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Practices for Construction-Ready Digital Terrain Models (2021)

Chapter:Chapter 3 - State of Practice

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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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Suggested Citation:"Chapter 3 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2021. Practices for Construction-Ready Digital Terrain Models. Washington, DC: The National Academies Press. doi: 10.17226/26085.
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11 State of Practice As noted in Chapter 1, an online survey questionnaire was built in Qualtrics and distributed by email to members of the AASHTO Committee on Construction (COC). Forty responses across 40 DOTs were received (Figure 6). Appendix B contains the aggregate survey results collected in Qualtrics. It also presents partially completed responses not included in the analysis in this chapter because of the lack of information provided. This chapter reports on results from key survey questions. Respondents were asked to identify the division in which they work and their corresponding role. As seen in Figure 7 and Figure 8, half of the respondents work in the construction division (50%) with a designated role of construction engineer or engineer manager (48%). Given that the survey was distributed to the AASHTO COC, this is not a surprising result. Survey recipients were asked, however, to distribute the survey to the individual in the DOT most knowledgeable of their processes for preparing and using DTMs in construction. With that request, 18% of survey respondents work in highway design, 18% work in computer-aided design and drafting (CADD)/support, 8% work in surveying support, and the remaining 8% self-designated as “other.” Figure 7 shows that respondents who selected “other” worked in more than one division specified in the survey, whereas Figure 8 shows that those who selected “other” had roles in administration technology implementation, design fields, and surveying. The survey consisted of five sections covering general DTM use, project-specific DTM use, user/non-user experience feedback, legal aspects, and designer/contractor interface. Each section of the survey, along with its results, is described in detail in this chapter. 3.1 General DTM Use This section covers the technical aspect of DTM at the DOT, uses in construction-phase applications, developer and end-user training, years of use, and maturity level of models. 3.1.1 DTM Usage Frequency Respondents were asked to indicate the number of projects that use DTM in a year. Only 15% of the surveyed DOTs indicated that DTM is used on fewer than 10 projects annually. Thirty- eight percent of the respondents reported that their DOT uses DTMs on 10 to 50 projects, 23% mentioned the use of DTM on 50 to 100 projects, and 25% indicated that they use DTM for more than 100 projects. Figure 9 depicts the DTM usage frequency of the 40 responding DOTs. 3.1.2 DTM Usage Timeline To capture how long DTMs have been used, respondents were asked to estimate the number of years their DOT has been using DTMs in the construction phase of projects. Results show C H A P T E R 3

12 Practices for Construction-Ready Digital Terrain Models State DOTs that participated State DOTs that did not participate Figure 6. Map of DOTs responding to the survey, N = 40. Note: Numbers do not add to 100% because of rounding. Figure 7. Distribution of the division of respondents in their DOTs, N = 40.

State of Practice 13 Note: Numbers do not add to 100% because of rounding. Figure 8. Distribution of the role of respondents in their DOTs, N = 40. Less than 10 10 to 50 50 to 100 More than 100 No Reply Figure 9. Breakdown of DTM usage frequency (by number of projects) per state, N = 40.

14 Practices for Construction-Ready Digital Terrain Models that 11% of the DOTs have been using DTMs for less than 3 years, 16% of the responding DOTs have between 3 and 7 years of experience using DTMs, 29% reported experience of 7 to 10 years using DTMs, and 45% of the respondents indicated that their DOT has been using DTMs for more than 10 years. Figure 10 depicts the DTM usage timeline of the 40 responding DOTs. 3.1.3 DTM Source The survey asked respondents to indicate the source of DTMs used in their construction projects. A few states outsource the creation of all DTMs, whereas a few create all models within the agency. For most states, however, the percentage of outsourced or in-house-developed DTMs ranges between 40% and 60%. The distribution of the results as indicated by respondents is shown in Figure 11, with the size of the data points being relative to the number of responses at that point (i.e., larger circles indicate more responses). 3.1.4 DTM Use Cases To determine how DOTs use DTMs, the survey provided respondents with 11 DTM use cases and asked them to specify if and how frequently their DOT practices each one. As shown in Figure 12, between 5% and 69% of respondents mentioned that their DOT does not use a particular DTM use case. The rest of the respondents (i.e., between 30% and 95%) indicated that a particular DTM use case is used by their DOT. Respondents whose DOT practiced a DTM use case rated the frequency on a scale of rarely, sometimes, often, and always. Figure 13 summarizes the results of the distribution of the respondents’ frequency of DTM usage and shows that DTMs are most commonly used for Grade Work (UC1), Automated Machine Guidance (UC5), and Survey Verification (UC3). Less than 3 More than 10 Between 3 & 7 Between 7 & 10 No Reply Figure 10. Breakdown of DTM usage timeline (in years) per state, N = 40.

State of Practice 15 Figure 11. Variation in the sources of DTM across DOTs, N = 32. Figure 12. DOT responses on the different use cases of DTM, N = 38.

16 Practices for Construction-Ready Digital Terrain Models DTMs are least used for Pavement Thickness Checks (UC9); QA/QC, Class Detection, or Reducing Plan Discrepancies (UC8); and Cost Analysis for Future Maintenance (UC11). Respondents also had the option to indicate additional DTM use cases that were not included in the survey. Respondents from the DOT in Pennsylvania (PennDOT), Florida (FDOT), and South Carolina (SCDOT) reported that their DOTs often use DTMs for design, production development and environment studies, and visualization, respectively. On rare occasions, FDOT also uses DTMs for construction sequencing, and SCDOT uses the models for hydraulic design. 3.1.5 DTM and Construction Inspection Because the use of DTMs can be extended to inspection, survey respondents were asked to indicate how frequently their DOT uses DTMs in construction inspection processes. As illus- trated in Figure 14, only 10% of the respondents said that DTMs were not used for inspection, 24% of the respondents indicated a rare usage of DTMs in the construction inspection process, another 42% reported that a DTM is sometimes used for inspection, and 24% mentioned that their DOT often uses a DTM for inspection. 3.1.6 DTM Training Provided to Construction Inspection Staff The survey investigated the type of DTM training that was provided to construction inspec- tion staff. The results illustrated in Figure 15 show that the majority of the surveyed DOTs (84%) provide informal peer training. Field-based and classroom-based training on hard- ware and software are also common types of DTM training provided by 65% and 52% of the responding DOTs, respectively. Fewer respondents (32%) reported that their DOT only provides reference material. Ten percent of the surveyed DOTs do not provide any DTM training to construction inspection staff. Figure 13. Distribution of the respondents’ frequency of using DTM, N = 38.

State of Practice 17 Respondents also had the option to report other types of training that were not listed in the survey. One respondent indicated reliance by the DOT on the training provided by a contractor or vendor when using GPS rovers for earthwork quantity checks. 3.1.7 DTM Handover To assess the readiness of DTMs handed over to contractors, the survey asked respondents to evaluate different aspects of these DTMs using a five-point Likert scale of very low (1), low (2), moderate (3), high (4), and very high (5). Respondents were asked to evaluate each aspect from two perspectives: (1) from the perspective of their DOT and (2) from the perspective of contractors as perceived by the DOT. The reasoning for this approach is that the researchers wanted to grasp not only the levels of quality and usefulness of the models Figure 14. Frequency of DTM use for construction inspection, N = 38. Frequency of DOT Response DT M T ra in in g M et ho ds Figure 15. End-user DTM training methods, N = 37.

18 Practices for Construction-Ready Digital Terrain Models that the DOTs created or received from designers but also the levels of quality and usefulness of the models according to the contractors. Given that the survey is distributed to DOT per- sonnel, the question had to be posed as seeking the perspective of the contractors as perceived by the agency. On average, all five aspects of DTMs that are handed over to contractors to use are reported to be moderately to highly acceptable from both perspectives, as shown in Figure 16. 3.2 Project-Specific DTM Use This section provides information on highway construction projects that use DTMs. It covers project size and type, delivery systems used, and the party responsible for construction inspection. 3.2.1 Project Size Respondents were asked to indicate the size of projects on which DTMs were used. As seen in Figure 17, 51% of the respondents indicated that their DOT uses DTMs for all projects regard- less of size. Other respondents reported that they use DTMs for specific project sizes (Figure 17). Respondents had the option to select multiple categories of project sizes that apply to their DOTs. DOTs that do not use DTMs on all projects use DTMs mostly on larger projects with sizes exceeding $5 million. A small percentage of DOTs (5%) uses DTMs for small projects that are less than $1 million. Note: Respondents used the following scale: Very Low = 1, Low = 2, Moderate = 3, High = 4, and Very High = 5. Figure 16. Respondents’ evaluation of DTM aspects as perceived by DOTs and contractors (from the DOT perspective), N = 33.

State of Practice 19 3.2.2 Project Type Respondents were asked to identify the types of projects for which their DOT uses DTMs. As illustrated in Figure 18, most responding DOTs use DTMs for widening corridors (87%), improving intersections (79%), replacing or constructing new bridges (71%), and rehabilitating roads (68%). DTMs are least used for road resurfacing (26%), bridge rehabilitation (37%), and safety improvement works such as shoulder widening, rumble strips, pavement marking/markers, and guardrail projects with minimal design (39%). 3.2.3 Project Delivery System As shown in Figure 19, respondents indicated that DTMs were mostly used with the traditional design-bid-build delivery method (91%) and least used with public-private partnerships (11%). There is moderate use of DTMs with the design-build (DB) and construction management/ general contractor (CM/GC) delivery methods at 54% and 29%, respectively. In the case example Frequency of DOT Response Si ze o f P ro je ct Figure 17. DTM use relative to project size (in $ millions), N = 37. Frequency of DOT Response Ty pe o f P ro je ct Figure 18. Use of DTMs by project type, N = 37.

20 Practices for Construction-Ready Digital Terrain Models discussions in Chapter 4, states note that DB and CM/GC, in particular, enable better DTM use in construction, given the ability to have early contractor involvement. 3.2.4 DTM Construction Inspection Respondents were asked to specify the party responsible for construction inspection on projects that used DTMs. Of the 36 respondents answering this question, 42% indicated that DOT staff members were responsible for construction inspection, 14% mentioned that con- struction and engineering inspection (CEI) consultants were responsible for construction inspection, and the remaining 44% said that both DOT staff members and CEI consultants were responsible. 3.3 User/Non-User Experience Feedback This section reports respondents’ perceptions of the benefits associated with DTM use and the barriers impeding its adoption, and summarizes their experience with DTMs. 3.3.1 DTM Project-Specific Benefits Respondents were provided with a list of six project-specific benefits for using DTM and were asked to rate each benefit using a five-point Likert scale of very low (1), low (2), moderate (3), high (4), and very high (5). Results are displayed in Figure 20. On average, respondents reported that Easier to Calculate Construction Quantities (SB1) and Earlier Identification of Plan Discrepancies and Conflicts (SB2) are the two project-specific benefits perceived to have high impact. Reducing Risk During Bidding for Contractors and/or DOTs (SB3), Improved Communication on the Project (SB4), Fewer Change Orders or Construction Revisions (SB5), and Fewer Project Delays (SB6) are reported to have a moderate impact on average. Respondents offered the following additional project-specific benefits: • Reducing labor needs, • Improving safety, Frequency of DOT Response Pr oj ec t D el iv er y M et ho ds Figure 19. DTM usage relative to project delivery methods, N = 35.

State of Practice 21 • Performing quantity verifications on contractors’ models, • Performing survey stake-out and grade control, • Identifying areas in excess materials in right of way, and • Staging. 3.3.2 DTM Long-Term Benefits Respondents were asked to indicate their level of agreement with a list of long-term benefits associated with the use of DTMs using a five-point scale of strongly disagree (1), disagree (2), unsure (3), agree (4), and strongly agree (5). Results are displayed in Figure 21. On average, respondents strongly agreed that Cost Savings (LB1), Improved Accuracy of Plans (LB2), Improved Documentation of Measurements in Database for Future References (LB3), Improved Communication (LB4), and Improved Efficiency of Project Construction (LB5) are long-term benefits that can result from using DTMs. Respondents also agreed, on average, that implementing DTMs can result in Fewer Claims and Litigation (LB6) over the long term. Respondents offered other long-term benefits, including improved time of completion, enhanced construction quality, and less reliance on paper plans. 3.3.3 DTM Hands-On Experience Respondents were asked to indicate if they had hands-on experience using DTMs on a highway construction project. Of the 38 respondents who answered this question, 58% per- cent have been directly involved in using DTMs, and the remaining 42% have not directly used a DTM. Respondents who indicated that they have had hands-on experience using DTMs on a high- way construction project were asked to elaborate on their experience and the use of the models. Figure 20. Weighted perception of project-specific benefits of DTM usage, N = 37.

22 Practices for Construction-Ready Digital Terrain Models Responses are grouped into major and relevant phases for a highway construction project’s life cycle and are provided in the following as direct quotes. • Design – Developed a DTM to support a new alignment on a multi-bridge project and a 4.5 million cubic yard earthwork project. – DTM file was created for quantity calculations and drainage models. – DTM experience includes building 3D models. – Made sure the specifications were in contract. The Road Construction Engineer went to Peer Exchange in NY and worked hand-in-hand with our designer for the model and with field personnel about implementing it. – Designed and modeled four projects that have been built ranging from 2 to 4 million dollars. We do not give the DTMs directly to the contractor, but we give them data pulled from the model. – Design uses DTMs to model roadway improvement and compares end area volumes to determine earthwork quantities. The existing and design DTMs are supplied to contractors [. . .] during advertising to assist with their bids. – Used DTM mostly during the design phase. – Used the existing DTM to create proposed design. – Our DOT is in the process of developing a DTM QC/QA policy to be able to hand over the DTM to the contractors. On previous projects, I have provided contractors with the DTM, and the projects were very successful. The key is not necessarily the design DTM, but the existing DTM, with concern of urban reconstruction and so forth. Many times, the existing DTM is not accurate enough for the urban projects that are flat. The key to being successful is the existing DTM, which holds the most accuracy for quantities at the end of the day. Great method on reconstruction of Interstates and in rural areas. Figure 21. Weighted perception of long-term benefits of DTM usage, N = 38.

State of Practice 23 • Surveying – Our office is responsible for producing DTMs by photogrammetry and LiDAR methods. – Have used kinematic equipment to obtain terrain model to determine areal measurements for vegetative establishment quantities. – The majority of grading/concrete paving in our state has DTM to be used by the contractor. The plans and x-sections still override any error in the DTMs provided. My involvement has been from doing the construction survey using DTM to creating DTM files to measure for pay. • Construction/Maintenance – Contractor used the design DTM files on all construction equipment, including paving operations for grade control. The project was a 12-mile, three-lane freeway concrete rubblization project and HMA [hot-mix asphalt] pavement was raised by minimum of 8 in. at the gutter line with 2.5%, 2%, and 1.5% cross-slopes at crown, which was the far-left lane white stripe between left shoulder and left lane. Left shoulder had a 4% cross slope in the opposite direction. Super elevation was provided on curves. All equipment was set up with computer control with DTM files from servers and run by LAN setup within the 12-mile section of I-295 Interstate. All material requisitions were calculated by the computer, and supplier chain was activated when material was low on proposed work. – DTM experience includes providing QC of contractor models; verifying contractor’s work using 3D model, staking; building model to QA/QC paper plans, specification; training field staff and field inspectors; and surveying hardware/software procurement and support. – Passed on the proposed DTM to construction inspectors to be used for cut and fill, slope verification, and slope limits. The DTM was also passed on to contractor for their use. – Provided checks of the design, existing, and as-built DTMs. – Used DTM for quantity take off for payment using surface-to-surface comparisons. – I was responsible for construction on a $1.1 billion project with multiple uses of DTMs. – I have been involved on the inspection side making sure the contractor is building the roadway to the correct elevation and location. 3.3.4 DTM Barriers From a list of 13 barriers, respondents selected those that impede wider adoption of DTM use on highway construction projects. Figure 22 shows the results. Respondents most frequently cited insufficient knowledge or training for inspectors (DOT or CEI), insufficient knowledge or training for office staff, and insufficient knowledge or training for field survey staff as chal- lenging roadblocks for adopting DTMs. As impediments to DTM adoption by their DOTs, respondents least often indicated that benefits of using DTMs are unknown, that the return on investment (ROI) is unproven, and incompatibility of existing hardware. 3.4 Legal Aspects To understand legal implications of DTM usage, the survey asked respondents to specify contractual language within either their DOT’s policy manual(s) or the contract documents regarding DTMs used in construction. The survey presented seven categories for this informa- tion. Respondents also had an eighth option in case they were not aware of any such contractual language used by their DOT. Actual written DOT policy language is not presented in this report so as not to endorse specific notes, provisions, specifications, or other policy language. Appendix B notes the specific agencies that responded to particular language included in Figure 23. Thus, if one is interested in knowing about policy guidance on file management protocols, the appendix will reference the agencies that may be able to share that information.

Figure 22. Perceived barriers to implementing DTMs, N = 37. Figure 23. Written language in DOT contract documents about the use of DTM, N = 36.

State of Practice 25 Although 11% of the respondents indicated that they were not aware of any written speci- fications related to contractors’ use of DTMs, the majority of respondents acknowledged the presence of language related to DTM in their DOT’s policy manual(s) or contract documents. Figure 23 shows these results. Among respondent DOTs having written policy manuals or contract documents addressing DTMs, the majority (91%) indicated that the DTM or XML files are supplied as “for information only” for contractors to use at their own risk. About half of the respondents (53%) mentioned that the extent of DOT liability of the accuracy of the DTM is included, and another 47% note that survey practices are included. Respondents were also asked specific questions about the use of DTM files as a legal contract document. Thirty-eight percent of the respondents indicated that they have not used DTM as a legal document, whereas 24% mentioned that they have. The remainder (38%) have not used DTM as a legal document yet but report that their DOT plans to in the next 1 to 5 years. Additional information on this question can be found in Appendix B. Respondents affirming that their DOT has used DTM files as a legal document were further asked to rank three contract documents—namely, 2D blueprints, written specifications, and 3D models/DTMs—according to their precedence when a conflict arises. As noted in Table 1, when all three elements were part of the contract documents, most respondents indicated that 3D models ranked third behind written specifications (ranked first) and 2D blueprints (ranked second) when a conflict arose. The 3D model was never the first document of record and had limited use (37%) as the secondary record document. 3.5 Designer/Contractor Interface This section covers DTM verification and modification processes performed by DOTs. 3.5.1 Verification of DTMs Only 14% of the respondents indicated that their DOT does not verify the accuracy of DTMs used in construction. Of the respondents who indicated that their DOT has a process to verify models, 78% indicated that they use field verification of survey points, 31% run model checks with the DTM used by the contractor, and 3% compare point cloud data (terrestrial/drone/ photogrammetry/LiDAR) to the DTM (as shown in Figure 24). In addition to the options Document Type Ranked First Ranked Second Ranked Third Written specifications 88% 12% 0% 2D blueprints 12% 50% 38% 3D models 0% 37% 63% Table 1. Contract document precedence when 3D model is included, N = 8. Frequency of DOT Response M od el V er ifi ca tio n M et ho ds Figure 24. Model verification methods performed by DOTs, N = 32.

26 Practices for Construction-Ready Digital Terrain Models provided in the survey, some respondents from South Carolina (SCDOT), Alabama (ALDOT), and North Dakota (NDDOT) indicated that their DOTs perform both field verifications and running model checks with contractors. Montana (MDT) also reported verifying models by performing internal QC checks. 3.5.2 Modification of DTM Models Respondents were also asked to indicate how models are updated when changes occur in the field. As shown in Figure 25, 56% of the respondents indicated that DTMs are not updated to an as-built condition, but 25% indicated that DOT or CEI personnel change the model to as-built conditions, 16% reported that the owner re-issues the design model and the contractor updates the construction model, and only 3% mentioned that their DOT relies on the contractor to change the model to as-built conditions. Frequency of DOT Response M od el M od ifi ca tio n M et ho ds Figure 25. Modification of DTMs as performed by DOTs, N = 32.

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Digital terrain models (DTMs) are three-dimensional (3D) models of the ground surface showing natural features such as ridges and breaklines.

The TRB National Cooperative Highway Research Program's NCHRP Synthesis 560: Practices for Construction-Ready Digital Terrain Models documents processes and strategies used by state departments of transportation (DOTs) for the use and transfer of DTMs from design into the construction phase of highway projects.

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