When surveying project sites with vegetation, you can use the Low-pass point grid to get 90% of your surface done in just a few minutes. After using the Low-pass tool, you might notice that ridges on your project site are not properly captured with points. You'll need to combine drawing tools to add points or breaklines on top of the ridge to get a surface that accurately captures the topography. We'll explain how you can quickly supplement the missing points and complete your surface in minutes.


In areas where you do not have vegetation, you won't have a problem with ridges not getting points because you'll typically use a different method—like using Q-Points or breaklines—to capture the topography.


Note: Virtual Surveyor is a toolbox. You will rarely get the entire job done only using one tool. The tools you use aren't the solution. The solution is that you are a surveyor, and you know how to use all the tools at your disposal to get your survey done properly and create an accurate topo surface. 


You can also read about how to get started with the Low-pass point grid tool: Low-Pass Points 


Overview


Creating a Low-pass Point Grid Over a Ridge

We've set up a dataset with a boundary drawn around an area that has multiple small ridges to highlight what you'll likely see in your projects if you use the Low-pass tool on a project site where ridges are included. With the dataset in the image below, you can see how the points are placed in a densely vegetated area. We can run the Low-pass point grid and see how it places points around the ridges. 


For a quick explanation of how we set it up: We created a boundary around the project site; then we set the Point Grid type to Low-pass and set the spacing to 10m (25ft) and ran the Point Grid to generate the points within our boundary.


Image showing that the Low-pass points did not fully capture the ridges in a vegetated area.


After running the Low-pass point grid, you can see that 90% of the work is completed. The Low-pass tool is designed to avoid placing points on surface objects by placing them on the lowest part of the ground. The side-effect is that ridges inside your boundary often don't get the points they need to properly detail the surface. If you triangulate the surface at this point, you will end up with some triangles that go under the ridges of the elevation terrain. You still need to use additional tools to add points to the ridges and complete your topographic survey.


Left image: Still shot of the Low-pass point grid generated over an area where the ridges have not been supplemented with points yet. Right image: If you create a surface (TIN) using just the Low-pass point grid and don't place any points on the ridges, the surface doesn't fully capture the terrain.

Low-pass point grid generated on an area with ridges. There are gaps on the ridges where points are still needed.

A generated TIN from the low-pass points showing that the triangles go under the terrain and are not capturing the ridges.


Options to Complete the Surface 

There are a few options and tools to use to help you add the necessary points on the ridges:

  • Draw breaklines using the Polyline tool. Draw your breaklines along the tops of the ridges to properly finish your survey.
    Image of a complete surface after running the Low-pass point grid and supplementing the missing ridges with breaklines (after using the polyline tool).
  • Draw points in manually. Use the Point tool to manually click along the tops of the ridges to add the points one at a time.
    GIF of a surface being completed after running the Low-pass point grid and supplementing the missing ridges by drawing points in manually with the Point tool.
  • Create the surface, then edit it to add the points. You can see the TIN update in real-time by editing the surface while adding the necessary points to accurately represent the terrain.
    GIF of a surface being edited after running the Low-pass point grid and supplementing the missing ridges by drawing points in manually with the Edit Surface (Add Vertex) tool.


You can see side-by-side examples of created surfaces showing the difference of having a TIN without any additional points added after using a Low-pass point grid, and a TIN that was supplemented with points and breaklines. 

Incomplete surface as a TIN still needing additional points to cover the ridges.

Complete topo surface (TIN) made after combining the Low-pass point grid tool with additional points or breaklines.


Here are the contours from the same TIN surfaces. The left image is the contours created using only the Low-pass point grid, and the image on the right is using both Low-pass point grid and breaklines (To see the differences more clearly, you can click on the images and use your keyboard arrow keys to toggle between the images).

Contours from a Low-pass created surface that did not capture the ridges.

Contours from a surface that was created using a Low-pass point grid and complemented with additional points and breaklines.


Tips and Tricks

  • Add more vertices to your polyline with a single click. If you used the polyline to add breaklines, you can quickly densify the breaklines to the same interval between vertices that you did for your Low-pass point grid to quickly add points with fewer clicks. You can then use Edit Vertex to manually add more vertices along each breakline as desired.
    Use the densify tool to quickly add more vertices along the breakline. You can then manually add more vertices to your polyline/breakline using the Edit Vertex.
  • Merge single point items with a multi-point item. If you manually add one point at a time with the Point tool, you can then combine (merge) the single points with your multipoint item and vice versa to work on or portray the points to however it best suits your needs.
    Merge or combine single point items with a multi-point item using the Merge Points tool.