The problem
The original question: type an address and see the parcel’s history, how it was split, when the building went up, what stood there before. Slovak open data has today’s cadastre, an address register and historic maps. Can they really be joined into a reliable story about a place?
What we built
One page for Trenčín’s old town. A slider moves between a historic map and today’s aerial photo. There are two historic maps to pick from: the Third Military Survey and a later Czechoslovak military topographic map at 1:25,000. Today’s parcels of three places are drawn on top: the town hall, the Trenčín Museum and the M. A. Bazovský Gallery. For each place the page shows three lists: what the sources tell us, what we don’t know and what we won’t claim.
Try your address (experimental). Type a street and number anywhere in Trenčín. The page looks up the address in the address register live, finds the current parcel the point falls into and loads the same historic maps and aerial photo around it. If the address is ambiguous or missing, the page says so instead of guessing.
Out of scope on purpose: addresses outside Trenčín, parcel history, owners and any dated change without a dated source.
How it works
- Data gate (Monday). A script checks three things per place: the address register returns exactly one point, that point lies in exactly one current parcel, and the historic map has an image there. A place that fails doesn’t go into the demo.
- Data bundle. Saved service responses become one JSON per place: observations with source, licence and the exact request URL, a list of unknowns and a list of claims we refuse. The same input always gives the same bundle.
- The page. A static viewer with no libraries reads only that bundle. Map images are fetched in advance; the page doesn’t call map services live.
What we learned
- We could not verify a parcel-level historic cadastre for Trenčín. Original cadastral maps exist in the archive, but we found none georeferenced with a clear licence for the scans. So “parcel history” became “place in time”: comparing the neighbourhood, not parcels.
- Open historic maps are scarce. GKÚ publishes only two openly licensed historic map layers: the Third Military Survey and the 1:25,000 topographic map. The First and Second Military Surveys and historic aerial photos for Slovakia are not available as open map services, as far as we found.
- The historic maps don’t say what year they are from. Neither service returns a sheet number or year, so the page says “year unverified”, not “100 years ago”. Today’s aerial photo at least carries a “2023” watermark.
- Not every landmark has an address. The Piarist church (Mierové nám. 43) and the synagogue (Štúrovo nám. 9) are not in the address register under those numbers, so they are not in the demo.
- Dates in the data are not always dates. All three addresses have “valid from” 0999-12-31, a placeholder. And the parcel date is the date of the data version, not of the building.
- Typed addresses work if you know the street list. The free-text geocoder returned nothing, but the register has only 276 street names in Trenčín, written as “Mierové nám.” or “Gen. Goliana”. Matching against that list, 20 of 20 test addresses resolved to the right point and exactly one parcel, whether typed in full or without diacritics. Typing only the surname of a street found 18 of 20; the other 2 are genuinely ambiguous, so the page asks which street. 881 of 9,203 Trenčín addresses have no street name at all and aren’t supported yet.
- The topographic map has a scale limit. The 1:25,000 map renders only at overview scale, and zoomed-in requests come back empty. So the page loads a wider area and zooms in itself.
- Address to parcel is a spatial match, not a legal link. The address register has no parcel number, only a building registration number. We find the parcel by checking which polygon the address point falls into.
What we measure
How far off the historic map is at control points (Matúš Tower, the parish church, the Town Tower, the Piarist church). We set the tolerance before measuring: 50 m, i.e. 2 mm on paper at an assumed 1:25,000 scale. If the median offset is above it, we show the maps only side by side and claim nothing from overlaying them. The result will be added here and in the demo.
Data sources and licence
All CC BY 4.0: address register (Ministry of Interior SR), cadastral parcels, register C (ÚGKK SR), Third Military Survey map and 1:25,000 topographic map (GKÚ) and the Slovak orthophoto mosaic (ÚGKK SR / GKÚ). This is a LabZone experiment over open data, not a service of these institutions.
Technologies
Node.js with no dependencies · WFS and WMS services of GKÚ, ÚGKK SR and the Ministry of Interior · static viewer in vanilla JS
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