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Use the ISO 6346 check digit as a consistency check after gate OCR reads a container number—not as proof that the container is genuine or matches a shipment. A reliable workflow separates three questions: did OCR read the characters correctly, does the resulting number pass the check-digit calculation, and does it match the relevant operational record?
What the ISO 6346 check digit checks
ISO 6346 is the standard for freight-container coding, identification and marking. ISO describes its identification system as intended for documentation, control, communications—including automatic data processing—and display on containers themselves. See the ISO 6346 public preview.
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A World Customs Organization handbook reproduces an extract from ISO 6346:1995 describing the familiar identification structure: three owner-code letters, one equipment-category letter, six serial-number digits and a check digit. That extract says the check digit validates the owner code, equipment-category identifier and serial number. Because it reproduces the 1995 edition, use it as historical structural guidance, not as confirmation of every requirement in the current edition. The WCO handbook provides that extract.
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Passing the check digit means the characters are internally consistent under the applicable calculation. It does not establish that OCR saw the physical marking correctly, that the number belongs to the container at the gate, or that the container is registered or expected in a particular transaction.
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Keep the three validation layers separate
| Layer | Question it answers | What it cannot establish alone |
|---|---|---|
| OCR and visual review | What characters does the image appear to show, and is the read sufficiently clear? | Whether the resulting number passes ISO 6346 calculation or matches an operational record. |
| Check-digit calculation | Is the check digit consistent with the owner code, equipment category and serial number under the applicable standard? | Whether the image was read correctly or the number identifies the expected real-world container. |
| Operational confirmation | Does the candidate match the terminal transaction, booking or other authoritative record relevant to the movement? | Whether the OCR image itself was clear; a record match does not replace image-quality review. |
Do not collapse these outcomes into one “verified” flag. Record which layer passed so that a checksum match cannot be mistaken for identity or transaction confirmation.
How to use the check digit after gate OCR
- Review the OCR candidate. Keep the image and recognition output available for review. If the read is uncertain or characters are visually ambiguous, request another read or send it for human review rather than silently substituting characters.
- Check the expected structure. Confirm that the candidate has the expected fields before calculating. The WCO’s 1995-edition extract describes six serial numerals and says missing leading places are filled with zeroes. Preserve those leading zeroes; the serial field is not an ordinary number to convert to an integer.
- Calculate under the applicable edition. Apply the check-digit method in Annex A of the current ISO 6346 text. ISO’s public preview identifies Annex A as covering numerical equivalents, the weighting factor, modulus, resulting check digit and a sample calculation, but does not expose the complete formula in the accessible material. Do not implement a remembered or copied formula without checking it against the licensed standard.
- Route a mismatch for review. A failed calculation means the candidate is inconsistent; it does not identify which character is wrong. Request another image/read or human review, then recalculate the corrected candidate.
- Confirm the transaction separately. If the number passes, compare it with the terminal’s expected transaction or another authoritative operational record whenever the workflow requires confirmation of the container’s identity or movement.
This is a practical gate workflow, not a procedure prescribed by ISO. The sources describe the identification system and check-digit purpose; they do not set an OCR-confidence threshold or prescribe a terminal’s exception-handling process.
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Which edition should an implementation follow?
ISO’s product listing identifies ISO 6346:2022, Edition 4, as published in April 2022. ISO also lists Edition 5 as under publication on its Edition 5 status page. The public preview available at the cited page does not provide enough calculation text to verify whether the algorithm changes in Edition 5. Before implementing or auditing edition-specific behavior, confirm the edition currently published and consult its complete text.
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What a passing result means at the gate
A passing check digit is useful because it can flag some inconsistent candidate strings before they move further through a workflow. It is not an OCR accuracy score, a guarantee that all recognition errors will be detected, or a registry lookup. The available sources provide no gate-OCR error rates or quantified detection performance, so a pass should be reported narrowly as “check digit consistent,” with visual review and operational confirmation tracked separately.
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