Albury, New South Wales
Australia
In one of my earlier posts (on February 23, 2010, to be prcise) , I showed you how you can create a simple TIN model of 3faces or 3d polylines from a set of points. Today, let us take this process a step further by drawing a 3d polyline across the TIN model and draping it over the model. By draping, I mean that it should automatically acquire interpolated Z values (elevations) at each point where it intersects the TIN triangles and create vertices along the polyline at these intersection points. This is a fairly standard and often-asked operation in many civil engineering designs.
It is easy to do this in GeoTools.
We will use the same DWG TIN_Points.DWG that we used for creating the TIN and build on it.
Step 1: If you have not already done it, open the DWG and create a TIN out of it using the 3d Polyline option. For help, click here.
Once you have the TIN containing traiangles of 3d polylines, you will now learn how to use a GeoTools feature to drape a 3d polyline across a TIN (or any set of intersecting polylines) and create a 3d profile polyline. The command to use is GT_DRAPEPOLY and you can find it under GeoTools->Build Tools->Drape a 3D polyline across a set of intersecting linear objects.
The GT_DRAPEPOLY command works on a simple principle. It takes a set of "Source" 3d polylines and compares them against a set of "Target" linear objects (like lines, polylines, arcs etc, could be 2d or 3d) and computes the intersection points between the Source and Target objects. It then adds vertices along the Source 3d polylines at each point where they intersect the Target. The vertices will be interpolated linearly along the segments on which they lie and at their true 3D intersection points. Now, is that not simple?
The GT_DRAPEPOLY command has no dialog box interface. It will just ask you select the Source and Target objects and will set out to do its work, churning out the intersection points and adding them to the polylines. You select the Source objects as shown in the figure below (see dotted [implicit] crossing selection window lines):
and select Target objects as shown in the image below (see [implicit] wndow selection window lines)::
I hope you got the idea now. You select the 3d polylines (Source) that are supposed to cross the set of linear objects like line, polyline, arcs (2d or 3d) etc, which os our Target. Once you hit ENTER, GT_DRAPEPOLY gets down to work and creates the intersections and populates the Source 3d polylines with intersection, interpolated vertices in between. When you grip the Source 3dpolylines, they should look like this (the result):
You can verify the results by listing the 3d polyline and checking the vertices. Now, you are ready to go further by creating a cross-section profile or doing a vertex elevation labeling or anything more you can do in GeoTools. All that and more another day, in another Tip of The Week.
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