Bentley Descartes Ground Extraction

Product(s):Bentley Descartes

Problem Description

Bentley Descartes Ground Extraction Information

Steps to Resolve
The Ground Extraction tool works in 3 steps.

1.)    The value of the "Largest Infrastructure Size" setting is used to create a grid over the Reality Model (Reality Mesh or Point Cloud). The grid's tiles must be big enough that each tile contains at least one ground point. Ground points (called "Seed Points") are found in each tile and a TIN is computed from those points. To validate seed points, output them to a DGN level by selecting the "Seeds" output method.


2.)    The second step consists of iteratively refining the TIN created in the first step. The "Terrain Variation" setting influences the number of iterations. A smaller value will create a flatter terrain. Getting this setting right may require a bit of trial and error.

The Terrain Variation value is a maximum, not a minimum. This is one of the factors that controls whether the algorithm continues to refine the terrain.

3.)    Once the TIN refinement is completed:

-If the user has chosen to output a "Terrain", the Geopak .Tin file is created and an STM is automatically generated.
-If the user has chosen to output a "Classified Point Cloud" then a threshold is used to classify all points of the point cloud that is within that distance from the TIN surface to the "Ground" class. (When the input is a Reality Mesh, a Point Cloud is automatically created and classified.)
One last thing that can influence the result. When working with a reality mesh, the model’s resolution is determined by the Export Resolution setting.
To change this setting:
1.  Select a reality mesh attachment.
2.  Open the "Properties" dialog.
3.  Navigate to the "Reality Mesh" section.
1.)  Largest Infra size. Do you just measure vertical with a building or horizontal?
You should measure the smallest side of the largest man-made infrastructure in the model.
2.)  Once you create a terrain model how do you change the contour lines? Meaning if it's default is a contour line every 1 foot then how do I change that to every inch for example?
Select the Scalable Terrain Model and open the "Properties" dialog. In the dialog's top pane, navigate to: Elements > Scalable Terrain Model > Calculated Features > Contours. Select the "Contours" node, in the properties area, fields named "Major interval" and "Minot Interval" are available to change the contour spacing.
3.)  What is Max. Triangle Edge? Does it affect the ground and created classification?
When the TIN is refined, the triangles become smaller and smaller. Max triangle Edge is a parameter that is used to stop the iteration. When a triangle's edge is smaller than this value, it is no longer refined. Classification on the other end is controlled by the "Threshold" value. All points that are between the refined TIN's surface and this value are classified as ground.
DEM/DSM Extraction
There is no way within ContextCapture to remove buildings/ trees and generate a pointcloud/mesh of only ground data.  You can use ContextCapture to generate the mesh and bring this into Bentley Descartes where you can use the Ground Extraction tool to create a point cloud of just the ground data as you require.
Discrepancies converting existing ground TIN to DTM for use in Roadway Designer

 Original Author:Frank McAllister