# Grade Modeling a Simple Intersection

In the video below we show how to create a simple grade model at a T Intersection using TBC tools and including an improved version of the Adjust Linestring Elevation command that will be released shortly.

The challenge with Intersections is determining what controls what i.e. what has the elevation control in the intersection. In most cases if you have a main line through the intersection, normally the centerline and cross slopes to Edge of pave control the elevations of the side road at the point where the two roads connect.

In this case we create the two alignments as linestrings, then we offset the two alignments by e.g. 12 for the Edge of pavement and whatever cross slope may be in play on the two roads. However where the 5 key lines (CL and EP on the mainline and CL and 2x EP on the side road) intersect you need to determine first what elevations you want at the 6 points where the lines cross.

Then you do Fillet arcs to put in the curb returns. These arcs will take elevations at the PC and PT points based on the source lines used to create the fillet arcs. This is a good start, however if there are slopes at play, which there will be, then the elevations along the arcs while correct at the PC and PT points will create a sag or crest in the curve that is incorrect - why you may ask - the reason is because the elevations along the arc are determined by linear interpolation along the arc vs interpolation on a sloping plane which is most likely what is required - the points that define the plane are the PC, PT and PI points of the arc.

The latest version of the Adjust Linestring command has two new modes - Plane Mode and Surface Mode. The Plane mode allows you to define a sloping plane using the PC, PT, PI points and then “Drape” the arc onto the plane and elevate points along the arc at intervals or using curve to chord tolerance value to better fit the arc to the designers intent.

Video shows you how - the new tools reduce the amount of thinking / hand calcs that you need to do to create better 3D linework for 3D machine models for Parking Islands, C&G in general, Parking bays and more.

2 Likes