The ability to take full advantage of GPS-guided machine control is largely dependent on the availability of accurate three-dimensional digital terrain models (DTMs) of both the existing and proposed (design) surfaces. These DTMs typically come from one of two places: either the surveying or engineering design firm creates and provides them to the contractor, or the contractor creates them (or has them created) from topographic or design digital data or paper drawings (or a combination of both). Let’s examine the typical lifecycle of surveying to construction-use data, from the data collection and reduction phase for existing ground DTM creation, to proposed design DTM creation, to DTM creation for use in stakeout and construction.

Data Collection

Enlarge this picture Figure 1. Description keys defined and uploaded to data collectorssimplify data entry in the field.

The tools for collecting field data for use in creating topographic surveys have come a long way in the last decade. The old rule-of-thumb that it takes an hour of office drafting time for every hour of field time is now obsolete. Firms taking advantage of these tools are seeing dramatic increases in both efficiency and accuracy, completing tasks that used to take a day in only an hour. To realize the results these new technologies can provide, business managers and owners must make the effort to develop surveying- and drafting-related standards and procedures and then to properly train their personnel on the instrumentation, procedures and data analysis. Otherwise, purchasing tens of thousands of dollars worth of hardware and software, and only utilizing a fraction of its capabilities, will result in an insignificant return on investment. Robotic total stations, RTK GPS and modern data collection technology can make the process of topographic surveying much more speedy, accurate and efficient.

Enlarge this picture Figure 2. Figure codes denote connectivity for planimetric featuresand allow post-processing software to "connect the dots" betweenfield shots, automatically creating linework in CAD drawings.

Prior to venturing out to collect field data, various components of the overall “data collection system” must be in place. These include both field and office components. For the field, description codes (or abbreviations) and figure codes (feature codes) must be defined and uploaded to data collectors. Description codes are short (numeric, alpha or alphanumeric) abbreviations that simplify data entry in the field. For example, STMH may be used to describe a shot for a storm sewer manhole. Figure 1 shows the description keys in Autodesk Land Desktop (www.autodesk.com) as an example.

Nearly all surveyors are familiar with description codes and all modern hardware supports description codes; their use is nearly identical from one device to the next. Less familiar are figure codes (similar to feature codes). Figure codes are a subset of the surveying command language and are entered as additional information in the field to denote connectivity for planimetric features, such as the edge of pavement or a gutter line. Entering codes such as BEG (BEGIN), E (END), CONT (CONTINUE) and so on allows the post-processing software to “connect the dots” between field shots, automatically creating linework in CAD drawings. In addition to manmade features, figures are also used to define natural features such as ridges, swales and bank toes. These figures are invaluable in speeding up DTM creation. Figure 2 shows figure codes in Autodesk Land Desktop.

Enlarge this picture Figure 3. Some feature code libaries can be directly accessed ondata collectors through mobile versions of Windows.

Trimble Geomatics Office (TGO) (www.trimble.com) uses libraries of feature codes, which provide a combination of point descriptions and feature type (point or line) functionality. Using data collectors running mobile versions of Windows, field crews select the feature codes from drop-down menus using a touch-screen interface or keypad. Figure 3 shows a portion of the feature code library file in TGO.

A common misconception is that the rodperson must alter his or her movements in order to use figure codes, completing one figure before starting another. With proper training, the movements and methods of the field crews are largely unaltered. Multiple figures can be “open” simultaneously and the rodperson can move randomly to collect shots.

Data Reduction

Enlarge this picture Figure 4. Many software applications transfer data from data collectorsto office PCs.

After the field work is completed, the data moves into the office. Many software applications are available for transferring data from the data collectors to office PCs, and most work on the same principal. A data cable connects the collector to the PC via a COM or USB port and the file is downloaded in a raw, unprocessed format. TGO allows users to download and adjust data collected in the field and prepare it for importing into a CAD program. Autodesk Survey comes with an optional Trimble Link module as well as Survey Pro by Tripod Data Systems (www.tdsway.com) that allows users to transfer data within the AutoCAD environment. Leica Survey Office (www.leica-geosystems.com) includes the Data Exchange Manager, shown in Figure 4 on page 15, for moving data between office and field equipment.

After the data is downloaded, various applications are used to analyze and validate the data. Then the software is used to create the DTM and contours and/or the planimetric features. Depending on the software used, the downloaded data file may need to be converted to a format recognized by the COGO (coordinate geometry) and/or drafting application. For example, .JOB files are converted to .FBK files so that figures can be automatically drafted. Most COGO applications recognize field data in ACSII format (and a myriad of other formats) and can import these files into a project database.

Training and proper implementation of standards and procedures by the field crew is of little value without complementary office processes. For successful and efficient creation of DTMs and linework, various components must be in place prior to importing field data into COGO or drafting applications. These include basic items such as consistent CAD layers (levels); a description key file that will expand the description codes into full descriptions, insert symbols associated with the point (e.g. a manhole) and place these items on the correct layers; a figure prefix library that allows the application to draft figures on the correct layers; and/or point groups that sort COGO points by properties such as description or point number range.

With the proper standards in place, creating existing ground surface DTMs and drafting planimetric features becomes a matter of importing the data files, creating a surface definition using the collected data, and reviewing and editing for accuracy and annotating. For example, using Autodesk Land Desktop and Survey, a DTM can be defined and contoured in minutes. Predefined point groups are used to sort ground shots into a group and breaklines (surface faults) are created directly from figures. The data must be analyzed and adjusted as required and then used to define, create and display an initial view of the surface. Edits to the collected data (e.g. adjustments of erroneous shots), creation of additional data (e.g. additional breaklines) and edits to the triangulated irregular network (TIN) that defines the surface are then used to refine and finalize the surface. Figure 5 shows a TIN of the existing ground in Trimble DTMLink.

Enlarge this picture Figure 5. Edits to the collected data, creation of additional data and edits to theTIN refine and finalize the surface.

Depending on the software application, creating and displaying the DTM may be separate tasks. Typically, displaying the DTM is done after the surface has been defined. A wide variety of display options are available, such as a regular grid of spot elevations; contours at regular elevation intervals; color-coded elevation or slope ranges; and a simple TIN. It’s important to note that in many applications the information displayed is extracted from the underlying DTM. For example, in Trimble DTMLink and Autodesk Civil 3D, contours are a display option of the DTM and dynamically update when the DTM is modified. GEOPAK Site and InRoads Site (www.bentley.com) provide similar functionality. However, in Autodesk Land Desktop, the contours are created separately after the DTM is finished and do not automatically change when the DTM changes. A well-defined DTM will result in accurate display. Conversely, a DTM created from insufficient or erroneous data will result in surface elements displayed differently than what may be expected. The surface creator should avoid the temptation to simply edit the displayed entities and should, rather, modify the underlying DTM to correct any problems with the surface. Figure 6 shows the same surface displayed with contours in Land Desktop.

If a topographic survey of existing conditions is the desired final product, the process of DTM creation ends here. Often, however, the existing surface model is used as the base for a proposed design and additional DTMs are required.

Proposed DTMs

Enlarge this picture Figure 6. Display of above surface with contours.

In some regards, the process of creating proposed DTMs is similar to creating DTMs that represent the existing ground. The major difference is the data used to create the surface. Where existing ground surface data is collected in the field, most proposed surface data is created as a result of the design process. At locations on the site where the proposed design matches the existing surface (e.g., at the site boundary), field-collected data can also be used for the proposed DTM. Proposed DTMs can be loosely grouped into one of two types depending on their intended use: design or construction.

Proposed DTMs for Design

Enlarge this picture Figure 7. An isometric view of a detention basin DTM.

Design surfaces are those used for and created as a result of the engineering design process. These surfaces are created by the design engineer and are typically used to quickly create proposed contours, label proposed slopes and spot elevations, and calculate earthwork volume. Design surfaces are not survey accurate and should not be used for purposes for which they were not intended. They are usually less detailed than a construction surface. However, these surfaces can serve as the starting point in creating construction surfaces.

Many factors influence the proposed grading design for a given site. Existing conditions and site constraints (e.g. natural features or drastic elevations changes); drainage and stormwater management issues; types of soils present; type of development (residential, commercial, roadway, etc.); and economic factors all affect the design process. The designer must have a strategy for addressing these factors and incorporating them into the grading plan. This strategic planning often begins not at the computer console but with a plot of the entire site and a red pencil. Although today’s software applications provide powerful grading design features, it is difficult to get the complete picture of a site, even on a 21" monitor. An oversized plot of the site allows the engineer to work with the entire site when developing the overall design concept. The detail work in grading a site is then accomplished using design software.

The key to the successful creation of a proposed DTM is to use the correct software tool for the component of the surface being designed. No one tool or function is appropriate for all design elements. Using a “divide and conquer” approach, the designer creates sub-components of the proposed surface, which are then combined to form a complete proposed DTM. As a broad generalization, a proposed site consists of a detention basin, roads, lots, open areas/berms, parking areas and building pads. Additional features such as streams, wetlands, etc. may also be present. Next, a few of these components will be examined.

Enlarge this picture Figure 8. Detention basin shown above in plan view with computer-generated contours created from the underlying DTM.

Many site designs today include a stormwater management facility in the form of a detention or retention basin. Products from many vendors, including Autodesk and Bentley, include hydrology tools for detention basin design. Although the details of this design process are beyond the scope of this article, many of the tools included with these feature sets can be used to create data for a proposed DTM. For example, a feature in Autodesk Land Desktop and Civil Design allows the user to grade the interior of a detention basin based on variables such as side slopes and required storage volume. Used in an iterative process, this results in both the design of a detention basin and a DTM. The DTM can then be used to generate contours and to calculate earthwork volume. Prior to the availability of these types of features, engineers were forced to draw basin and berm contours by hand, which were then manually drafted in a labor-intensive process. Small changes in design would require re-drawing and re-drafting, consuming even more time. Figure 7 shows an isometric view of a detention basin DTM; Figure 8 shows the same basin in plan view, with computer-generated contours created from the underlying DTM. This particular basin incorporates a retaining wall, adding to the angular appearance of the contours.

Another common design element is a roadway. Residential sites have networks of roads for access to home sites. In addition to parking lots, larger commercial and industrial sites often have main access roads. Nearly all civil design packages include (or offer as an add-on) roadway design functions and capabilities. Roadway surface creation begins with the design of the horizontal and vertical alignments. These vertical and horizontal elements serve as the backbone for the rest of the right of way (ROW) or access-road design. A typical road cross-section consists of design elements such as a standard width, cross-slope and curbs (of course these features may vary over the length of the road and the software can accommodate these variations), all of whose positions are related to the vertical and horizontal alignments. During the design phase, the engineer might only be interested in centerline, top of curb and ROW elevations. (Gutter and edge of pavement elevation are a function of curb geometry, and therefore would be redundant information on a grading plan). So, a simplified model of the road surface would suffice. The left side of Figure 9 on page 19 shows the left half of the actual top-surface geometry of a proposed road, including adjacent areas up to the ROW line. The right side shows a simplified half-section that the engineer might use to extract the needed data.

A number of different methods are available for creating a ROW surface. Some products, including those by Autodesk, CAiCE (www.CAiCE.com) and Bentley allow users to create surfaces from a roadway cross-section template. Many shareware and third-party add-ons to popular software create ROW surfaces from three-dimensional lines offset a certain distance and elevation from the centerline vertical alignment.

Products like Autodesk Civil 3D and Bentley GEOPAK Site use dynamic relationships between design elements. A design change to the vertical alignment results in a similar change in surface geometry. This allows the engineer to easily consider design alternatives and make revisions.

The engineer continues designing the other surface components and then assembles them into a complete proposed DTM. Figure 10 shows a portion of the completed proposed surface design in isometric view and shaded.

Enlarge this picture Figure 9. The left side of this drawing shows the left half of theactual top-surface geometry of a proposed road, including adjacentareas up to the ROW line. The right side shows a simplified halfsectionthat the engineer might use to extract the needed data.

After creating a proposed design DTM, many engineering analysis and design tasks are much easier to accomplish. For example, locating areas of the site that are outside acceptable minimum and maximum slope ranges can be identified in minutes by displaying the DTM with color ranges based on slope. Spot elevations at points of interest on the surface can be extracted directly from the DTM, eliminating the time and inherent inaccuracies of manual interpolation and annotation. Drafting tasks are also simplified with the creation of a proposed design DTM. Labeling those same spot elevations used to be done by creating a text entity based on a red-lined markup. Now, dynamic labels are used to call out spot elevations based on the underlying DTM. Proposed contours are drafted in seconds instead of hours and labeling of contours can be completed in minutes.

The above-mentioned applications all use a TIN to create a DTM. The DTM-Contour Module of SurvCADD XML2 by Carlson Software (www.carlsonsw.com) takes a different approach. Rather than require a TIN, users have the option of generating contours directly from 3D polyline breaklines. This eliminates the need to create a TIN prior to contouring and has the additional advantage of utilizing field-generated figures.

Earthwork volume calculations are normally an important part of any site design. Most of the design applications provide tools to perform these calculations. InSite SiteWork (www.insitesoftware.com) provides a CAD tool it calls “paperless take-off,” which allows users to create DTMs from 2D or 3D CAD files (DWG or DXF), which are then used for volume calculations. (See Figure 11 on page 22.)

Proposed DTMs for Construction

Enlarge this picture Figure 10. A portion of the completed proposed surface design inisometric view and shaded.

Creating proposed DTMs for use in construction operations such as stakeout or GPS-guided machine control is the final step in the DTM process. (Technically, there may be subsequent steps such as an as-built survey and use in a GIS system.) As mentioned earlier, the main difference between design and construction DTMs is the level of detail. Another difference is the need for proposed additional DTMs, such as a proposed mass earthwork subsurface DTM or an existing ground DTM with the topsoil stripped. In most cases, these surfaces do not show up on any engineering plans, but are major components of the development of a site.

Depending on the needs of the design engineer, the requirements of permitting agencies and/or the direction given by the developer, the design DTMs may be of sufficient detail that they can be modified for use as a construction DTM. For example, a particular city may require that proposed contours be shown for the entire development. This would then necessitate the need for a more detailed proposed design DTM. Additionally, in many locations around the country, mass earthwork grading plans are a requirement, and therefore DTMs representing these surfaces are created by the engineer.

Enlarge this picture Figure 11. This design application by InSite SiteWork allows users to create DTMsfrom 2D or 3D CAD files, which are then used for volume calculations.

Whenever possible, the engineers, contractors and developers/owners should work together to ensure that digital data created in the topographic survey and design phases are available during the construction phase of a project. This implies including DTMs (and DTM data such as COGO points) as well as CAD drawing files when transmitting information from the engineer to the contractor. A number of formats and methods are available for sharing data. A common exchange format is the DXF (Drawing eXchange Format) file format. Using this format, three-dimensional TIN lines, contours and other data can be exported from one COGO program or design application and imported into another. A more recently developed method is the use of LandXML (www.LandXML.org), an industry specific implementation of the XML (eXtensible Markup Language) schema. LandXML provides a means to import and export data in a format such that the data is not degraded in the transfer process. Since LandXML is an open schema, applications that support it theoretically use an identical format.

If the digital data for the design DTMs is available, this is an excellent place to begin creation of the construction DTM. However, great care must be taken when using DTMs created by others and the construction documents should be referenced frequently to ensure that the DTM corresponds to what is shown on the plans. Assuming that correlation is high, construction DTMs can be developed by creating additional proposed surface data and rebuilding the DTM to reflect these additions.

Those creating DTMs for construction purposes have traditionally used software applications developed directly by hardware manufacturers or by software companies whose primary focus is the construction end of a development project. Many of these applications also include a component for working with real-time GPS. Topcon’s TopSite (www.topconmc.com) is a CAD-based software program used to create a DTM from imported files from most popular design programs, including Trimble Terramodel, Autodesk Land Desktop, and Bentley InRoads and GEOPAK. SiteModel 3D from AGTEK Development (www.AGTEK.com) is used for DTM design and earthwork analysis. It can read and write DXF and DWG file formats.

Enlarge this picture Figure 12. A typical cross-section detail for lot with a basement. Asshown, the mass grading geometry in the area of the house is differentfrom the finished grade.

In the examples above, the design DTM did not include data for the mass earthwork subsurface. Only the top finished surface was modeled. For the detention basin component of the DTM, breaklines (surface faults) were used to model the top surface and all areas of the detention basin were treated the same. In reality, areas of this basin above the normal water level (NWL) will receive six inches of topsoil respread after the mass earthwork operations. Therefore, the mass earthwork DTM needs to reflect this. Using standard CAD editing commands, the contours representing the basin above the NWL can move   -0.5' in the z-direction (down 0.5'). These modified contours can then be added back to the DTM definition to create the mass earthwork surface for the detention basin component.

  Other areas of the DTM may require edits that are more complicated than that described above. Accurately modeling the mass earthwork surface for the individual lots in a residential subdivision might involve more than simply lowering the polyline that represents the building footprint. Although the subsurface elevations usually reference the top surface (e.g., the pad may be a given depth below the top of foundation) the geometry of the subsurface is often quite unlike the finished surface. Figure 12 on page 23 shows a typical cross-section detail for lot with a basement. As shown, the mass grading geometry in the area of the house is different from the finished grade.

One method for adding this additional detail to the construction DTM is to create a simple block that consists of two rectangular drawing entities: one for the front of the subsurface pad and another for the rear of the pad. These entities are positioned relative to one another such that they maintain the geometric relationship called for by the design. In the example above, the front pad is -1.0' below the top of foundation (TF) and the rear pad is -2.0' below the front pad. The block is created with an insertion point at the front middle, 0.5' above the front pad. The block can then be inserted into the drawing at the correct elevations in relation to the TF by selecting elements on the design DTM.

Utilizing various features of the software applications, enough additional detail can be added to the design DTM surface definition to create a useable construction DTM. However, digital data is not always available or of sufficient quality, so some construction DTMs must be created from “scratch” using the engineering plans. In some ways, creating a construction DTM using the plans is easier than creating the design DTM, because all parameters are fixed by the design, thus eliminating the iterative processes of evaluating differing design configurations and reviewing for design flaws. Conversely, it is more time-consuming to create a construction DTM without the benefit of the design DTM as a starting point.

A common procedure for creating DTMs from paper drawings or from 2D CAD files (without access to DTM data) is digitizing. With paper-only drawings, the modeler uses a digitizing tablet to trace surface features such as contours or spot elevations, assigning elevation information as the entities are created. As one would imagine, this is a tedious and time-consuming process. A similar procedure is “heads-up” digitizing. In this scenario, paper drawings are scanned to raster (non-vector) images and inserted as a background into the modeling software. Depending on the capabilities of the application, the user manually, semi-automatically or automatically traces the raster image, creating vector entities with elevation data that can then be used to create a DTM. A third method is to use 2D CAD drawing files as the basis for creating DTM data. Using the paper drawing or CAD files to determine contour or spot elevation data, the model creates breaklines, COGO points and other entities that become the basis for a construction DTM. For example, the engineering firm may provide a CAD file of the grading plan. However, the spot elevations shown at building corners are just plain text entities labeling the elevation. The DTM creator can set a COGO point at these locations in the drawing, assigning the label elevation to the COGO point elevation. The modeler can then go on to create breaklines to define surface swales, berms, etc.

After the construction DTM has been created, the data then returns to the field for use in grading operations. Most often this has been in the form of traditional grading stakes that are used to determine cut and fill at given locations across the site. A recent trend that has gathered considerable momentum in the last few years is stakeless grading via either GPS- or robotic total station-guided machine control. This procedure provides the ultimate in time savings because grading operations are performed without the need for the traditional grading stakes. Onsite survey control is still required, but most of the operation is handled in an automated fashion. Earthmoving machines are equipped with antennas and receivers as well as a graphical operator interface inside the cab. The construction DTM is transferred from the office PC to the earthmovers via a small memory chip and the equipment in the machines has a display that guides the operator, providing real-time 3D location data of the blade. Onboard software compares the actual location of the blade to the desired location as determined by the construction DTM. The blade height is then set either manually by the operator or automatically by the control equipment tied into the earthmover’s hydraulic system. Manufacturers of the hardware and software systems have gone to great lengths to ensure ease of use while maintaining the highest level of accuracy.

Conclusion

Digital Terrain Models form the backbone for almost all development projects. The initial existing conditions DTM provides the engineers, property owners, developers and contractors with the knowledge of current site constraints and with a basis for design. Proposed design DTMs allow the engineer to increase precision while decreasing time spent by modeling and evaluating multiple options, reviewing for quality and adherence to design parameters and revising the site in a reduced amount of time. Construction DTMs used by contractors and estimators allow for accurate quantity take-offs and cost estimates to help win bids and deliver high-quality construction in less time.

The life of digital terrain models and the data used to create them spans across the entire breadth of a project. From initial topographic survey to engineering design, and on to estimation and construction, data is successively built up and has become increasingly valuable. Individual firms at each step of the process can realize tangible productivity and profit gains by training on and fully implementing the software applications they use. The firms that learn to master the skills needed to utilize this data will be the ones that succeed.