Feature line elevations can be obtained from a surface and can also be relative to a surface, so if the surface is updated, the feature line is updated.
The update behavior of relative feature lines is different depending on whether the feature line was set to be relative to a surface when it was created, or whether it was created at absolute elevations and then set to be relative.
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Watch Video: Use Relative Feature Lines to Model a Surface
Set feature line elevations to be relative to a reference surface and add the relative feature lines to a different surface as breaklines. |
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Behavior of feature lines created to be relative
The following illustration shows a feature line in green drawn so that it is follows the surface elevations (shown in a dashed gray line) at a positive elevational offset. This feature line was created so that it is relative to the surface.
If the surface is edited, the feature line is updated automatically and maintains the elevational offsets, as shown in the following illustration.
Behavior of feature lines created at fixed elevations, and then set to be relative
The following illustration shows a feature line that was drawn at fixed elevations with no relationship to the surface. If the surface below it is edited, the feature line elevations will not update.
The feature line is then set to be relative to the surface. This creates a relationship between the feature line and the surface but does not reset the feature line elevations to follow the surface.
However, the relative elevational offsets between the feature line and the surface will now be maintained if the surface is edited, such as if the elevation is lowered, as shown in the following illustration.
Using relative feature lines as data in surface definitions
The following table contains information about using relative feature lines as data in surface definitions in AutoCAD Civil 3D 2018 and the AutoCAD Civil 3D 2018.1 Update.
| AutoCAD Civil 3D 2018 | AutoCAD Civil 3D 2018.1 |
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In AutoCAD Civil 3D 2018, if you want to use a relative feature line as breakline or boundary data in a surface, make the feature line relative before defining it as a breakline or a boundary. A non-relative feature line that is defined as data in a surface cannot be made relative to any surface. |
In
AutoCAD Civil 3D 2018.1, a non-relative feature line that is defined as a breakline or a boundary in a surface can subsequently be made relative to a surface (other than a surface in which it is defined as data).
Note: In
AutoCAD Civil 3D 2018.1, a feature line cannot be made relative to a surface in which it is defined as breakline or boundary data.
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In AutoCAD Civil 3D 2018, if a relative feature line is defined as breakline or boundary data in a surface, its relative surface association cannot be changed. |
In AutoCAD Civil 3D 2018.1, if a relative feature line is defined as breakline or boundary data in a surface, its relative surface association can be changed (to a surface other than one in which it is defined as data). |
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In AutoCAD Civil 3D 2018, avoid adding a relative feature line as a breakline or a boundary to the same surface to which it is relative. |
In AutoCAD Civil 3D 2018.1, a relative feature line cannot be added as a breakline or a boundary to the same surface to which it is relative. |
In either AutoCAD Civil 3D 2018 or AutoCAD Civil 3D 2018.1, issues can occur as a result of circular dependencies if feature lines that are relative to different surfaces are then added as breaklines or boundaries to each other's surface definitions. For example, if Feature Line 1 is relative to Surface 1, and Feature Line 2 is relative to Surface 2, issues can occur if Feature Line 1 is defined as data in Surface 2 and Feature Line 2 is defined as data in Surface 1. In such a situation, rebuilding each of the surfaces will cause the other one to become out of date.
Editing relative feature lines
Feature line editing commands are supported for relative feature lines.
When you open the Grading Elevation Editor for a feature line that is set to be relative to a surface, you will see the selected surface listed in the Grading Elevation Editor, and the editable feature line points will be shown as being relative to the surface.
You can specify relative elevation options by using the following features in the Grading Elevation Editor.
For a feature line that was not previously set to be relative to a surface, when you open the Grading Elevation Editor and then select a surface from the Relative to Surface list, the Elevation Derived From column is initially set to Absolute Elevation for all points and does not automatically change to Relative to Surface. Use the drop-down list to select Relative to Surface for the points that you want to be relative.
Updating elevation points on a relative feature line
When you create a feature line and assign elevations based on a surface, you can select the Insert Intermediate Grade Break Points check box. This creates elevation points along the feature line where the feature line crosses surface TIN lines. These points are shown as green circles in the following illustration.
If you edit or otherwise move a relative feature line, or if the surface it is relative to is updated, elevation points are not automatically added or removed on the feature line to match the surface TIN lines, as shown in the following illustration.
You can update the elevation points on the feature line by using the Insert Elevation Point and Delete Elevation Point editing options in the Grading Elevation Editor or on the Edit Elevations panel of the Feature Line ribbon tab.