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 accelerate your topographic ground shot placement by placing points at areas of lower elevation with specific spacing between points. Knowing how and when to use the Low-pass Point tool can help save you a lot of time.


The Low-pass Point tool has some extra intelligence built into it when compared to the Rectangular Point Grid tool. Low-pass Points uses a spacing parameter to help it place points at ground level where you need them most. 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 Spacing parameter. 


The final output of Low-Pass Points is a Multi-point item, which contains all of your points in one project item. You can find the Low-pass Points tool as part of the Point Grid tools in the HOME tab of Virtual Surveyor. 



Overview


Quick Usage Guide

  1. Create a Boundary around the area you wish to run the Low-pass tool.
  2. With the boundary selected, go to the HOME tab and set the Type to Low-pass.
  3. Set the Spacing you want between the points.
  4. Click on the Point Grid button to generate your Low-pass Points.
  5. A progress bar in the bottom left of the software displays and points are created on the Elevation Terrain.
  6. Your Low-pass Points are added as an item in the Project View as a Multi-point item.


How Low-Pass Points Work

The most common uses of the Low-pass tool is for Topographical (TIN / Contour) modeling over environments with surface features such as vegetation, urban buildings, construction yards, etc. 


Important: The Low-pass tool is not suitable for fully closed canopy environments.



  • Create an effective Boundary: Draw the boundary around the area you wish to use the Low-pass Points tool. Your boundary(s) should cover the desired area for the survey and can be anything from simple, complex, or broken into multiple areas to only run Low-pass where appropriate. In the sample images above, 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.


  • Set appropriate Spacing: Select the Low-pass type in the Point Grid tools area and set the Spacing. It is important to set your Spacing depending on the context of your survey area. Often, your most ideal spacing between points is just a bit wider than the most common surface feature (e.g. building, tree canapy, etc.). This allows the points to be placed on the ground around your features. For this example, we measured the diameter of the trees—the most common features—to determine an appropriate Spacing for using the Low-pass tool.


  • Iteratively create Low-pass points to find the correct Spacing: You don't need to measure your features to find the appropriate spacing parameters. Most of the time, it is actually faster to iteratively test results directly. E.g. start at a general spacing setting such as 30ft (10m) and adjust the spacing to be further apart or closer together depending on the results you get. If you see multiple points on top of your features, delete the Multi-point item and change the Spacing appropriately.


  • Points might be created on top of buildings or trees: A 30ft (10m) Spacing on this dataset has created points that mostly fell on the terrain. A few areas where the surface features had larger areas than 30 x 30ft—such as the house or tree groupings in the Northeast corner—have a few points that were created on them due to their size being over 30ft wide. If points end up on top of trees and buildings, you can always just simply delete them. Or delete the Multi-point item and try using the Low-pass feature again with smaller spacing.

 

  • Avoid Spacing parameters that are too small for your dataset. Keep in mind, if you create spacing that is significantly smaller than the width of a typical surface feature, you'll end up with too many points that are created on the surface features instead of the ground. In the example image above, we created spacing at 10ft (3m), which is a considerably smaller spacing than the width of the tree canopies for this dataset.


Tips and Tricks

  • The Low-Pass Points work best for environments containing multiple surface features or vegetation. Using it on bare earth environments may not give you the results you expect as only the lowest areas of the ground are captured. Using Q-Points is a much more efficient tool for bare earth projects. 
  • You'll need to combine tools with Low-pass when working on ridges or slopes (e.g. the Polyline tool for breaklines or manually placed points with the Point tool) to highlight important elevation changes and finish creating your surface.
  • When using Low-pass over areas with a significant slope, you'll need to balance your spacing parameters as the Low-pass tool will group your created points 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 the tool there. Use other Point Grid types in the other areas where Low-pass may not be suitable.


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