CAD foundations · Lesson 10 · Approx. 7 minutes including practice
Datums and references
A datum provides an agreed reference for describing or checking a feature. Datum selection relates to function and inspection and belongs with the responsible technical authority. Consistent references reduce contradictory interpretation.
Step 1: Approved reference system
A documentation workflow, not an equipment operating sequence. Animation starts only when you choose Play.
Keep the reference system consistent
A datum is part of an agreed reference system for describing geometry or checking features. It is not simply the nearest convenient edge on a view. The selected references need to reflect the applicable functional and inspection requirements, and their definition belongs to the competent technical authority. Documentation should preserve the relationship between datum symbols, controlled features and relevant notes. Conflicting references can make a drawing difficult or impossible to interpret consistently.
Prepare and check the work
Start by identifying the approved datum scheme and the relevant drawing convention. Check that each referenced datum is actually defined and that symbols remain attached to the intended feature after edits. When views are rearranged or a model is replaced, verify those associations again. Record unresolved questions rather than adding a symbol to fill a perceived gap. If supplier information uses a different reference system, obtain an approved mapping before copying dimensions into a new drawing.
Worked example · demonstration only
A drawing refers to datum C but only A and B are identified. That is a missing requirement, not a formatting preference. Raise a query with the source authority and keep the issue visible in the review register until the drawing can be interpreted as intended.
Guided practice and review evidence
Use only a demonstration dataset for this exercise: Identify the stated datum references on an approved drawing and map the related features. Before starting, write down the question the finished record should answer and identify the source information you would need on a real job. Keep a separate list of missing inputs. Prepare a first draft, then compare it with the topic-specific checks above. Ask another person to follow your references without explaining them verbally. Record where they hesitate, what information they cannot locate and any different interpretation. Keep a brief review log with four columns: observation, proposed action, responsible reviewer and closure evidence. Revise the example and repeat the information-finding check. Your evidence of completion is the revised demonstration record plus that log, not simply time spent reading. Do not upload client information to the academy or practise on operating equipment.
What good completion looks like
A useful result can be understood by someone who did not prepare it. They can identify its purpose, applicable item, source and review status, and they can distinguish confirmed information from unresolved questions. Pay particular attention to this failure mode: A convenient modelling origin is not necessarily the inspection datum. Check the output in the format the recipient will use. A native file, spreadsheet and exported PDF can expose different problems. Keep the source revision and the issued output linked, and record any limitations of the handover. On a real project, follow the customer's authorised checking and approval arrangements before the information is used. The knowledge check below tests understanding of this lesson; it does not assess practical competence or authorise regulated work.
Practical workflow
- Establish the source: identify the approved input, document revision and person responsible for its technical content. Record missing information as an open question.
- Prepare a small representative example before applying the approach to a whole project. Check identifiers, units, references and the intended reader’s needs.
- Ask the nominated reviewer to check technical meaning. Capture the response, resolve comments and issue only under the agreed document-control process.
Review checklist
- Can another reader identify the source and applicable revision?
- Are assumptions and unresolved decisions clearly recorded?
- Is there evidence of the required review before issue?
Educational material and demonstration examples. Not accredited training, practical competency assessment or authority to perform regulated work. Use current approved sources and your responsible technical authority on real projects.