Zurich Leads on Duplicate Image Replacement in City Archives — But Rivals Are Closing the Gap

Zurich's municipal archive has begun a systematic cull of duplicate digital images from its public-facing databases, a project that archivists say has already flagged tens of thousands of redundant files stored across the city administration's servers since the first major digitisation drive in 2018. The effort is one of the most structured of its kind among mid-sized European cities, according to institutional documentation reviewed by The Daily Zurich.

The timing matters. Swiss federal data-management guidelines updated in early 2026 placed new obligations on cantonal bodies to audit redundant records before the next budget cycle. For Zurich's archive, housed on Alfred-Escher-Strasse and responsible for everything from nineteenth-century cadastral maps to recent planning photographs of the Hardbrücke district, that means confronting a backlog that has accumulated for nearly a decade.

What Zurich Is Actually Doing

The programme, run jointly by the Stadtarchiv Zürich and the city's Department of Digital Transformation, uses a perceptual hashing system — software that generates a compact fingerprint for each image and flags near-identical copies — to surface duplicates before human reviewers decide what to delete, merge or retain. Pilot work focused first on the Langstrasse and Aussersihl neighbourhood documentation series, two collections that saw intensive re-photographing during the tram-network expansion between 2019 and 2023. Early internal estimates put the duplication rate in those collections at roughly 34 percent, though the final verified figure will emerge once the phase-two review concludes later this year.

The project also feeds directly into Zurich's broader Wohnungsnot response. Planning departments drawing on photographic evidence to approve or contest building permits in tight-supply neighbourhoods like Altstetten and Schwamendingen need clean, deduplicated imagery to avoid legal challenges based on outdated or contradictory visual records. A single address appearing twice with different metadata has, in at least three documented cases since 2022, complicated permit-review timelines at the Amt für Baubewilligungen.

How Other Cities Are Handling the Same Problem

The challenge is not uniquely Swiss. Amsterdam's Stadsarchief, which manages one of Europe's largest municipal photograph collections — more than 800,000 digitised images — launched its own duplicate-detection programme in 2023, deploying open-source tooling developed partly in collaboration with the University of Amsterdam. The Dutch institution has moved faster on automation but faces a different legal context: Dutch public-records law gives the archive wider discretion to permanently delete redundant files, whereas Swiss cantonal archiving law requires a more conservative retention default.

Vienna's Wiener Stadt- und Landesarchiv, which sits comparably to Zurich in both collection size and administrative structure, is still in a scoping phase as of mid-2026. City documents obtained through a routine public-records request show that Vienna has budgeted approximately €180,000 for a feasibility study due to report in the fourth quarter of this year. Zurich, by contrast, has already spent that scoping phase and is into active remediation — giving it a meaningful operational lead over its Austrian counterpart.

Hamburg and Copenhagen, both cities of comparable administrative complexity, have focused their recent digital-records investments primarily on optical character recognition for text documents rather than image deduplication. That reflects a different prioritisation rather than a failure, but it does mean Zurich and Amsterdam are currently the two European municipal archives with the most operationally mature duplicate-image workflows.

ETH Zurich's Digital Humanities Lab has provided technical consultation to the Stadtarchiv project, connecting academic image-processing research to a real-world archival problem. The collaboration, which began formally in the spring of 2025, is being documented as a potential model for other Swiss cantonal archives in Bern and Basel, which face similar duplication pressures but have not yet committed funding.

For residents and researchers, the practical outcome will be a cleaner, faster public image portal — the Bilddatenbank accessible through the city's online services — expected to relaunch with deduplicated content in the first quarter of 2027. Institutional users such as law firms working on property disputes in Zürich-West, or journalists pulling historical images of Bahnhofstrasse for reference, should find search results less cluttered and more reliably dated. The city's technical team has recommended that anyone currently using the archive for time-sensitive research download and locally save any images they rely on before the portal migration begins in October 2026.