This Client Server article is republished in its entirety from 2003 for reference purposes.
By Patsy Gant, Technical Documentation Coordinator, Bentley Civil, Bentley Huntsville Office
03 June 2003
In recent years, the digital terrain model (DTM) and related technology has laid the cornerstone for engineering design projects worldwide. Using civil engineering software applications such as Bentley InRoads and Bentley GEOPAK, users have created highly accurate DTMs for much of the Department of Transportation (DOT) infrastructures within the realm of each state. Over 90 percent of United States DOTs have standardized on Bentley's civil engineering software applications to meet their design needs.
The DTM simulates the surface of the earth in corresponding real world coordinates and in salient feature recognition. By combining the DTM with analysis and design software, users are able to project their physical presence onsite.
Unquestionably an invaluable aid to the engineering community, the time and costs associated with DTM creation is often prohibitive. Fortunately, continuing advances in data collection, photogrammetry, remote sensing, Global Positioning Systems (GPS), and conventional surveying methodologies are systematically reducing the time and costs associated with creating DTMs.
Acceptance of the DTM by the civil engineering community at large has demanded that certain criteria be met. More specifically these criteria are related to the data capture phase of the DTM, wherein the greatest investments in financial outlay and time are recognized. Criteria to be considered when evaluating collection methods and systems include the following:
- Costs
- Ease of use
- Rate of capture
- Applicability
- Accuracy
- Repeatability
The evolution of data collection methods and tools, combined with new computing hardware and accompanying software, has steadily improved. A revolutionary data collection system known as the Combined Approach is expected to be available on the commercial market soon. The Combined Approach weaves together several new technologies developed in recent years.
Evaluation of all available data collection methods using the criteria identified above clearly demonstrates the profound effect the Combined Approach will have upon the approach to DTM data capture by the civil engineering and surveying communities. A discussion of the items to consider as this transpires is provided herein.
A Brief History of the DTM
A fundamental understanding of the DTM is necessary to legitimately discuss the various data collection approaches. Initially the DTM was developed for use with large-scale mapping projects. Since that time, it has been ever evolving, developing, and adapting to meet the needs of a multi-discipline workplace. As the use of the DTM has increased, likewise it has evolved to include a variety of applications, including the following:
- Civil engineering and surveying
- Mapping
- Close-range industrial photogrammetry
- Conventional photogrammetry
- Satellite based photogrammetry
- Architecture
- Landscape architecture
- Mechanical modeling
- Public safety
- Medical arts
Development
The civil engineering DTM was specifically designed to develop a true representation of the earth's surface. The original DTM, generally referred to as a triangulated irregular network (TIN), contained point data that was formed into triangle faces by connecting triangle legs point to point. Such point-to-point connections can be determined from several algorithms.
One of the most common algorithms used in civil engineering utilizes Delauney's Criteria, which states that the triangles are formed when the smallest possible circle is formed between points, with no other point inside the circle. The TIN treated all points as equals when forming the triangles, which required that a large number of points generate a "valid" surface model. To form consistent triangle faces that would yield accurate models, users needed to have a great deal of points in the model as well as to maintain symmetry among the points in the model. It is very important to understand that the TIN could only be as accurate as the triangle faces that were formed from the points provided, utilizing Delauney's Criteria for triangle formation.
The next generation of DTM, the Triangulated Topological Network (TTN), evolved from widespread commercial and military usage. The TTN took into consideration that various point and linear types could exist within a model that should have differing influences on the way the triangle legs were formed. TTN implementation recognized the difference between a random point and a linear point series. Users could apply simplistic rules to differ from the random point to a linear series of points. It was at the TTN stage of the DTM life cycle that the need for huge numbers of random points and symmetry of points could be overcome. This was possible via the simple modification to Delauney's Criteria that states:
"Linear point series shall be treated as ‘barriers,' prohibiting triangle legs from crossing them."
Another fundamental aspect of the linear feature was that it enabled users to densify, with straight x/y/z interpolation, between the original linear vertices to strengthen the DTM symmetry. The linear feature also enabled users to define key grade breaks in the DTM that could not be crossed with triangle legs. This greatly streamlined data collection methods and decreased the number of points required to ensure a valid DTM.
Feature definitions and identities that resulted from the TTN include:
- Random - This is a point with no specific control to consider in triangulation.
- Breakline - This is the most basic linear feature.
- Contour - This is a more advanced linear feature that is at a constant elevation. It can generate a hybrid "inferred" breakline by analyzing near-contour lines in peninsula and ridge areas.
- Interior - This is a closed linear feature that can exclude specified areas of a DTM. Such areas (a building, for example) might be excluded because they do not contain adequate information to form triangle legs or should not be considered as part of the actual ground DTM.
- Exterior - This is a closed linear feature that specifies (clips) the limits of a triangle formation for a DTM.
The Intelligent DTM
The DTM has evolved far beyond the standard TTN model to include all other features that exist in the real world and even as record data. Examples of physical features that can now be stored in a DTM are underground and overhead utilities, drainage structures and standard construction pay items. Examples of record features that can now be stored in a DTM are computed right-of-way and limits of construction lines, deed lines, and easements.
DTM Creation/Manipulation Software
Before discussing the data collection process, users must understand how the end product (the DTM) can be acquired. The actual creation of a DTM can be accomplished using Bentley InRoads or GEOPAK civil engineering software applications. Both applications have been developed specifically for the civil engineering community at large. These applications have been developed based upon a worldwide user base, which provides feedback to developmental staff regarding tools necessary to effectively complete their projects. Over a decade of valuable user interaction has helped implement what is today a very robust solution to any DTM creation or editing requirement.
The Bentley civil engineering applications provide flexible, high-level, double-precision DTMs that contain intelligent features. Each feature within the intelligent DTM knows whether it is a breakline, random point, contour, exterior boundary or interior boundary and can annotate itself based on its feature definition. In addition, users can set up a style for each type of feature. Then, when the feature is displayed, it automatically displays the style correctly.
Because feature display is repeatable, users do not have to depend on the CAD file for proper representation of features. Features such as underground utilities and guardrails can be added to the DTM without being considered in the triangle formation of the surface model. Feature querying and reporting via XML offer design quantities for estimating.
Data Sources for DTM Creation
The civil engineering industry has used several methods to create DTMs. Some of the most common methods include:
- Conventional field survey data collection, including:
- Total stations and data collector combinationsusing trigonometric leveling methods
- Differential levels and digital levelsusing baseline and cross-section methods
- GPS using RTK (Real Time Kinematics)to collect points and linear features with surveyors using receivers mounted on range poles
- GPS using RTK to collect points and linear features with vehicle-mounted receivers
- Photogrammetry data collection, including:
- Low to mid-range altitude aerial cameras with ground control points for x,y,z
- Low to mid-range altitude aerial cameras with airborne GPS for x,y,z control
- High altitude satellite methods
- Close range industrial
- Collaborative technology:
- Vehicle-mounted GPS RTK combined with Gyroscope and Laser Sensors (referred to as the Combined Approach)
History of Data Collection
To understand how technology has steadily improved over the years, a brief review of the history of civil engineering and surveying tools into the market for data collection reveals the following:
| Approximate Date | Method Available | Standard Deliverables |
| Early 1300s | Jacob Staff and Compass | Hand drawn scrolls and tablets |
| Early 1800s | Transit and Chain or Stadia | Hand drawn maps and parchments |
| Late 1800s | Plane Table Alidade | Field drawn topo and planimetric maps |
| Late 1800s | Photogrammetry | Hand drawn maps initially, evolving to DTMs, CAD files |
| Early 1900s | Tacheometer | Hand drawn maps |
| 1950s | EDM with Theodolite | Hand drawn maps and DTMs |
| Late 1900s | Total Station | Hand drawn maps, CAD files and DTMs |
| Late 1900s | GPS with RTK | DTMs, CAD files |
| Early 2000s | Combined Approach | DTMs |
Business Model for Data Collection
As in all business, the civil engineer and surveyor must consider the economic ramifications involved with data collection for DTM creation. Three major considerations immediately come to light when determining the best data collection method:
- What is the initial startup cost?
- What is the expected standard system accuracy?
- What type of comparison (Production Ratio) exists between the available methods?
The following table provides information on the three items listed above, with the Production Ratio being equivalent to the time it would take to accomplish the same task, with one being the fastest:
| Method | Initial Startup Cost | Accuracy | Production Ratio |
| Total Station | $20,000 | 1-cm | 30 |
| Total Station with GPS | $45,000 | 1-cm | 20 |
| Photogrammetry | $55,000 | 3-cm | 10 |
| Combined Approach | $500,000 | 1-cm | 1 |
Different Methods: Pros and Cons
Additional considerations are evaluated when deciding on the best method to use for data collection. A discussion relative to the pros and cons is as follows:
| Method | Pros | Cons |
| Total Station | Lower startup costs. Availability of experienced resources. Learning curve quite small due to several available people in the market. | Safety, traffic, inclement weather |
| Total Station with GPS | Median startup costs. Easier to bring in control points. Easier to collect data in open areas using kinematic mode. Very accurate data due to ability to bring in control easily. | Safety, traffic, inclement weather, canopy, lack of localized control |
| Photogrammetry | Mid-range startup costs, as long as most of work is sublet to other consultants for flights, control surveys, and bundle adjustments. Open areas can be flown in minimal time versus field surveying. Best advantage for method is derived when collecting data from models that do not require high accuracy, as used in general mapping and planning. | Canopy, lack of localized control, seasons, inclement weather, availability of planes or helicopters |
| Combined Approach | This system extremely fast and accurate. Unlike GPS, canopy is not a problem. To be cost effective, many jobs must be done to recoup initial startup cost. Once initial investment had been met, system will far exceed any other method for collecting data along a drivable corridor. | Lack of localized control |
The Combined Approach
The series of photographs and documentation presented here outline the basic methods using the Combined Approach.
Image 1
The ARAN® Automatic Road Analyzer, pictured in Image 1, is collecting data using the Combined Approach. Roadware Group Incorporated, located near Toronto, Canada, manufactures this vehicle. The bar mounted on the front of the vehicle collects data samplings along the roadway. Notice that the bars can be adjusted inward toward or outward away from the vehicle, based on the desired width of data collection.
The vehicle obtains coordinate values to a known point in the vehicle, from which the reference bar is mathematically associated. The known vehicle coordinate and direction of travel is obtained both from standard GPS RTK, as well as from inertial measurements from Interferometric Fiber-Optic Gyroscopes and Silicon Accelerometers.
The ARAN Automatic Road Analyzer also has several on-board cameras that can collect roadway and adjoining data simultaneously as the DTM data is being collected. These photographs can be archived and retrieved using the same coordinate system as used with the DTM, making it very easy to both review the DTM and the photographs of the area after the data has been processed.
Safety First
Image 2
As illustrated in Image 2, the surveyor is no longer required to be in the direct flow of traffic. Immediate benefits include the following:
Surveyor remains out of traffic
Traffic is not delayed due to surveyor being in roadway
Data that was too dangerous to obtain is now available
Collection time can be increased in problem areas that require special focus
Data for intersections that would have been very dangerous can be collected without any problems
Overlay projects that would not have been surveyed due to traffic issues can be driven with accurate quantities computed and used in bid letting, saving very large sums of money for several large DOTs
Post Processing: GPS and Gyro at Work
Image 3
Image 3 illustrates a GPS receiver working directly with the GPS/Gyro mounted on a vehicle. Relative to this item, it is noted that:
Relative coordinates are adequate for GPS, depending on project requirements.
Absolute coordinates can be obtained from a CORS station or known monumentation.
Heavy canopy is overcome with the redundant Gyro data being collected at the same time as the GPS data.
Continuous data capture is possible, even if GPS lock is interrupted for as much as three miles, with post processing algorithms using the Gyro data to compute accurate coordinate values.
Projects that normally could not be used for GPS can now be completed with the Combined Approach method.
Data collection needs to be on unobstructed surfaces, such as roadways.
Data outside of the roadway, such as ditches and banks, still is recommended to be collected using more conventional methods, such as Photogrammetry or total station surveys.
Accuracy
To maintain a high level of design accuracy, the survey data collected in the field must be sufficient in quantity to cover all surface undulations. In addition, it must be appropriately tied to the specified horizontal and vertical datum. The most reliable conventional method for confirming data validity is to physically perform a trigonometric field survey using static GPS to set the x,y,z control points and a standard total station to collect data via observing points on a road. As stated when discussing DTM creation, by collecting linear point series at grade breaks (also called breakline surveying), it is possible to build a very accurate DTM. This DTM can then be used to confirm the results of the Combined Approach.
The longitudinal profile view shown in Figure 4 was created with Bentley's civil engineering software using a field survey and a Combined Approach DTM that was collected at a northeastern DOT site with the author present on both the field survey and the vehicle that collected the data. The results of this test illustrate very clearly the undeniable accuracy that can be obtained with the Combined Approach.
Image 4
Summary
The DTM has undoubtedly become the foundation for many civil engineering design projects throughout the world. Bentley's civil engineering software applications, InRoads and GEOPAK, provide the tools necessary to create DTMs from any of the data collection methods. By utilizing the powerful DTM creation tools with the new Combined Approach to data collection, it is possible to increase the open corridor data collection Production Ratio by up to 30 times that possible via a total station survey.
Possessing the power to more efficiently collect data to create these highly precise DTMs stands to revolutionize the manner in which highway overlay projects and resurfacing jobs are completed. No longer will a surveyor be tasked to work within the traffic flow. No longer will traffic literally shut down so that surveying jobs can be completed. Lives will be saved and limited governmental improvement budgets can be extended further to provide more services to their constituents. The acceptance and application of the Combined Approach and forward moving technology promises exciting benefits to the civil engineering community and the infrastructure at large.
See Also
GEOPAK TechNotes, FAQs and Support Videos
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