> ## 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.

# Flatness

> GD&T flatness of a plane feature, with no datum

Flatness constrains the form of a plane. In ASME Y14.5 the tolerance zone is the space between two parallel planes, and the surface must lie between them. Flatness has no datum. A face can be flat and still be tilted relative to the rest of the part; that tilt is [Perpendicularity](/voyager/voyager-perpendicularity), [Parallelism](/voyager/voyager-parallelism), or an angle.

The Flatness Tool in [3D Auto-Dimensioning](/voyager/voyager-auto-dimensioning) evaluates one **plane** from the [feature set](/voyager/voyager-feature-extraction), whether the plane was extracted or [authored](/voyager/voyager-manual-feature-authoring). It does not accept cylinders or other kinds.

<Frame>
  ![Use the Flatness Tool to measure the flatness of a plane](https://lumafield-support-assets.s3.us-east-1.amazonaws.com/flatness.gif)
</Frame>

<h2 id="what-the-number-means">
  What the number means
</h2>

The reported flatness is the width of the zone that contains the evaluated surface. A smaller number is a flatter face. The Viewport draws that deviation as a heatmap on the plane, and the callout carries the value and a pass or fail state against the tolerance you entered.

**Erosion** drops surface points within a distance of the selection's edge. Edge points from extraction are often noisier than the body of the face. A small erosion keeps the evaluation on the face you care about. It does not change the plane feature itself.

<h2 id="workflow">
  Workflow
</h2>

<Steps>
  <Step title="Select Flatness">
    Choose **Flatness** from the Toolbar in [Dimensioning Mode](/voyager/voyager-modes#dimensioning-mode), or right-click a plane and choose **Add flatness control…**.
  </Step>

  <Step title="Choose a plane">
    Click the plane. Eligible planes highlight on hover.
  </Step>

  <Step title="Set erosion and tolerance">
    Enter an erosion distance, in millimeters, and a tolerance. Erosion can stay at zero.
  </Step>

  <Step title="Submit">
    Submit to add the control to the feature set. The heatmap and the callout appear on the plane.
  </Step>
</Steps>

<Tip>
  Erosion and tolerance can be edited after the control exists. The heatmap and the pass or fail state update without a new extraction.
</Tip>

<h2 id="interpreting-results">
  Reading the result
</h2>

Use the callout controls to move the leader. The flatness value does not change. For a very small tolerance, increase erosion slightly before you decide the face failed. The edge of an extracted plane is the usual source of a number that looks worse than the face.

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

* [3D Auto-Dimensioning](/voyager/voyager-auto-dimensioning)
* [Feature Extraction](/voyager/voyager-feature-extraction)
* [Manual Feature Authoring](/voyager/voyager-manual-feature-authoring)
* [Cylindricity](/voyager/voyager-cylindricity)
* [Perpendicularity](/voyager/voyager-perpendicularity)
* [Parallelism](/voyager/voyager-parallelism)
* [Direct Dimensions](/voyager/voyager-direct-dimensions)
* [Datum Definition](/voyager/voyager-datums)
* [Profile](/voyager/voyager-profile)
* [Voyager Modes](/voyager/voyager-modes)
* [Voyager Release Notes](/platform/voyager-release-notes) and [Monthly Highlights](/platform/monthly-highlights)


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