Tokyo's Digital Archives Are Riddled With Duplicate Images — Here's What the Numbers Actually Show

Tokyo's push to digitise its public records has produced an unexpected bill. Across the metropolitan government's network of libraries, ward offices, and cultural institutions, duplicate image files now account for a significant share of total digital storage load — a problem that administrators at the Tokyo Metropolitan Library in Minami-Azabu have been quietly working to quantify since late 2025.

The timing matters. Japan's Digital Agency, established in September 2021 and headquartered in Chiyoda ward, has been pressing municipal bodies nationwide to consolidate and clean their data repositories ahead of a broader inter-government information-sharing framework due to go live in fiscal year 2027. For Tokyo, that deadline is no longer abstract. Ward offices from Shinjuku to Koto are now under pressure to audit their holdings and eliminate redundancy before migrating to shared infrastructure.

The Scale of the Problem

Duplicate image replacement — the process of identifying, flagging, and systematically swapping repeated files with a single canonical version — sounds routine. The numbers make it less so. Industry benchmarks for large municipal digitisation projects suggest duplicate image rates of between 15 and 30 percent are common in first-generation scanning programmes, where the same physical document may have been photographed multiple times across different projects with no central deduplication layer in place.

Tokyo digitised more than 1.2 million archival documents through its Tokyo Digital Archives program between 2018 and 2024. If the lower end of that duplication range applies, that implies roughly 180,000 files that are exact or near-exact copies consuming storage unnecessarily. At current enterprise cloud storage rates — which in Japan typically run between ¥3 and ¥5 per gigabyte per month for government-tier contracts — even a modest average file size of 8 megabytes per image means the redundant load could be generating hundreds of thousands of yen in avoidable monthly costs across the system.

The Sumida City Office digitisation unit and the Toshima Ward Cultural Properties Division are among the bodies understood to be piloting perceptual hashing tools — software that generates a compact fingerprint for each image and flags near-duplicates even when file names or metadata differ. Perceptual hashing can reduce manual review time by more than 60 percent compared with traditional file-comparison methods, according to published benchmarks from the National Institute of Informatics, based in Hitotsubashi, Chiyoda.

Why Clean Data Now Carries a Price Tag

The financial case for duplicate image replacement is straightforward. The administrative case is more complicated. Many of the duplicates in Tokyo's system exist because different departments commissioned separate scanning runs of the same physical material — Edo-period cadastral maps, wartime evacuation records, mid-century planning blueprints — without checking whether the item had already been processed. Merging those records now requires not just deleting a file, but verifying which version carries the most accurate metadata, highest resolution, and correct provenance tagging.

That verification step is labour-intensive. The Tokyo Metropolitan Library has estimated internally that a full deduplication pass across its 400,000-item digital collection would require approximately 2,000 staff-hours if done manually — a figure that drops to around 300 hours with automated tooling, though human review of flagged pairs is still required.

The yen's persistent weakness against the dollar — the currency has traded above ¥155 per dollar for much of 2026 — has pushed up the cost of dollar-denominated cloud contracts and imported server hardware, adding a further incentive to reduce storage bloat before contract renewals in the October 2026 fiscal cycle.

For institutions still in planning stages, the practical path forward involves three steps: running an automated hash-comparison pass to generate a candidate duplicate list, assigning a metadata specialist to review flagged pairs, and establishing a single canonical file registry before any data migration begins. Organisations that skip the third step and migrate first tend to replicate the problem at higher cost on the new platform. The Digital Agency's technical standards unit has published guidance on this sequence, and ward-level IT officers can access the framework through the agency's portal in Chiyoda. The deadline to submit migration readiness assessments is March 31, 2027.