SPURIA // FIELD NOTES

The Survey Plane Already Flew — Timestamping with Open Aerial Archives

National aerial survey programmes photograph entire countries on a fixed cadence and publish the frames as open data — in Denmark's case with capture timestamps accurate to the second. When and how to use Skråfoto, PDOK, and Norge i bilder to date a scene instead of guessing.

/geoint/#geolocation#satellite-imagery

Several European countries photograph their entire territory from survey aircraft every one to two years and publish the result as open data. The imagery is sharper than anything you’ll get from satellite tiles — around 10 cm per pixel — and it comes with metadata Google will never give you: the exact capture date, in the best case the exact second.

That turns these archives into two different tools. As a map, they’re a better base layer for feature matching than aged satellite imagery. As a clock, they can date anything visible in a frame — a parked vehicle, a construction stage, a moving aircraft — to the survey pass. If your scene is in a covered country, check the archive before you reverse-engineer a date from shadows.

At a glance

Every cell below was checked against the provider’s own pages, metadata records, or API responses on the date in the last column.

ArchiveCoverageCapture timestampEarliest yearLicenceAccessLast checked
Skråfoto (Klimadatastyrelsen)Denmark, incl. BornholmPer frame, UTC, to the second2017 (passes 2017, 2019, 2021, 2023, 2025)CC BY 4.0Viewer; STAC API (v1.0 and v2) with Cloud-Optimised GeoTIFFs; token required — from a Dataforsyningen account, or the viewer’s public one2026-09-25
PDOK / Beeldmateriaal (Het Waterschapshuis, Kadaster)NetherlandsMosaics: year only. Photo-centre index: date and time of day per photo, to a fraction of a second (time zone not stated)2016 for the PDOK mosaics; 2012 for the photo-centre indexCC BY 4.0PDOK WMS/WMTS, beeldmateriaal.nl viewer and data portal; no key2026-09-25
Norge i bilder (Kartverket)NorwayPhoto date (day) per orthophoto project; a project can carry several dates; no time of day1935 (around 4,000 projects)Not open data: free to view; downloads and WMS under the Norge digitalt licenceViewer for everyone; WMS/WMTS and downloads for Norge digitalt parties only2026-09-25
swisstopo aerial images (LUBIS)SwitzerlandPer photo, date only1921 (black and white); collections run to 2010swisstopo OGD terms: free, commercial use allowed, credit swisstopomap.geo.admin.ch layers; STAC API at data.geo.admin.ch with scans as GeoTIFF; no key2026-09-25

Only Skråfoto and the Dutch photo-centre index put a time of day on a frame. Norway and Switzerland reach back much further, but to the day at best — and swisstopo’s current orthomosaic, SWISSIMAGE, is labelled by year only.

The archives

Denmark — Skråfoto (Klimadatastyrelsen). The strongest of the set. Five camera directions per pass — nadir plus obliques facing north, south, east, west — roughly every two years since 2017, with a per-frame capture timestamp to the second. Behind the viewer sits a STAC API; the token is published in the viewer’s own /config.js and is public by design. Querying every frame covering one coordinate is a single request:

T=<token from config.js>
curl -s "https://api.dataforsyningen.dk/rest/skraafoto_api/v1.0/search?token=$T" \
  -H 'Content-Type: application/json' -d '{
    "collections":["skraafotos2017","skraafotos2019","skraafotos2021",
                   "skraafotos2023","skraafotos2025"],
    "intersects":{"type":"Point","coordinates":[<lon>,<lat>]},
    "limit":500}' \
| jq -r '.features[] | "\(.properties.datetime) \(.properties.direction) \(.id)"' | sort

Each item links a Cloud-Optimised GeoTIFF, so you can window one corner of a 20544 × 14016 px frame over HTTP with /vsicurl/ instead of downloading it. Licence is CC BY — credit Klimadatastyrelsen when you republish.

Netherlands — PDOK (Beeldmateriaal). Nationwide orthophotos, updated yearly at 7.5–25 cm, CC BY. The mosaic layers are labelled by year only, but the index behind them is not: the beeldmiddenoverzicht on beeldmateriaal.nl, published for every year since 2012, lists each photo centre with its capture date and time of day to a fraction of a second. The mosaic is built, in principle, from the central part of each photo, so the nearest photo centre points to the source frame. Even at year level, a yearly cadence at that resolution still brackets construction, demolition, and vegetation changes tightly.

Norway — Norge i bilder (Kartverket). Orthophoto projects with per-project capture dates, often multiple projects per region per decade. Date granularity is the acquisition period, not the frame — good enough to answer “was this building there in 2019”. Unlike the other two, it is not open data: anyone can view it for free, but downloads and the WMS/WMTS services are reserved for Norge digitalt parties.

Sweden and Finland run comparable national programmes (Lantmäteriet, Maanmittauslaitos) with more friction — some products need registration. Worth knowing they exist; not worth learning until a case lands there.

The workflow

Geolocate first, then query. The archives index by coordinate, so the where has to come before the when. One point on your scene is enough.

Enumerate everything over that point. Don’t browse the viewer year by year — pull the full frame list from the API and read it as a timeline. Ten survey passes over one runway is a hundred-line table, not an afternoon of clicking.

Search every camera direction. The lesson that cost me the most time on the Billund case: I filtered for nadir frames because “aerial survey” meant “straight down” in my head, found nothing, and started doubting the challenge. The frame I needed was a north-facing oblique. Obliques also see façades, signage, and vehicles that nadir geometry hides.

Respect the viewing geometry when you measure. In a 45° oblique, the ground is foreshortened but an object hundreds of metres above it is not foreshortened by the same factor — an aircraft can appear 1.5× oversized against the terrain scale. Measure on the ground plane, or convert first. An anomalous measurement is a clue about the imaging geometry before it’s a clue about tampering.

Cross-check the clock against the sun. A to-the-second timestamp is falsifiable for free: compute the solar position for that instant on SunCalc and compare it against shadow directions in the frame. If they disagree, you’ve misread the metadata — or the frame.

Worked examples

The rule of thumb: in the Nordics and the Netherlands, assume a timestamped photograph of your scene already exists. The question is not whether someone imaged it — it’s which pass, which direction, and what was standing in frame when the shutter fired.

Sources