A datum line is a point of reference or standard for comparison. The term is primarily used in reference to surveying and engineering, where it is used in the measurement of depths and heights.
In general, a datum line is a tool used by engineers to ensure the accuracy of measurements. Engineers use the datum line as a horizontal base line to exactly measure various heights. Datum lines are valuable in the development of railway plans, as well as other complex engineering drawings. These lines are critical to guaranteeing measurement precision and avoiding errors from the start of a project.