How TerrAlert Works: A Simple Guide

How TerrAlert Works: A Simple GuideTerrAlert Team · 2026-09-09This is a straightforward walkthrough of what TerrAlert actually does, step by step, describing only functionality that exists in the product today.Step 1: You define an area to watchYou draw a boundary, an Area of Interest, on a map in the dashboard, or submit one programmatically as GeoJSON through the REST API if you are on a plan with API access. You also choose which categories of change to watch for at that location: deforestation, construction and development, flooding, fire, vegetation loss, or urban expansion.Step 2: TerrAlert fetches new satellite imagery as it becomes availableTerrAlert uses Sentinel-2 optical satellite imagery, sourced through the Copernicus Data Space Ecosystem, the European Space Agency's public, open-access programme. As new imagery of your area is captured and becomes available (which, per Sentinel-2's revisit schedule and typical cloud cover, is usually every several days for a genuinely clear, usable image; see How Fast Are Satellite Alerts, Really?), TerrAlert retrieves and processes it.Step 3: A change-detection model compares each new clear pairOnce two clear images of the same area exist, a "before" and an "after," an AI model (a Siamese U-Net architecture, for anyone curious about the technical detail) compares them and identifies areas that differ. The result is a set of candidate change polygons, each with an estimated area and a confidence score.Step 4: Findings are corroborated where possibleBefore most detections become alerts, TerrAlert checks them against independent data sources rather than relying on the optical model alone: NASA's FIRMS active-fire product, the GLAD and RADD forest-loss alert systems, NASA's GIBS flood-extent data, and, on Pro and Enterprise plans, Sentinel-1 radar coherence change, which is specifically useful for construction because radar sees through cloud cover that would otherwise delay an optical-only finding. This corroboration step is the main reason TerrAlert's false-alarm rate stays manageable. See Understanding False Alarms for why that matters more than a single headline accuracy number.Step 5: You get alerted, only for what clears your thresholdEvery Area of Interest has a configurable minimum-confidence threshold. A detected, corroborated change above that threshold triggers an alert, delivered by email or by HMAC-signed webhook, including automatic detection of Slack and Microsoft Teams incoming-webhook URLs, which get a plain chat message instead of the raw signed payload.Step 6: You review it in the dashboardEvery detection includes a before/after split-view comparison, the estimated area affected, the confidence score, and the category of change. From there, the decision about what to do next, investigate, escalate, dismiss, is yours. TerrAlert surfaces the finding; it does not make the call for you.What TerrAlert deliberately does not claim

It does not claim real-time detection. See the honest numbers on the FAQ.

It does not publish a single headline accuracy percentage, because a benchmark number does not represent performance across every real-world location. Corroboration and a configurable threshold are the actual reliability mechanism.

It does not classify why a change happened, only that it happened and which broad category it resembles.

Key takeaways

TerrAlert's pipeline is: define an area, fetch new clear imagery, run change detection, corroborate against independent sources, alert above your threshold, review in the dashboard.

Corroboration against independent public data sources, not a single model's confidence score, is the main lever TerrAlert uses to keep false alarms manageable.

Detection speed is bounded by real satellite physics (revisit schedule and cloud cover), not by TerrAlert's software, and the product is upfront about that.

FAQDo I need to be technical to use TerrAlert? No. Drawing an area and choosing categories to watch requires no technical background. The REST API is there for teams that want to integrate it into their own systems, on Pro and Enterprise.What happens if no clear imagery is available for a while? Nothing gets reported for that area until a new clear image exists, which is correct behavior, not a system failure.References

ESA: Copernicus Data Space Ecosystem

NASA FIRMS

World Resources Institute: Global Forest Watch (GLAD/RADD)

Ready to see it on a real area? Start free or read Getting Started With TerrAlert.All posts