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Use the ISO 6346 check digit as a consistency check on a container number read by gate OCR—not as proof that the OCR is correct or that the container is the one expected for a transaction. A reliable workflow checks the image and recognized characters, validates the number’s structure and check digit, then confirms the result against the relevant operational record when identity matters.
What the ISO 6346 check digit verifies
ISO 6346 defines a system for coding, identifying and marking freight containers. ISO describes its intended uses as documentation, control and communications—including automatic data processing—as well as display on containers themselves (ISO 6346 public preview).
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 digits and a check digit. That extract says the check digit validates the owner code, equipment-category identifier and serial number. Because this is a 1995-edition extract, treat it as a description of that edition—not confirmation of every detail in the current edition (WCO Handbook on the Customs Convention on Containers).
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In a gate OCR workflow, a passing check digit means the recognized fields are internally consistent under the applicable calculation rules. It does not establish that the camera read the markings correctly, that the number belongs to the physical container, or that the container matches a booking, manifest or registry entry.
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Keep the three validation layers separate
| Layer | Question it answers | What it does not establish |
|---|---|---|
| OCR and image review | What characters appear to be printed on the container? | That the recognized string is structurally valid or belongs to the expected transaction. |
| ISO 6346 check-digit validation | Is the check digit consistent with the owner code, equipment category and serial number under the applicable standard? | That OCR selected the correct characters or that the container is registered or expected. |
| Operational-record confirmation | Does the number match the relevant terminal transaction or authoritative record? | That the camera read was flawless; operational matching is a separate check. |
Do not collapse these outcomes into one “verified” status. Record what was actually checked—for example, OCR reviewed, check digit passed, or operational record matched—so downstream users can distinguish a checksum result from confirmation of identity.
How to validate a number read at the gate
- Retain the image and OCR candidate. Keep the source image or a suitable crop alongside the recognized string so a reviewer can compare the output with the markings.
- Check the expected structure. The WCO’s 1995 extract describes three owner-code letters, one equipment-category letter, six serial digits and a check digit. Confirm the applicable current standard and local input rules before enforcing that structure in production.
- Normalize without changing meaning. Preserve all six serial positions, including leading zeroes. The WCO extract notes that missing leading places in the serial number are filled with zeroes. Do not silently substitute a visually similar character or discard a position to make a candidate fit.
- Calculate using the applicable Annex A rules. ISO identifies Annex A as covering numerical equivalents, the weighting factor, modulus, resulting check digit and a sample calculation. Use the exact rules in the licensed, applicable edition; do not implement a formula recalled from memory or infer one from a preview that does not expose the full calculation.
- Handle the result according to what it means. For a mismatch, reacquire or inspect the image and route the candidate for human review rather than silently correcting it. For a pass, treat the number as internally consistent, then compare it with the expected transaction or authoritative record if the operation requires identity confirmation.
This is a practical workflow, not a gate procedure prescribed by ISO. The sources do not establish an OCR confidence threshold, terminal exception process, or performance rate for particular recognition errors.
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What to do with ambiguous or failed reads
Low-confidence or visually ambiguous characters
Visually similar characters can create OCR ambiguity. Preserve the alternatives or flag the uncertain position for a better image or human inspection; do not let a passing check digit silently decide between characters. A checksum may expose some inconsistent candidates, but the available sources do not establish that it detects every error class or provide a measured detection rate.
Malformed candidates
If the candidate does not fit the expected fields, do not run a check-digit test on a string that has been padded, truncated or altered without a documented rule. Return it for image review or exception handling. Keep the original OCR output so normalization choices remain auditable.
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Check-digit mismatch
A mismatch means the candidate fails the applicable consistency calculation. It does not identify which character is wrong. Re-read the image or request human review; do not change a character merely to force a pass.
Passing check digit but record mismatch
A check-digit pass cannot resolve a disagreement with a booking, manifest or other operational record. Treat that as a separate identity or transaction exception and resolve it against the relevant authoritative data.
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Which ISO 6346 edition should an implementation use?
ISO’s product listing identifies ISO 6346:2022 as Edition 4, published in April 2022, and says it is expected to be replaced by Edition 5 (ISO 6346:2022 product listing). ISO’s separate status page lists Edition 5 as under publication at stage 60.00 (ISO Edition 5 status). These listings describe the status reported by ISO, not a guarantee that it remains unchanged; check the current published status before making an edition-sensitive implementation decision.
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