environments containing many surface features. Using it on bare earth environments may not give you the results you expect as only the lowest areas will be captured, and not the breaklines and important elevation changes.
- When using over areas with significant slope, you will need to supplement your survey as the Low-Pass Points will group down slope.
- If creating a topographical surface as your final export, display your surface as Contours and enable to 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.