How do COLMAP, Meshroom, Metashape and RealityScan establish real-world scale and units for a photogrammetry result?

Partially verified: the curator re-read S15, S19; S24, S25, S26 were not re-read by the curator and rest on the producer's pinned reading. Reconstructions have no inherent real-world scale: COLMAP geo-registers by fitting a similarity transform to known camera-centre positions, and Meshroom's SfMTransform can scale to real-world size (its auto_from_landmarks mode only fits landmarks into a [-1,1] box, i.e. normalises rather than measures). RealityScan's one-distance rule and Metashape's markers/scale bars are producer-read only. Curator qualifications: Partially verified: the curator re-read S15, S19; S24, S25, S26 were not re-read by the curator and rest on the producer's pinned reading. Curator GET 2026-10-23 of the sources not re-read (mechanical source evidence, not semantic verification): S24: HTTP 200, bytes equal to the producer pin; S25: HTTP 200, bytes differ from the producer pin; S26: HTTP 200, bytes differ from the producer pin. Docs read 2026-10-22; no reconstruction run. RealityScan help host unresolvable to curator (DNS); Metashape manual not re-fetched. Limitations: Docs read 2026-10-22; no reconstruction run. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://github.com/colmap/colmap/blob/bd1fcf654d2dd8fefa1466999c190a246f83f4b9/doc/faq.rst",
      "note": "COLMAP doc/faq.rst at bd1fcf65 (raw), Geo-registration L410-413; meshroom-manual SfMTransform.rst (raw)"
    },
    {
      "url": "https://github.com/alicevision/meshroom-manual/blob/480bb414b6330d077a30f84e5131f62e426c91cd/source/feature-documentation/nodes/SfMTransform.rst",
      "note": "COLMAP doc/faq.rst at bd1fcf65 (raw), Geo-registration L410-413; meshroom-manual SfMTransform.rst (raw)"
    },
    {
      "url": "https://www.agisoft.com/pdf/metashape-pro_2_3_en.pdf",
      "note": "COLMAP doc/faq.rst at bd1fcf65 (raw), Geo-registration L410-413; meshroom-manual SfMTransform.rst (raw)"
    },
    {
      "url": "https://rshelp.capturingreality.com/en-US/tutorials/scaling.htm",
      "note": "COLMAP doc/faq.rst at bd1fcf65 (raw), Geo-registration L410-413; meshroom-manual SfMTransform.rst (raw)"
    },
    {
      "url": "https://rshelp.capturingreality.com/en-US/tutorials/georeferencing.htm",
      "note": "COLMAP doc/faq.rst at bd1fcf65 (raw), Geo-registration L410-413; meshroom-manual SfMTransform.rst (raw)"
    }
  ]
}