Zurich Leads on Duplicate Image Removal — But Other Cities Are Closing the Gap

Zurich's city administration confirmed this spring that its GIS mapping division, operating under the Stadtentwicklung Zürich directorate on Amtshaus IV near Werdmühleplatz, had completed a first full audit of duplicate imagery across the municipal geoportal — a project quietly underway since late 2024. The audit identified several thousand redundant raster layers embedded in the publicly accessible map infrastructure, slowing query times and inflating storage costs at the city's data centre in Altstetten.

The timing is not coincidental. Across Europe, city governments that invested heavily in digital infrastructure during the Covid years are now confronting a shared hangover: rushed data imports, parallel vendor contracts, and poor version control have left civic image repositories bloated with duplicate files. Zurich, with its tradition of meticulous public administration and a housing and urban planning crisis demanding sharper data tools, had particular reason to move fast.

What Zurich Is Doing Differently

The city's approach centres on a two-stage automated deduplication pipeline, built in partnership with ETH Zurich's Institute of Cartography and Geoinformation on Hönggerberg campus. The first stage uses perceptual hashing — a standard computer-vision technique — to flag visually identical or near-identical aerial and street-level images. The second stage routes flagged files to a small team of data stewards inside Geomatik + Vermessung Zürich, the city's official surveying office on Lindenhofstrasse, who make final deletion decisions.

What separates this from simpler deletion scripts used elsewhere is the audit trail. Every removed file is logged with a timestamp, the automated confidence score, and the name of the steward who approved removal. That record is publicly accessible under the city's open-data commitment. For a city where direct-democracy culture makes administrative transparency politically non-negotiable, that audit log is as important as the deduplication itself.

Compare that with Amsterdam, which in 2025 ran a broadly similar cleanup of its Digitale Stad geoportal but faced criticism from municipal councillors after it emerged that no deletion log had been retained — meaning historical aerial images from the IJ waterfront development area had been permanently lost. Vienna's MA 41 geoinformation office has taken a more conservative route, archiving all duplicates rather than deleting them, which preserves history but has reportedly left storage costs largely unchanged. London's Ordnance Survey-aligned borough systems remain fragmented enough that no single deduplication standard exists across the 33 boroughs.

The Numbers Behind the Problem

Storage is real money. The European Commission's 2025 report on urban digital infrastructure estimated that duplicate and obsolete imagery accounts for between 18 and 27 percent of total civic geodata storage in mid-sized European cities — a range Zurich's own audit broadly confirmed internally, according to documentation published on the open-data portal at data.stadt-zuerich.ch. For a city the size of Zurich, even modest reductions translate to meaningful savings in cloud licensing and energy costs at a time when the city's climate action targets — set under the Netto-Null 2040 framework adopted by city council — put digital infrastructure energy consumption under new scrutiny.

Zürich West, the rapidly densifying district where new residential towers are reshaping the skyline along Hardstrasse, has generated a disproportionate share of new aerial imagery in recent years as construction monitoring flights multiply. That volume pressure makes the deduplication work more urgent, not less, as the housing crisis drives continuous urban change that planners must track accurately.

The ETH partnership is scheduled to publish a peer-reviewed methodology paper by the end of 2026, which will make the pipeline's technical specifications available for other cities to adopt. Copenhagen and Barcelona have already made informal inquiries, according to documentation from a March 2026 European urban data consortium meeting held in Bern — though neither has committed to a formal pilot.

For Zurich residents, the practical effect is already visible: query times on the public map portal dropped noticeably after the first deduplication batch was applied in April 2026. Anyone using the city's planning maps to track building permits in Oerlikon or flood-risk overlays along the Limmat should notice faster load times. The broader lesson — that data quality demands the same institutional rigour as data collection — is one Zurich is positioning itself to export.