> ## 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.

# Alignment

> Place a scan on its CAD model so datums and frames can be copied between them

Alignment places a scanned part on its nominal CAD model. Once the two occupy the same space, datums and Frames defined on one side can be copied or linked to the other. [Profile](/voyager/voyager-profile) and [Position](/voyager/voyager-position) measure against that CAD. Alignment is not a replacement for rotating an [ROI](/voyager/voyager-region-of-interest): an ROI sets the part's axes in the volume, and Alignment sets where that volume sits on the CAD.

Alignment is in the Toolbar in [Dimensioning Mode](/voyager/voyager-modes#dimensioning-mode).

<h2 id="what-the-fit-is-doing">
  What the fit is doing
</h2>

Voyager searches for a rigid placement of the scan on the CAD: a rotation and a translation, with no scale. **Run best fit** returns one or more candidates. Each candidate reports:

* **RMS** — how far the matched surfaces sit from the CAD, as a root-mean-square distance.
* **Scan coverage** — how much of the scan found a counterpart on the CAD.

Step through candidates with **Previous fit** and **Next fit**. A low RMS with poor coverage can mean the fit locked onto one face and ignored the rest of the part. Prefer the candidate that covers the surfaces you will datum from.

**Adjust by hand** is there when the best fit is close but wrong. **Rotate** and **Translate** move the scan. **Undo adjustment** returns to the candidate you started from. The panel marks a placement **adjusted** after you move it.

If the search finds nothing, the panel says the best fit found no alignment. Check that the CAD is the same part, that it has finished processing, and that the scan is not still dominated by a fixture.

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

<Steps>
  <Step title="Import the CAD">
    Upload the nominal `.STEP` file. See [Nominal CAD Import](/voyager/voyager-nominal-import). The Alignment picker only lists CAD that has finished processing.
  </Step>

  <Step title="Open Alignment">
    In Dimensioning Mode, choose **Alignment** and select the CAD model.
  </Step>

  <Step title="Run best fit">
    Choose **Run best fit** and review RMS and scan coverage. Step to another candidate if the first one sits on the wrong face.
  </Step>

  <Step title="Adjust if you need to">
    Use **Rotate** and **Translate** to seat a face that the search left tilted, then leave the adjustment in place.
  </Step>

  <Step title="Copy datums and frames">
    Define [datums](/voyager/voyager-datums) and a Frame on one side. **Copy to CAD** or **Copy to scan** creates the counterpart. **Link** pairs a scan feature with the CAD face it landed on. **Unlink** breaks that pair. **Unlink CAD** drops the alignment itself.
  </Step>
</Steps>

A copy is refused when the other side has no matching face, when the names collide with a different kind of datum, or when the counterpart is already linked elsewhere. The panel names the reason. Link the blocked faces first, or pick the CAD face yourself — CAD faces are selected the same way as scan faces.

**Compare with CAD** opens the aligned pair so you can see the scan against the model while you work. **Back to scan** or **Back to CAD** returns to that side.

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

* [Datum Definition](/voyager/voyager-datums)
* [Nominal CAD Import](/voyager/voyager-nominal-import)
* [Profile](/voyager/voyager-profile)
* [Position](/voyager/voyager-position)
* [3D Auto-Dimensioning](/voyager/voyager-auto-dimensioning)
* [Voyager Release Notes](/platform/voyager-release-notes)


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