In tough economic times, you need more choices; it’s often difficult for an employer to justify time for training, and we want you to have an avenue for finding answers when you need them. Our tutorials aren’t product demonstrations. They’ve been created in response to client questions and concerns arising from the use of AutoCAD Civil 3D, as well as other technologies, in “the real world.”

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Join me in welcoming the Grade Extension by Reference tool in Civil 3D 2010.

Grade Extension by Reference

From Help:
Specifies the elevations of a feature line, survey figure, parcel line, or 3D polyline by extending the grade of a segment on another feature.

Scott McEachron
www.121pcs.net

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The first triangulated irregular network program was written by Randolph Franklin at Simon Fraser University in 1973 and was based upon the concept of Delaunay Triangulation. This method of triangulation is how Civil 3D builds a TIN.

The triangulation method was invented by Boris Delaunay in 1934. Based upon his definition, the circumcircle (The circle that passes through all vertices of a polygon, in this case, a triangle) of a triangle formed by three points can’t contain points (or vertices) other than the three that define it. Other points are only permitted at the perimeter, or “border” of the surface.

Circumcircle

Delaunay Triangulation

The usefulness of Delaunay Triangulation is only as good as the points mathematically analyzed. More points = better triangulation (to a degree, of course). For our purposes, that means the more representative the shots taken in the field are to the actual terrain, the better the triangulation. Bad shots = bad triangulation. Too few shots = bad triangulation. The goal of a good topographical survey is to collect as much information about feature changes as possible. In Civil 3D, the points are analyzed by various mathematical algorithms, and the result is Delaunay Triangulation. Breaklines and borders are added manually (or via figure commands) as a means of establishing the path at least two vertices of a triangle must take, even if that leads to a Delaunay violation.

The algorithms that do the math do the hard part. It’s human intervention that adds the accuracy to the network. Notice I’ve not made a distinction between “existing” triangles, or “proposed” triangles. The software doesn’t care, it’s just doing mathematical analysis. Therefore, when modeling terrain, you can use all the tools at your disposal to model both. In modeling existing ground, you are limited by features that physically exist. In modeling proposed ground, you’re limited only by your creative approach and knowledge of both the software, and the analysis tools it uses.

Civil 3D Library
Looking for Grading & Terrain Modeling tutorials? Check out The DTM Branch in the Library. These downloads aren’t software demonstrations. They were created for clients using AutoCAD Civil 3D in the real world. Registration is required.

©2010 Paving the Way. All Rights Reserved.

To download Google Earth, click here.

Cheers!
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If you’ve used the 2008 or 2009 edition, we’d appreciate your feedback TODAY as we’ve already started. Drop a comment here, or at scott.mceachron@121pcs.net.

Scott McEachron
www.121pcs.net

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The first step in the process is to import the data. There are several methods for initializing the dialog box, but I use the MAPIMPORT command. I created a quick (and I do mean quick – it may not be the prettiest thing you’ve seen, but it’ll get the point across) video that outlines the process:

Step 1: Import data

The second step is to save the drawing, start a new drawing, attach the first using the MAP Taskpane, query the contours, and physically alter their elevations using the object data. I’ve created a short video that outlines the process:

Step 2: Attach, Query, Alter Properties

Once you get the hang of it, the process of importing shapefiles is painless and exceptionally powerful.

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Generic Links

Think about this:  You’re modeling a complex object, or a complex site.  You’ve tried to work with grading objects all day, but you’re crashing.  But based upon yesterday’s post, the software is simply trying to building triangles, and you’ve introduced something… well… unusual.  It would be nearly impossible to trouble-shoot every place one could possibly send the software into a mathematical tailspin, so you may as well plan for it – plan for the worst, hope for the best.  Software builds triangles, in order to manipulate triangles we create breaklines.  Corridors create feature lines, feature lines can be extracted from corridors, and with a right click they become breaklines.  So if you find yourself in a complex grading situation, get back to basics.  Grading objects not working out for you so much?  Try building a corridor with generic links.  The pond below crunches all day – curve, reverse curve, compound curve, into a reverse curve, all tangent to one another and the object closes.  If the object is too small, you’ll hit a tight spot in the small curves and if you’re not careful, you’ll spin your wheels for hours trying to figure out what’s wrong.

Generic Link Pond

I created the pond above with generic links.  Doesn’t that mean I had to create an alignment and profile the pond?  Yup.  Is there anything wrong with that?  Nope.

I’ve had one common question in the past, and it’s related directly to the pond:  Why can’t I add curves to a terrain model?  When I grade around a curb and gutter, can’t I use a polyline with a curve in it?  Nope.  Why not?

Triangles aren’t round.

Scott McEachron
www.121pcs.net

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As far as I’m concerned, an exercise in grading needs to begin with an exercise in terrain modeling. Whether existing or proposed ground – it’s all about the breaklines.  If you’ve ever had the pleasure of manually creating contours on the board from a grid of points based upon hand-written notes in a field book, then chances are you’ll have a better understanding of how breaklines work (if you do, then you’ll likely remember how to construct a circle through 3 points – the old way). 

Imagine four points that define the four corners of a square.  When connected by lines, any three points define a triangle. You can create 2 triangles in a square, and there are two possible solutions – either the hypotenuse of the triangles runs southwest to northeast, or northwest to southeast.  The algorithm running the math behind the scenes builds triangles, but it’s human intervention that ultimately determines triangle direction.  Terrain modeling isn’t a science, it’s an art; you can’t let the software run wild building triangles, you need to help define their direction based upon your understanding of field conditions.

What do breaklines do?  Breaklines effectively tell the terrain modeler “do not cross this path with a triangle.”  Take a look at the following images, notice the triangles (in the image to the right, the edges are covered by the yellow breakline):

But take a closer look at the shots, they’re CL (centerline) shots.  You and I know the typical roadway has a well defined crown (unless you’re in Texas where it’s common to see a well defined parabolic crown), but how does the software know?  It doesn’t.  How do you tell the software to create a crown?  You create a breakline.  Notice the orientation of the triangles after the addition of a breakline to the model.

Notice how the contours more accurately represent ground conditions? That’s what terrain modeling is about – you’re a triangle traffic cop.

So it sounds like I’ve nailed existing ground, but what about modeling proposed ground – is there a difference? Absolutely not. It’s still about directing triangles. Sometimes simple polylines work, sometimes we use feature lines, sometimes we use points, sometimes we flip faces or swap edges (telling triangles to direct themselves in the opposite direction), sometimes we use hard core grading objects or corridor models. There isn’t a right way or a wrong way – there isn’t necessarily a right tool, or a wrong tool.

I think often the problem is this: you don’t know what you don’t know. So as you’ve been trained to do (I call it “the way we’ve always done it” syndrome), you contact your local reseller to take a class in grading. And 9 times out of 10, that’s exactly what you’ll get – grading… sites, feature lines, grading objects – how to push buttons. But if the course were instead a study in “terrain modeling,” you’d open your eyes to a whole new way of approaching digital land development, of which “grading” is a small part.

Civil 3D Library
Looking for Grading & Terrain Modeling tutorials? Check out The DTM Branch in the Library. These downloads aren’t software demonstrations. They were created for clients using AutoCAD Civil 3D in the real world. Registration is required.

©2010 Paving the Way. All Rights Reserved.

SRTM = Shuttle Radar Topography Mission, it’s what drives the topo behind Google Earth.

On February 11, 2000, the Shuttle Radar Topography Mission payload onboard Space Shuttle Endeavour launched into space. Leica made the EDM, Daimler Crysler Aerospace made the radars. With its radars sweeping most of the Earth’s surfaces, SRTM acquired enough data during its ten days of operation to obtain the most complete near-global high-resolution database of the Earth’s topography. A 10 day mission almost 9 years ago. How often is the data updated? It’s not… for the most part. While the data may be more accurate than the standard DEM derived from a USGS QUAD, it’s simply not practical for design use. The project was aiming to maintain an accuracy of 16 meters vertically.

Keep in mind, the SRTM project had no ground control. Up and over every building, every highway, every house, every tree. Several algorithms were used to interpolate data in void areas (as well as other areas), but make no mistake, the areas were interpolated.

Google Earth offers us an incredible resource for downloading topo data, but be advised, while the availability of the data is significant for many reasons, the data has been massaged, interpolated, massaged again, and algorithmed to death.

Feeling rather geeky? Check out this article by Frank Clowes of Penn State University (Update – July 25, 2009: This article is no longer available). At the bottom of the page, you’ll also find a link to an SRTM kmz file for use in Google Earth that will give you direct access to the SRTM data. Keep in mind the formatting is a recognized USGS format.

To download Google Earth, click here.

Civil 3D Library
Looking for Grading & Terrain Modeling tutorials? Check out The DTM Branch in the Library. These downloads aren’t software demonstrations. They were created for clients using AutoCAD Civil 3D in the real world. Registration is required.

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1. Civil 3D adoption is catching on as people realize the strength in the software.
2. Typically the end of the year/winter is a great time to plan/implement changes in production (typically due to the weather).
3. Vista, 64-bit computing, and new Autodesk products are right around the corner.

Someone once told me there are approximately 200 changes in the software with each new release (quite likely the same person that told me the average user pans and zooms 700 times in an 8 hour day – quit typing/picking and use the wheel). I’ve said it before – I think Civil 3D 2007 is production ready, but your success is directly proportional to the openness of your mind; Civil 3D is not an upgrade – I’m amazed by it every day.

TIN Volume surfaces, for example: A TIN volume (we used the “Composite Method” to create this in Land Desktop) surface provides an exact difference between the base and comparison surfaces. Therefore, the Z value of any point in the volume surface is precisely the difference between the Z of the comparison surface at that point and the base surface at that point. This is true whether the comparison and base surfaces are both grid or both TIN, or one of each. If you display and label the contours of a TIN Volume surface, it stands to reason the labels will reflect the depth of cut/fill.

But a volume surface is a persistent object – it’s dynamic. If either your base or your proposed surface changes, so will your Volume surface. But what are the ramifications? If you display your TIN Volume surface as a colored map of cut vs. fill, and place a legend in your drawing. Then as your existing and/or proposed surfaces change, so will the map, and so will the legend – dynamically.

Cheers!

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