What is essential for ensuring accurate elevations in construction plans?

The correct choice focuses on bench marks, which are crucial for establishing accurate elevations in construction plans. Bench marks are fixed reference points with a known elevation, serving as a basis for measuring and maintaining consistent heights throughout the construction process. They ensure that all elevations are relative to a common standard, which is essential for proper grading, drainage, and ensuring that structures meet design height requirements.

In construction, having accurate elevations is vital because it affects the overall integrity of the project, including aspects like site drainage, alignment of utilities, and even the structural soundness of buildings and roads. Bench marks provide the reliability needed to ensure all measurements are consistent, thus making them an indispensable tool in the field.

Other options like blueprints and design specifications, while important for other aspects of construction and design, do not specifically address elevation accuracy. Project schedules relate to time management and task organization but do not provide any foundational references for determining elevation levels. Therefore, bench marks are specifically essential for ensuring accurate elevations in construction plans.