A datum is a reference framework used to describe positions or elevations consistently.
Definition
A datum is the reference framework that gives meaning to measured positions or elevations. Survey and mapping work commonly identifies horizontal and vertical references, because coordinates or elevations from different datums may not align without transformation. The project requirements, governing authority, and intended deliverable determine which references apply. A surveyor or engineer documents the datum and any conversion assumptions so teams can combine records, design files, and field observations consistently. A datum may be specified in a firm's scope or client standards; owners should ensure it is clear in survey and design deliverables. It is not a coordinate system by itself: a coordinate system describes how locations are represented, while the datum provides a reference framework. The ASCE provides resources for civil engineering professionals.
How owners use it
- Owners specify the required horizontal and vertical references in survey requests and design standards.
- Project managers check that incoming survey files identify their datum before combining them with project data.
- A drawing set has 2 elevation sources, and 1 uses a different datum; 2 - 1 = 1 source needs conversion review.
- Teams document transformations when translating field measurements into the coordinate references used for design.
