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Suggested Citation:"Summary." 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:"Summary." 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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1 Practices for Construction-Ready Digital Terrain Models Facilitated by advancements in computer-aided design (CAD) technologies, highway designers have had three-dimensional (3D) modeling capabilities for decades. Digital terrain models (DTMs) are 3D models of existing terrain contours created with the input of surveying technologies. DTMs provide a useful reference layer for designers to overlay their proposed work with existing elevations. They also include rich information for contractors. Despite those benefits, departments of transportation (DOTs) have seen inconsistent practices and results from moving these design models to the construction field. This synthesis documents those practices, challenges, and successes across the United States through a national survey and case examples. The objective of this synthesis study is to document current DOT processes and strate- gies for the effective use and transfer of DTMs from design into construction of highway projects. Information was gathered from a literature review, survey of DOTs, and follow-up case examples. An electronic survey was created and distributed to the voting DOT members of the AASHTO Committee on Construction, and 40 completed responses were received. Subsequent case example interviews conducted with DOTs in six states provided additional details. Five of those states have significant experience using DTMs in construction, and one state has adopted many e-construction initiatives but has limited experience with DTMs in construction. Results of the study show that about half of the responding DOTs have been using DTMs for more than 10 years and on more than 50 projects annually. The most common use cases of DTMs were for grade work, quality measurements, and survey verification. Respondents also indicated that they occasionally use DTMs for inspection. DOTs mostly rely on informal peer training, and more than half provide field- and classroom-based training. Half of the responding DOTs use DTMs on all projects regardless of size and mostly for corridor widening, intersection improvements, new or replacement bridge work, and rehabilitation projects. The most common project delivery system for projects that use DTMs is still design-bid-build. DOTs noted benefits and challenges associated with DTMs. The primary short-term benefits include simplifying the calculation of construction quantities, enabling early identification of plan discrepancies and conflicts, reducing risk during bidding for con- tractors and DOTs, and improving communication on projects. Long-term benefits include providing cost savings, improving accuracy of plans, improving documentation measure- ments in databases for future measurement, enhancing communication, improving effi- ciency of project construction, and reducing claims and litigation. DOTs reported that the major issues for further implementation of DTMs in highway construction include insufficient knowledge or training for inspectors, office staff, and survey staff. S U M M A R Y

2 Practices for Construction-Ready Digital Terrain Models The majority of respondents indicated that DTMs are provided “for information only” for contractors to use at their own risk. Only eight of the responding agencies have used a 3D model as a legal contract document; however, the 3D models do not take precedence over written specifications and two-dimensional plans when conflicts arise. Most DOTs verify their own models by using field verification of survey points, but more than half of DOTs indicated that DTMs are not updated to an as-built condition. The findings presented in this synthesis are aggregated from 40 different DOTs and provide an overview of the current state of practice of DTMs. Future research could be conducted to expand the scope of work and to survey contractors about their DTM needs and practices. Additionally, as DOTs continue to invest in modeling and sharing infor- mation digitally, it becomes critical to further investigate the legal issues associated with using models as contract documents.

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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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