Daniel Ewald, landscape architect MNLA

AutoCAD – Placing roads in sloping terrain

For a landscape architect it is fundamental to know how to place roads and pathways in sloping terrain correctly. Last year we learned a quick and easy way to do this in AutoCAD.

In your map, you select your starting point from a contour, and draw a circle (C) with a radius of your chosen road slope – i.e., if the road should be sloping at 1 by 11, set the radius to 11 meters. From the point this circle crosses the next contour, in the preferred direction, continue this process.

 

This is also a great visual reminder that the gentler your slope is, the longer your road will be.

When all the contours have been circled, you can draw a rough line between each point. This marks the center line of the road.

Rough center line.

Now you can create curves where the road turns. I converted the polyline into a spline here, however a better approach is to use polylines and connect them with arcs (ref. the comment section below). The curve radiuses on these should be between 25-50. Afterwards, use MEASURE to add points along the line, for instance every ten meters, and annotate with cote heights (I cheated in this example to skip the math – my map had no Z values). Offset (O) from the line your given road width, add sidewalks if required, and the road is ready for your road engineer to do the necessary calculations.

Road complete for review and calculations.

Filed under: AutoCAD, Studies, Tricks of the trade, ,

9 Responses

  1. Knut Hallgeir Wik says:

    Good tip, Daniel. This really is a good start to see what is possible in a steep terrain.
    Just wanted to add something. You could do more before handing it over to the road engineer. Instead of a spline, a polyline with arcs is more useful in the prosess following next. Both in your own work and for the engineer.
    And if you went on in Civil 3D you could pick up this centerline as an Alignment and start working dynamcially with the side-terrain for the road. And then you are in total control of the road

    • Daniel Ewald says:

      Thanks for the input, Knut! Appreciated – and you’re totally right about the splines – I started writing about radiuses and then my notes lacked what you just mentioned 🙂 I’ll update the post to make it clearer.

  2. karlbarrett says:

    That’s really helpful. Thanks! Be great to see more tutorials they are so helpful. :0)

    • Daniel Ewald says:

      Happy to hear it! I’ll be sure to include some more tips and tricks on the blog, while they’re still fresh – I’ve found it’s the very best way to remember them 🙂

      • karlbarrett says:

        Yes me too. I’d be lost without my blog. It was never intended to have any type of audience just a way I could reflect on my work and remember! ;o)

  3. Stephen says:

    How do you determine the radius of the circle using a given slope percentage? e.g. Designing a 10% slope, 15% slope… etc
    Do contour intervals need to be considered?

    • Daniel Ewald says:

      By converting the percentage to a 1:G gradient first: G = (100/P%). You still place your road according to the contour intervals – the circles meet the contours and thus marks the point from where you place your next circle. Of course, this assumes you are working with as little terrain intervention as possible. If not, you can align your circles more freely and adjust your terrain afterwards.

  4. Clive says:

    I have been looking for this answer for some while (i.e. radiuses)!

    I am starting with a 3D terrain in Google Sketchup, with 1m intervals.

    To what extent, if at all, can this placing process be automated please?

    Thanks

    • Daniel Ewald says:

      Glad to hear you find my post useful.

      I’m not sure how to automate this process, if it indeed is possible.You still have to consider the direction in which you want your road to turn, and I assume developing an automated script for this would be challenging.

      However, this method is for placing a road with as little terrain intervention as possible – and of course, you will rarely be working on a project allowing the space required to plan roads so gently. In more usual circumstances, one can plot out the course more according to given limitations and use the array function to place your circles in a preferred direction, or along a path, adjusting the terrain afterwards.

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