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Terrain Models in Land Desktop

By Brian Morse, December 12, 2003

Is it worth the effort to build accurate terrain models? Yes, it is. I think of all the automation features in Land Desktop and Civil Design that you can use when you have a terrain model to work with - Profiles, Cross Sections, Spot Elevation Labels, and Volume Reports, just to name a few.

Shaded 3D faces from a terrain model of a road intersection, created using Autodesk Land Desktop and Civil Design. Click on image to enlarge.

Accurate Existing Conditions

One of your most important investments in a project is the creation of an accurate terrain model of existing conditions. This is your foundation for the rest of your design decisions throughout the project. For example, knowing where the existing highs and lows are on your site helps you evaluate current storm water runoff characteristics.

Road Centerline Profile showing existing ground and proposed alignment. Click on image to enlarge.

Do it right, or not at all
It is obvious that you expose yourself to liability when you base your design decisions on false information. An inaccurate terrain model is false information. It is a waste of time trying to automate the creation of profiles when the underlying data is not correct. Like placing a building on an unstable foundation, you are headed for larger more expensive problems at some point in the future. Put the effort into making the existing terrain model represent the truth. The benefits will follow you through the rest of the project.

It’s worth the effort
After hearing or experiencing the horror of bogus terrain models many people stop right here and decide to use manual methods of representing existing conditions through the use of spot elevations and contours. They ask, “Why make this great effort to produce a terrain model only to have it represent unusable junk?” They are right. Don’t build a terrain model from original survey points alone. Use breaklines to help the terrain modeler to correctly interpret the existing conditions. Generate 3D faces and view them using the 3D Orbit command to check for defects. Does it sound like more work? It is at the beginning of a project when you are processing the initial mapping data. Is it worth the extra effort? Unquestionably, YES. You quickly recoup that initial time investment many times over in time saved later. When it comes time to make design changes, calculate volumes or cut cross sections you will save enormous amounts of time and effort.

Plan view of a terrain model of a road intersection.

Finished Ground

A large reason for developing an accurate terrain model of existing conditions is to assess the flow of water over a site. In your final design, you can effectively utilize the naturally occurring water flow characteristics thereby reducing the impact and cost of your development.

I can do it faster by hand
Traditional site grading techniques used a manual process of calculating individual spot elevations or hand drawing contours. Grading a site using these methods can take some people less time than using Land Desktop for the first draft. I have heard many people say, "I can do it faster by hand." Think about this for a moment. When you take the simpler and sometimes faster approach for the first draft of your grading plan you are cutting yourself off from the far more efficient editing tools to produce the second or third draft. You leave yourself with no option to use the tools for automatically regenerating Profiles, Cross Sections, or Contours. After grading the site (using slower manual methods) you are faced with tedious volume calculations by hand.

Shaded 3D Faces from a Terrain Model of a Road. Grading was done using Road Templates in Civil Design. The skirting feature produces the visual effect of slicing through the earth's crust. Elevation Banding with Color Coding helps you visualize the highs and lows much better than contours. Click on image to enlarge.

Seeing is believing
Many people just cannot visualize what a site will look like by looking at a plan with spot elevations and contours. With a terrain model you can create a 3D view. Now you have the ability to display the site at any angle and in 3D. Now you have a greater impact on people’s understanding of either existing conditions or design concepts. Viewing your site in 3D immediately brings design flaws to your attention. Avoid costly mistakes, client misunderstandings, contractor’s “field adjustments”, and missed deadlines due to last minute revisions.

The Payoff
When you have developed accurate terrain models you avoid the drudgery of manual drafting and design. Compare the amount of effort you spend editing spot elevations and contours to reflect a design change. Using Civil Design to produce Profiles and Cross Sections based on an accurate terrain model are much faster than creating them manually. Comparing grading techniques using manually generated spot elevations to those using Civil Design's Grading Object is like comparing small arms firepower to nuclear weapons.

Land Desktop provides evaluation tools to check your work, editing tools to modify your work rapidly, and automated annotation tools which derive their display data from the terrain model. Tying the Finished Ground terrain model to existing conditions (a.k.a. daylighting) is automated through a variety of commands in Land Desktop and Civil Design.

You can evaluate more design options when there is less effort involved in making design changes. Balancing volumes on a site is based on facts rather than gut feelings. You have proof that the site volumes balance because you are comparing accurate terrain models. You are evaluating the entire site not just a few cross sections. You can choose any location to place a spot elevation and feel confident that it is correct.

Getting Started
Many people own Land Desktop and do not use it. Why not? <insert clever excuse here> Learning the techniques of terrain modeling is a continuous process. You build one skill upon another. The single most important step is to get your learning process started.

About the Author

Brian Morse is a Certified Autodesk Instructor specializing in the use of Autodesk Software in the Civil Engineering and Land Surveying Professions. Over the past 10 years Mr. Morse has handled more than 7,000 support calls and contributed technical solutions to the Autodesk online knowledge base, while working for Softdesk/Autodesk technical support. He has trained some 800 students in the past 4 years while working for a major Autodesk reseller and now as an independent consultant. Mr. Morse has plenty of real world Civil/Survey experience reaching as far back as 1983. He has performed and managed topographic and boundary surveys, produced hand-drafted and CADD drawings for residential and commercial land development projects continuously over the past 20 years. Information about his services is available at www.BrianMorse.com.

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