Tech Tips

Exclusion Polygons

This Tech Tip explains how to use exclusion polygons if you have a strip survey or a survey with blank areas where generated contours create odd looping contours.


The Problem

When you show generated contours, the contours show correctly.

Settings Menu > Display Settings

The quick contours.

When you generate contours and show the generated contours, this is the result.

Settings Menu > Display Settings

The generated contours.

The Solution

To resolve this issue we define exclusion polygons for the areas where there is no survey data.

Terrain Mode > Model Menu > Exclusion Polygons > Define

The program will prompt:

Click inside the empty area, the program will track the perimeter, prompt for a name

and add an exclusion polygon.

You can continue adding exclusion polygons, or press Esc to finish.

Once the polygons are defined you can view them if necessary.

Model Menu > Exclusion Polygons > Display

If there is an error about unable to track, then reduce the empty area into smaller sections, by using breaklines to join points from opposite ends.

The next step is to re generate the contours

Terrain Menu > Generate Contours

Make sure you click on the option to Use exclusion polygons (PolyFences), then click OK

You can now select which polygons you want to use. I will use all the polygons, select by left clicking on the name to select.

Click OK, once you are happy with your selection.

The generated contours, showing the correct contours.

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