From the Ground Up

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posted 01/12/07 by Jack Strongitharm Sight Distance Analysis

Ok, Civil 3D currently does not have a button to check this part of Civil design (it is still only three years old), but it does not mean that you cannot check sight distance.

The procedure outlined in the next two week’s Friday Blogs outlines a simple idea to check visibility now.

Your model will contain a existing ground surface and also will require a surface to the top of your road if you are looking at road design. (Its not all about roads, such as can I see that Wind turbine from my house!)

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With the road design though you typically check from the drivers eye (excuse the left hand drive for all non UK, Irish, other left hand drivers!). So we can export a feature line from our corridor.

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Offset that line to the distance from the carriageway edge, such as 1.5m or to your country standards

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Draw line to distance required. Then rotate it to touch your sight line offset line you created

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Create a new surface, then paste surface EG and then Road Top, to create one combined final surface. (A great technique!!)

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Turn surface style off for clarity, (use a style with all turned off)

Create a new site and call it ‘Sight Lines’ or what you want
Note: This step overcomes the relationship between crossing alignments and feature lines.

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Convert line to a feature line, then drape to the surface, but uncheck ‘insert intermediate gradient break points’

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Using the feature line level editor, raise the points above the surface to the required heights, such as 1.05m and 0.26m at the target.

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At this point you can view the sight line in a quick profile

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As you can see this fails in a big way. (I made sure it failed of course!)

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You can then grip edit your sight line in plan to the next position, re-drape to the surface and raise the end levels and check the Quick profile.

Next Friday, I will show how to create depth/ isopachyte contours along this line to map out how far out or clear your design is.

Jack Strongitharm

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