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Finding and Repairing DTM Problems

Updated July 09, 2021

This article applies to:
  • Terrain 3D
  • Terrain Forestry
  • RoadEng Civil
  • RoadEng Forestry
This Knowledge Base article is an excerpt from one of our tutorial files. The files referenced in the article, as well as the full tutorial document, are available in our Tutorial Installer.  

In this exercise, you will use the 3D window to help find problems with a DTM. You will also remove bad data points from the model and tag critical features as breaklines. It is possible to find all the problems with this model by looking carefully at the contours (especially as they are closely spaced). However, the 3D window often makes this task quicker and easier.

You should already be familiar with moving around in the Plan and 3D windows (previous two exercises).

Note: Refer to Getting Started section for file install folders (<RoadEngCivil> and <Defaults and Layouts>).

  1. Open the Terrain Module.

  2. File | Open <RoadEngCivil>\DTM\Topo with issues.terx.

  3. View | New Window | 3D from dropdown. A 3D window will appear on your screen.

  4. Use menu View | Tile Vertically to show 3D and Plan windows side by side.

Removing a Bad Point From the Model

  1. Adjust the Plan and 3D views until you can see the bad elevation point shown below.

Figure 1: Bad Elevation Point Displayed in 3D, Plan and Feature Properties Windows

  1. Select the bad point in the Plan window with the mouse . You know you’ve selected the correct point when the 3D window shows the current point on top of the anomalous spike (figure above). Note that the Status window shows that this point is a 3D modeled point – it is part of the TIN surface.

At this point you could delete the feature but then there will be no record of this point. Instead we will remove it from the TIN model.

  1. Open feature properties panel using one of the following methods:

  1. Feature Tools | Feature Properties…

  2. Right Click | Modify Selected Feature(s) | Properties…

  3. <Ctrl-E> - speed key or hot key.

Figure 2: Feature Properties Dialogue Box

  1. Clear the Modeled property so the point feature will no longer be part of the model.

  2. Press OK to accept the change and close the dialogue box.

  3. When warned that “existing triangles will be cleared” respond OK.

Note: The above procedure is typical of most Terrain Module operations:

First, select features of interest (sometimes the current feature and current point are important).
Second, use the Modify Selected Feature(s) menu to do something to the selection set.

  1. Select the Terrain Modeling | Generate TIN in the tool bar to open the Terrain Calculation dialogue box (see Creating a DTM with Contours exercise above). The settings for this dialogue box were configured when this file was created; you don’t need to adjust anything.

  2. Press OK to recalculate the DTM and the contours. Note that the anomalous spike in the model has disappeared.

Defining Breaklines

  1. Adjust the Plan and 3D views until you can see along the curve in the road shown below.

Figure 3: 3D and Plan Contours Depicting Bad
Triangles Caused by Missing Breaklines.

What looks like a land slide in the figure above is a triangle formed by connecting centerline survey points with their nearest neighbor, a top of bank point. We know that the shoulder of the road should be a smooth and continuous line; in terrain modeling terms, this is a breakline. Some typical breaklines are listed below:

  • Road shoulder

  • Ditch bottom

  • Top of cut

  • Toe of fill

  • River bank

  1. Select the EP feature as shown in the figure above. Note that the properties the Status window indicate that this is NOT a breakline.

  2. Open the Feature Properties dialogue box (as in step 7 above) and Turn ON the Breakline property.

  3. Press OK to accept the change and close the dialogue box.

  4. Recalculate the Terrain Model: Terrain Modeling | Generate TIN (as in steps 11 and 12). Note that the Model looks a little better.

  5. Terrain Modeling | Delete TIN | check Delete Contours box. This will make the following step easier.

  6. Find other features that should be tagged as breakline (EP, TOE, TOB), and repeat the steps above. Note that you can use the <shift> click (or click and drag) technique to select more than one feature at a time and then change their properties all at once.

Figure 4: Model after Features made Breaklines and Model Re-Calculated

  1. File | New. Do not save changes.

Digital Terrain Modelling