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Drafting Spiral Curves From Old Plan Data

Here are steps that can be followed to draft spirals in MicroStation with data that is usually available on old plans. Refer to Spiral Curve Data for definitions of spiral curve terminology. It should be noted that, depending on the accuracy of the plan data, this method may not work as expected.

Given:

Δ (delta angle)
Dc (Ds)
Ts
Ls
Lc

  1. Extend a straight line from the TS point in the same direction as the tangent line a distance of Ts.
  2. Make a copy of this line and rotate it by 180°-Δ. This will give you the tangents for the entire spiral & curve combination.
  3. place spiral tool imageUse the Place Spiral tool on the Create Curves toolbar (shown at right) to draft the first spiral section. If this toolbar isn't displayed, it can be found under Tools > B-spline Curves > Create Curves.

    create curves toolType = Clothoid

    initial degree = 0 (tangent)

    final degree = Dc (Ds)

    Length = Ls
  4. You will be prompted for the start point, select the TS location. Next click at the opposite end of the tangent line. The third point can be an approximate location of the end of the spiral. It defines the direction of the curve.
  5. Repeat step 4 for the spiral at the other end of the curve.
  6. Use the Place Arc tool with the Edge method to draft the circular portion of the curve. Specify the curve length (Lc) and start/end points of the curve. If you receive an "invalid element" message, it may be due to rounding of the Lc value when it was computed for the old plans. You may have to adjust this value to get the curve to plot.

New Hampshire Department of Transportation
PO Box 483  |  7 Hazen Drive  |  
Concord, NH  |  03302-0483
Tel: 603.271.3734  |  Fax: 603.271.3914