You draw a boundary on a map around somewhere you care about — a concession, a construction site, a stretch of forest, a supplier’s land. TerrAlert then fetches new Sentinel-2 satellite imagery of that area as it becomes available, compares each new clear image against the previous one, and tells you where the ground has changed.
It is built for the case where nobody can visit often enough: land you do not own, sites you audit rather than run, or areas too remote or too numerous to inspect.
Realistically, several days — and it is worth understanding why, because no satellite service can honestly promise same-day.
Sentinel-2 passes over a given point roughly every two to three days, not daily. Imagery becomes downloadable around half a day to a day after that pass. Cloud then removes a share of what is left, more in cloudy seasons and regions. A change can only be found by comparing two clear images, so what matters is the gap between consecutive clear passes, which can be anywhere from a few days to several weeks.
So for most areas, expect a detection within roughly a week of a change, sometimes faster after a run of clear weather, and longer in a persistently cloudy region. Anyone quoting you minutes for optical satellite monitoring is describing the alert delivery, not the observation.
Sentinel-2 Level-2A optical imagery at 10 metres per pixel, from the European Space Agency’s Copernicus programme, accessed through the Copernicus Data Space. It is open data, so there is no per-image licensing cost and no minimum commitment.
Detections can also be checked against public signals: NASA FIRMS active fire, GLAD and RADD forest-loss alerts, and NASA flood maps. A signal of the matching kind on or near a detection, dated between its two images or a few days after, counts as independent verification of it; a signal of another kind nearby is context only. Construction and urban expansion have no such source at present.
We do not publish an accuracy figure, because we do not yet have one we would stand behind. The model has been evaluated on public benchmark imagery, but not on a labelled set built from our own Sentinel-2 pipeline, and quoting the benchmark number as though it described the live system would be misleading. Building that evaluation set is current work.
What we do instead is make every detection checkable. Each one carries a model score you can filter on, a before-and-after pair of the actual images, and where available a public signal recorded nearby. The intended workflow is that you look at the pair of images and decide, rather than trust a number.
Any size. The satellite sees the ground in 10 m squares, so for an area smaller than 10,000 m² or narrower than 100 m we watch a box at least 100 m across around it, shown dashed on your map, and tell you about change at or next to your area. The smallest change we can report is a patch roughly 50 m across; for areas larger than about 5 hectares the smallest reported change grows with the area, up to 5,000 m². A small change to a single building will not be found. Our Terms do not allow watching private homes or people.
At the other end, the maximum area per boundary depends on your plan, from 200 km² on the free tier up to 3,000 km² on Pro.
Nothing useful, which is the honest answer. Optical satellites cannot see through cloud. A scene where cloud covers the area is recorded but not compared, because comparing a cloudy image against a clear one produces confident nonsense.
In practice this means monitoring is faster in dry seasons and slower in wet ones, and some areas go weeks without a usable image. The comparison view shows the cloud share of each scene so you can see exactly why a period is quiet.
By email on every plan, or, from Starter up, by webhook to a URL you supply. Webhooks are signed with HMAC-SHA256 so you can verify the payload came from us, and a Slack or Microsoft Teams incoming-webhook URL is detected automatically and posted as a readable chat message rather than raw JSON.
You set a minimum model score per area, so you decide where the line sits between hearing about everything and hearing only about the clear-cut cases.
Yes. API access, meaning programmatic use with your own token from the API keys page, is included on Pro and Enterprise. It is a REST API authenticated with a bearer token, covering your monitored areas, detections and alert history, with a general limit of 200 requests a minute and lower limits on some actions, all listed in the API guide. Detections can also be exported as CSV or GeoJSON from the interface on any plan.
Free for one area up to 200 km² with weekly checks, with no card required. Paid plans run from $149.99 a month for three areas up to $599.99 for twenty, with larger areas, more frequent checks, webhook alerts, longer history and API access. Enterprise is priced per deployment.
Pricing is flat monthly, in US dollars and excluding tax, which is added at checkout where it applies. Paid plans renew every month until you cancel, which you can do at any time from the Billing page. There is no per-image or per-scene charge, because the underlying imagery is open data.
Your boundaries and detections are yours. Every detection can be exported as CSV or GeoJSON, and from the Account page you can download all your account data as one JSON file, including your areas with their settings and your detections with their geometry, or delete your account, which takes effect immediately.
If a paid subscription ends, your full history is kept for 60 days so you can move to another provider or take it with you; after that the Free plan’s 30-day history window applies.
The underlying satellite imagery is Copernicus open data, free for anyone to use under its own terms.
See pricing or the TerrAlert overview.