How Copenhagen's Street Archives Ended Up Full of the Same Photo Twice

Copenhagen's municipal archive holds tens of thousands of photographs documenting the city's built environment — from the red-brick facades of Nørrebro to the canal-front warehouses of Christianshavn. The problem, confirmed in planning documents circulated among city departments this spring, is that a significant portion of those records contain duplicate images: the same photograph filed under multiple entries, inflating the archive's apparent size and burying genuinely unique records beneath layers of repetition.

The issue matters now because the City of Copenhagen is midway through a broader digitisation push tied to its Fælles Byrum strategy — the shared public-space framework that has guided urban development planning since 2021. Architects, heritage officers and neighbourhood councils rely on photographic records when assessing renovation permits and new construction along protected streetscapes. Duplicate entries slow that process down, and in a handful of documented cases have led to review panels examining the wrong version of a building's condition history.

How the Duplicates Accumulated

The roots of the problem stretch back to at least 2014, when the municipality began migrating analogue slide and print collections into the Københavns Stadsarkiv's digital management system. The migration was handled in phases, with different city departments uploading materials independently and without a unified tagging protocol. Stadsarkivet, located on Nyrnberggade in the Ørestad district, has since acknowledged the lack of a single controlled vocabulary caused the same image to be assigned different file identifiers each time a new department uploaded its own copy.

The Teknik- og Miljøforvaltningen — the city's technical and environmental administration — compounded the problem when it began its own parallel documentation effort along Strøget and the inner Vesterbro renewal zones after 2017. Field photographers working for subcontractors shot many of the same façades that Stadsarkivet had already catalogued, and those images entered the system as new records rather than duplicates of existing ones. By the time the discrepancy was flagged internally in late 2024, estimates within the administration suggested that duplicate or near-duplicate images accounted for a meaningful share of entries in certain neighbourhood subcategories, though the city has not yet published a precise figure.

What the City Is Doing About It

Since January 2026, Stadsarkivet has been piloting an automated deduplication tool developed in partnership with the IT University of Copenhagen on Rued Langgaards Vej. The tool uses perceptual hashing — a technique that generates a compact digital fingerprint for each image — to flag pairs and clusters of visually identical or near-identical files. Archivists then review flagged clusters manually before any record is deleted or merged, a safeguard insisted upon after a 2023 incident in which automated cleanup at a comparable archive in Hamburg incorrectly retired a set of unique historical negatives.

The pilot covers roughly 12,000 images from the Frederiksberg and Indre By subcollections and is expected to conclude by the end of September 2026. If the error rate in the automated flagging stays below three percent — the threshold set by the project steering group — the tool will be rolled out across the full collection through 2027.

For residents and professionals who rely on the city's public image portal, the immediate practical advice is straightforward: cross-reference any photograph retrieved from the Stadsarkivet portal with its acquisition date and original source department before using it as documentary evidence in a planning submission. The archive's own guidance page, updated in April 2026, now flags records from the 2014–2019 migration window as potentially subject to ongoing review. Heritage consultants working on protected buildings in Nørrebro and along the Frederiksstaden axis have been specifically advised to request the original accession file — not just the digital thumbnail — when the image predates 2020.

The deduplication project will not be cheap. The IT University pilot alone carries a contract value that city procurement documents list at 1.4 million kroner. Whether that buys a clean, reliable record of how Copenhagen looks — and looked — is a question the archive expects to answer by early next year.