> ## Documentation Index
> Fetch the complete documentation index at: https://support.lumafield.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Datum Definition

> Define plane, axis, and midplane datums, and rank them into a Frame

A datum is the perfect reference a measurement is taken from: a plane, an axis, or a midplane. In ASME Y14.5 a datum is not the scanned surface. It is the counterpart that surface is allowed to touch. Voyager builds that counterpart from one or more features in a [feature set](/voyager/voyager-feature-extraction), or from faces on imported CAD.

Datums live under the feature set in the Dimensioning tree, in the **Datums** group. A **Frame** ranks those datums — primary, then secondary, then tertiary — into the datum reference frame that [Profile](/voyager/voyager-profile), [Position](/voyager/voyager-position), [Perpendicularity](/voyager/voyager-perpendicularity), and [Parallelism](/voyager/voyager-parallelism) measure in.

<Frame>
  ![Datum defined from a detected feature](https://lumafield-support-assets.s3.us-east-1.amazonaws.com/Feature_Datum.png)
</Frame>

<h2 id="supported-types">
  Datum types
</h2>

* **Plane** — one or more plane features. Several faces can belong to one plane datum. Hold the modifier key the panel names to add another face.
* **Axis** — one cylinder. The datum is the axis of that cylinder.
* **Midplane** — the plane halfway between two plane features.

CAD faces are picked the same way as scan faces, once the CAD has finished processing. See [Nominal CAD Import](/voyager/voyager-nominal-import).

<h2 id="how-a-plane-datum-sits">
  How a plane datum sits on the part
</h2>

A new plane datum uses a **tangent** fit. The datum plane rests on the high points of the surface. That is the contacting simulator ASME Y14.5 and ISO 5459 describe, and it is the default for every new plane datum.

A plane datum can also be:

* **GG** — least squares, the average of the surface
* **GC** — the minimum zone, the middle of the narrowest pair of parallel planes that contain the surface

An axis datum can be least squares, minimum zone, or a mating size (the smallest cylinder around the pin, or the largest cylinder inside the hole). A midplane uses least squares or a mating size. The tangent fit is planes only.

<h2 id="datums-on-part">
  Defining a datum
</h2>

<Steps>
  <Step title="Select Datum">
    Choose **Datum** from the Toolbar in [Dimensioning Mode](/voyager/voyager-modes#dimensioning-mode), or right-click a feature and choose the datum action for its kind.
  </Step>

  <Step title="Choose where it is built">
    Select the feature set, or a CAD model that has finished processing.
  </Step>

  <Step title="Choose the type and the features">
    Pick Plane, Axis, or Midplane, then click the feature or features. Eligible features highlight on hover.
  </Step>

  <Step title="Label and submit">
    Assign a label such as `A`, `B`, or `C`. The label is stored in capitals. Submit to add the datum to the feature set.
  </Step>
</Steps>

<Frame>
  ![Datum defined on an imported STEP file](https://lumafield-support-assets.s3.us-east-1.amazonaws.com/StepFile_Datum.png)
</Frame>

<h2 id="frames">
  Frames
</h2>

A Frame is the ordered datum reference frame. Rank the datums that locate the part: **primary**, then **secondary**, then **tertiary**. The primary orients the part. The secondary stops rotation about that orientation, or one translation. The tertiary stops the last translation the first two left free. You do not have to fill every rank. [Position](/voyager/voyager-position) and [Profile](/voyager/voyager-profile) require a primary and treat the other two as optional. [Perpendicularity](/voyager/voyager-perpendicularity) and [Parallelism](/voyager/voyager-parallelism) require a primary and allow a secondary only when it clocks the zone.

Choose **Frame** from the Toolbar. Select the feature set or the CAD model the Frame belongs to, then pick a datum for each rank or click its callout. **Clear rank** empties one rank. If a Frame with the same datums already exists, the panel names it instead of creating a duplicate.

Build the Frame once. With the scan [aligned](/voyager/voyager-alignment) to the CAD, **Copy to CAD** or **Copy to scan** creates the counterpart, and **Link** pairs the two. Profile and Position then measure in the pair.

<h2 id="adjusting-leaders">
  Leaders
</h2>

Each datum draws a leader and a labeled flag. Use the callout controls to move the leader. The datum value does not change. Clean placement matters once several datums and GD\&T checks share the view, including in [Reporting Mode](/voyager/voyager-modes#reporting-mode).

<h2 id="related-pages">
  Related pages
</h2>

* [Alignment](/voyager/voyager-alignment)
* [3D Auto-Dimensioning](/voyager/voyager-auto-dimensioning)
* [Feature Extraction](/voyager/voyager-feature-extraction)
* [Manual Feature Authoring](/voyager/voyager-manual-feature-authoring)
* [Profile](/voyager/voyager-profile)
* [Position](/voyager/voyager-position)
* [Perpendicularity](/voyager/voyager-perpendicularity)
* [Parallelism](/voyager/voyager-parallelism)
* [Nominal CAD Import](/voyager/voyager-nominal-import)
* [Voyager Release Notes](/platform/voyager-release-notes) and [Monthly Highlights](/platform/monthly-highlights)


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