the art of digital terrain modeling - welcome | augicv31-2 digital terrain modeling combines aspects...

December 2-5, 2003 ◊ MGM Grand Hotel Las Vegas

The Art of Digital Terrain Modeling

Gary Rosen

CV31-2 Digital Terrain Modeling combines aspects of art and science to develop 3D representations of land surfaces, both existing conditions, and proposed designs. Learn a wide range of tips and tricks, methods, and techniques to unlock the magic that is the inevitable future of the civil design industry.

About the Speaker: Gary Rosen is president of Electric Pelican Ink CADD Consulting Services, and is a 24-year veteran of the civil/survey industry. He has spent 16 of those years working with Softdesk and Autodesk software. Gary spends most days providing on-site consulting and training to “Land Nuts” in New England. He is the author of INSIDE Softdesk Civil, Learning Land Desktop, and two training videos, Autodesk Land Desktop in a Nutshell, the Movie, and Autodesk Civil Design in a Nutshell, the Movie. [email protected]


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An Opening Thought Digital Terrain Modeling is nothing less than the central key to the future of Civil Engineering and Surveying Computer Aided Design and Drafting. While nearly all applications designed for this mission have the ability to perform the task, the vast majority of the actual implementation of this function in the industry is relegated to the mundane task of generating contours in drawings that represent existing conditions. Only a redefinition of the industry with the total implementation of Digital Terrain Modeling at its core will have the richness of data required to unlock the true potential of the awesome technology that is just over the horizon. So let’s go for it. Overview – Understanding LDT Surfaces In all of the previous lessons in this seminar we have covered the theory and practice, form, and function of many different critical aspects and components of LDT. It is the implementation of Surfaces, however, that drives a vast array of LDT functionality. Whether creating or modifying Surfaces, or deriving information from defined Surfaces, such as Profiles, Cross Sections, and Volumetric Calculations, much of the functionality in LDT, and almost all of the functionality within Civil Design, involves their use. For this reason a solid working knowledge of LDT Surface theory and practice is critical to the successful implementation of the product line. Existing or Proposed? A very fundamental concept to understand about LDT Surfaces is that LDT draws no distinction between digital terrain models that represent existing conditions in the physical world, and digital terrain models that represent proposed design surfaces that have not yet been constructed. All of the rules and techniques of digital terrain modeling are applied to both of these equally. Surfaces are also known as DTM’s , or Digital Terrain Models, or TIN’s, Triangulated Irregular Networks. Surface modeling within LDT is a process of three-dimensional triangulation of distinct points in space. This point data can be generated from a wide variety of sources, but ultimately all of the points are distinctly connected in three-dimensional space. Therefore, it is the triangulation that determines the actual shape of the Surface.


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GIGO and QIGO Any given collection of 3-D points can generate a nearly infinite number of variations in triangulation, and it is the triangulation that determines the shape of the Surface. In Surface triangulation, there is a unique phenomenon taking place. Many of you are probably familiar with the very old and fundamental computer concept known as GIGO, or Garbage In Garbage Out, meaning if you put bad data into the system, bad data will come out. The idea holds true to this day in LDT, and for that matter, I think it is safe to say for all other computer applications as well. In LDT, however, there is another possibility, which I will call QIGO. Quality In Garbage Out. A perfectly accurate collection of survey points can yield a digital terrain model that does not accurately resemble the original ground, if the point data has not been triangulated, connected in three dimensional space, correctly. If two shots taken at the opposite edges of a crowned roadway wind up directly connected to each other in the TIN, the crown of the roadway is missing at that location. The points along the crown need to be directly connected to each other with Surface triangulation to form the crown line along the length of the roadway. Forcing specific points to be connected in the TIN is accomplished through the use of Breaklines. I started working on a Survey crew in 1979, and it was a very different industry then, but an interesting thing to realize is that now, as then, the whole process of accurately representing an existing ground surface still starts with the decision made by the field crew of where to collect the data. Even though those locations can now be collected with standard optical instruments, a robotic collector, reflectorless technology, or GPS, the “shots” taken is where the process all starts, whether it ends in contours drawn on paper with a pencil, or a digital terrain model of the surface stored within the memory of a computer. Once created, Surfaces can also be visualized using a wide assortment of methods, including Contours, Polyface Meshes, Elevation Ranges, Slope Ranges, and 3D Grids. Surfaces can also be passed into other applications, like 3D Studio VIZ or MAX, to be incorporated into photo-realistic renderings.


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TME Using the Terrain Model Explorer The Terrain Model Explorer is where the majority of the activities related to Surface Modeling take place. In fact, many critical parts of the process can only be accessed here. However, some related Terrain functionality is available exclusively from the Terrain pulldown menu, and some is accessible from both places. The Terrain Model Explorer is a Windows Explorer-type user interface, designed to make working with Surfaces more intuitive, and make viewing of Surface data more accessible. Open the drawing Terrain1, which is associated with the Lesson 18 Project dataset. This Project has a populated Point Database, and a Point Group called TOPO to be used to build a Surface. On the Terrain pulldown menu, pick the first item, Terrain Model Explorer. The TME will appear on the left side of the screen. The left side of the TME contains a tree-structured display of the Project’s Surface data.

The Terrain Model Explorer. Note that there are 2 major categories of Surfaces, Terrain and Volume. Terrain Surfaces are any Surfaces defined by the user. As described in the introduction of this lesson, there is no distinction in LDT between Surfaces based on data representing existing conditions and Surfaces comprised of data designed to represent proposed design improvements. They are all considered Terrain Surfaces. Volume Surfaces are developed by the software when doing volumetric calculations. The right side of the TME displays more specific information about whatever item is selected on the left side. Right-click on the word Terrain. On the cursor menu, pick Create New Surface. LDT generates a new Surface and calls it Surface1. This is actually a new folder in the current Project’s DTM folder. The new folder contains files designed to receive and store the Surface Data supplied by the user.


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The right side of the TME shows the new Surface and lists it’s Status as having “No Data”.

The initial conditions of a new Surface. Pick on the expand box (with the small plus sign in it) next to the Surface name on the left side of the TME. A tree structure unfolds with 3 branches; TIN data, Edit History and Watershed. The TIN Data branch contains six branches, one for each of the six categories of TIN data discussed above.

Further expansion of the Surface data tree. Right-click on each of the six types of TIN data. A different cursor menu appears for each one. Each is designed with options to work with that type of data. Some of the cursor menus have only one item to choose, others have more. Notice under “Point Files” the “Add Points from AutoCAD objects” item is followed by a small black triangle. This indicates an additional flyout menu is accessed by that choice.


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The Point Group cascaded menu.

The Point Files cascaded menus.

The DEM Files cascaded menu.


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The Contours cascaded menu.

The Breaklines cascaded menus.


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The Boundaries cascaded menu. Right-click on the name of the Surface on the left side of the TME. On the cursor menu, pick Rename. Rename the Surface to EG1, (for Existing Ground). Pick OK. The Surface is renamed, as are the corresponding external data files designed to hold the TIN Data for the Surface.

Renaming Surface1 to EG1. TIN Data is the raw material that Surfaces are constructed from. There are currently six distinct categories of TIN Data available.

Point Groups – Point Groups are created within the Point Group Manager, and then selected in the Terrain Model Explorer. Point Groups are carefully constructed to include only those Points that are supposed to contribute to the Surface Model, excluding all others. A single Surface can have any number of Point Groups specified for use.

Point Files – There are two sub-categories of Point Files. The first is an ASCII text file, listing some combination of Point Numbers, Northings, Eastings, Elevations and/or Descriptions of a set of Points. These files can be Imported directly into TIN Data files, bypassing their entry in the Project Point


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Database. The potential drawback of this method is that all Points listed in the file are used as Surface Data, and so no filtering is possible.

The other sub-category of Point File data is AutoCAD Objects. AutoCAD Points, Lines, Blocks, Text, 3D Faces, and Polyfaces can be entered in here as Surface data. The critical coordinates of each of these is used as Surface Data, i.e. the endpoints of the Lines, the Insertion point of the Blocks and/or text, the corners of the 3Dfaces, the vertices of the Polyface.

DEM Files – These are Digital Elevation Model files, available from the USGS, corresponding to most if not all of the US Quads. These files can be easily downloaded from the web and Imported directly into the LDT Terrain Model Explorer and used as Surface Data to build a Digital Terrain Model of that area.

Contours – AutoCAD polylines, 2D or 3D, or LDD/LDT Contour Objects can be entered here as Surface data. The vertices of the objects are used essentially as a large collection of point data. In addition, if the “Create as contour data” box is checked in the Contour Weeding dialog box, the contours will be treated as Breaklines. This can help generate a more accurate Surface. Contour weeding and supplementing is the process of removing unnecessary vertices from very densely populated polylines with many close vertices, or adding data to scarcely populated polylines with few vertices and large distances between them. The process is unique to Contour data.

While Contours are still a common source of Surface data, they do have several inherent problems. The first is that they often generate a vast amount of data, and so produce very large Surface files, which can be slower and generally more difficult to work with. The other problem is that Contour data alone produces flat spots at the bottom of all of the low areas, and at the top of all of the high areas. Similarly, points on contours representing a ditch or berm type of feature tend to interpolate to other points on the same contours, with of course the same elevations, causing flat spots on the Surface. LDT can minimize this problem by selecting the “Minimize flat triangles resulting from contour data” check box in the Build Surface dialog box. After an initial set of Contour is entered, if you enter more contour data you will prompted as to whether this additional data be added to the Contour Data Surface file, or overwrite it. There is also an option under Contour data to simply Delete all Contour data, and start over. Breaklines - Also known as Faults in earlier versions of the software, are essentially AutoCAD 3D Polylines. Breaklines serve two purposes. First, the vertices of the polylines are used as a data source. Secondly, Breaklines force the triangulation of the Surface to occur in a certain direction. Points connected by Breaklines must be connected as edges of triangles in the Surface, thereby forcing interpolation along them. Therefore, if the vertices of the 3D Polyline connect Points that are already in the Project Point database, the Breakline is contributing no new data. In this case, it is only the forced shaping of the Surface by the Breaklines that makes them so valuable. Any given set of 3D points can generate a wide variety of Surfaces, simply based on how the points are connected. The more points, the more combinations that are possible. Breaklines force the


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interpolation of the data to better represent the desired Digital Terrain Model. This can be a model of an existing condition, or a model of a proposed improvement. A common method used to define Breaklines is to connect existing Project Points. Alternatively, existing polylines can be used to define Breaklines. Using either method defines Breakline data in external files. Once Defined, Breaklines can therefore be Listed, Imported, or Edited from any Drawing attached to the Project Dataset. Boundaries - The final category of TIN Data that can be used to influence the Surface are Boundaries. There are 3 types of Boundaries: Outer, Hide and Show. Outer boundaries are placed around a Surface, and are used to eliminate incorrect interpolations between points at the perimeter of the data source. Any TIN line an Outer Boundary crosses is deleted from the finished Surface. Hide Boundaries create holes in the Surface. If a Hide Boundary is used to “Make Breaklines”, the resultant hole in the TIN will be the exact shape of the Boundary. This is useful for house sills or bodies of water. Show Boundaries bring back part of a Surface that falls inside a Hide Boundary, as in an island condition. All six of these TIN Data types can work in concert to provide Surface data to the Digital Terrain Modeler, as long as they don’t contradict each other. If two data sources define the same Horizontal location with different elevations, the Surface will be flawed and unexpected results may occur. The six categories of acceptable TIN Data are like the six food groups that the Digital Terrain Modeler can digest. You just have to figure out which food group(s) your data fit into.


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The Key Components and Capabilities

Building Surfaces

“Building” a Surface is the process of producing a Triangulated Irregular Network of interpolations between groups of points in three-dimensional space. The coordinates of these points are those supplied by all of the TIN Data used in the process.

Surface Breaklines Breaklines are added to a Surface to force the triangulation to occur along distinct linear features on the ground, such as edges of pavement, flowlines of ditches or streams, tops or toes of slopes, crowns of roadways, or curbs. To represent these types of features accurately, the triangulation of the Surface must form along them, not across them. Once the TIN Data is specified, the Surface is Built, which is essentially the triangulation of that data. The algorithm within the modeling engine connects the supplied TIN Data points based on proximity. Based on the juxtaposition of the data points used, Surface triangulation may form along linear features naturally. However, if the triangulation does not follow these types of features, it must be forced to do so by the addition of user-defined Breaklines.


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Surface Boundaries LDT supports three types of Surface Boundaries - Outer, Hide, and Show Boundaries. Outer Boundaries, as the name implies, are used to limit the interpolations between points at the outer edges of a Surface. Surface triangulation will commonly take place between these points by “jumping” across areas with no data, forming areas of the Surface that are incorrect. These interpolations need to be eliminated, and an Outer Boundary is the best method available to accomplish this. Outer Boundaries can either be drawn well outside of the Surface, coming in only to cross Surface Lines to be deleted, or they can be drawn directly on top of the outermost Surface Lines that are to be retained. Hide Boundaries are used inside the body of the Surface to create a hole in the Surface of a specific shape. Two common uses of Hide Boundaries are for building footprints, or bodies of water. Show Boundaries are used to reveal Surface areas inside of empty areas created by Hide Boundaries.


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Surface Editing Although Digital Terrain Modeling in LDT has many different tools and techniques that may be employed, it is still a sequential process, regardless. The first step is to specify what data is to be used as a source for the model. This is known as TIN data. The next step is called “Build”, which is the process of 3-dimensional triangualtion of the specified data to construct a Triangulated Irregular Network, or TIN. These first 2 steps are mandatory, but there is a third optional step, which is known as Surface Editing. Surface Edits are executed after a Surface is Built, for the purpose of changing the triangulation that was derived from the Building process itself. When a Surface is Edited, it is changed in real time. It does not need to be re-Built to incorporate the change.


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Surface Borders and Sections A Surface Border is generated by LDT, and shows the size, shape and location of a Surface. You could think of it as a shrink-wrap of the Surface. While Surface Borders and Surface Boundaries share a very similar name, they are in fact totally different in use and intent. Surface Boundaries are drawn and defined by the user, for the reasons covered in Lesson 21. Surface Borders can only be generated automatically by LDT, based on defined Surfaces. The only user input involved is the decision whether to generate the Border as sets of 2D or 3D lines, or as single 2D or 3D polylines. A very useful feature in LDT R3 and 2004 is the ability to generate a dynamic section through a surface from any simple AutoCAD geometry, such as a line, arc, polyline, or even a circle. The section illustrates the terrain through a surface directly over the selected object. The section is also dynamic, in that, if the sampling object is moved or edited, the section is updated automatically. The section appears in a view window, but can be imported into the drawing file as real geometry. Also in R3 and 2004, if you have defined more than one Surface in the same area, the Section Viewer can display Sections of both Surfaces simultaneously. In R2i, R2 and R1, this dynamic sectioning can be done with a special object called an aecc_section, but it can only display one Surface and there is no way to easily import it into the drawing.


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Contours Topography has been traditionally represented as Contours in maps and civil engineering drawings for a very long time. The Contours represent lines of constant elevation. Once a Surface has been constructed in LDT, it, too, can be represented in a drawing as Contours. Surface Contours are created in LDT either as LDT objects called aecc_contours, or as AutoCAD polylines. Aecc_contours’ appearance and behavior are based on Contour Styles. Each time Contours are generated for a defined Surface, the Contour intervals are set, both a Major, or index, Contour interval, and a Minor, or intermediate, interval.

Slope Analysis Since Digital Terrain Models in LDT are essentially defined as a collection of triangular spaces in 3-dimensional space, it is a relatively straightforward calculation for LDT to determine the slope of any given triangle. This leads to a variety of slope analysis tasks that LDT can perform, such as:

1. Assigning the triangles to groups based on slope 2. Adding arrows to the drawing within each triangle, pointing in the direction of the slope 3. Delineating watersheds and drawing their borders in a drawing 4. Tracking the probable path of a drop of water on a Surface 5. Labeling the Surface slope at any given point, or between 2 points


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Watershed Delineation

Water Drop


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Slope Arrows

Slope Labels


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Slope Ranges as 2D Solids


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Elevation Ranges Elevation Ranging is similar to Slope Ranging, but divides a Surface into slices based on a starting and ending Elevations instead of percent slopes. This can help to visualize the Terrain and easily see what areas are at the same elevation. The process of generating a Grid of 3D Faces is somewhat analogous to draping a mesh over a TIN. The result is a smoother visualization of the Surface than is seen when looking at the actual triangulation itself.


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3D Visualization Civil engineering and Surveying have always been performed in 3D, inasmuch as all the data involved is always three-dimensional, however most graphical representations of this work have traditionally been presented in 2D, from a Plan View. Working with Survey and civil engineering data in a digital environment affords the opportunity to easily start visualizing it in 3D, simply by employing some simple tolls within LDT. This lesson is designed to help you move in that direction with your work. I have personally been involved recently in presenting several projects to local planning boards and technical review boards in 3D, and my prediction is that this is going to rapidly become more the norm than an anomaly.

the art of digital terrain modeling - welcome | augicv31-2 digital terrain modeling combines aspects...

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