
Vertical Curve Question
I have a question about the true geometric nature of a handdrawn vertical curve as I would construct one in a profile in Autocad.
My PVI is in the middle of a 70' long symetrical vertical curve. The back tangent is positive 1% the forward tangent is negative 1%.
I calculated the actual elevation in the middle of the curve by calculating the parabola constant as follows: Constant = ftangent grade rate  btangent grade rate / 2 x Length. This gives me a constant of 0.014.
I then take the constant (0.014) and multiply it by L/2 squared, giving me a value of 0.24
I passed an arc between the BVC, 0.24 below PVI (midpoint) and the EVC.
Then I went to calculate a tangent offset at 20' in from the BVC which I did as follows.
Tangent offset = Parabola Constant x 20' squared which rounds to 0.08
When I dimension a tangent offset from my grade rate to curve in Autocad I am getting 0.12 as opposed to 0.08
Vertical Curve Calculations are relatively new to me and my major concern is whether or not I can geometrically construct the curve in the manner I did in the profile: Arc > Begin Point (BVC) > Midpoint (Tangent offset at PVI) Endpoint (EVC).
I have no idea what I am doing wrong here, and no one in my office can seem to give me a good answer. Any help would be greatly appreciated for this road design novice. Thanks.


I would trust your calulations, not what you're getting in AutoCAD. A vertical curve can not be represented by a circular arc, if I understand what you're doing. I believe the most accurate way o draw it is to plot a lot of points, then draw a pline through the point, then "pedit "fit".
A much quicker way, & supposedly accurate, is to draw a polyline from endpoint to VPI to endpoint, then PEDIT>>Spline. For it to be a proper parabola, set SPLINETYPE to 5 before pediting, and to get a smooth curve set SPLINESEGS to 15 or higher


Thanks for your response. So what your saying is that effectively a vertical curve is closer to an infinite amount of joined line segements as opposed to one continuous entity.


Yes, since Autocad doesn't have a "parabola" entity. maybe in 2008?
but a "spline fit" polyline with SPLINETYPE set to 5 apparently is a parabolic shape (I learned this from others  tested it once & it seemed to be accurate).

Posting Permissions
 You may not post new threads
 You may not post replies
 You may not post attachments
 You may not edit your posts

Forum Rules