The Tracker monitors a set of mapped objects (polygons) for change over time using bi-temporal Sentinel-1 SAR. For each object it compares acquisitions across dates, flags pixels that changed beyond your thresholds, and records an event when enough pixels change.
A pixel is flagged when its backscatter changes beyond an absolute or relative threshold between two dates, in the direction you care about (decrease, increase, or both). An object becomes an event when enough flagged pixels accumulate, and is confirmed once the change persists.
Tip: Decrease detection on VH is the classic setting for spotting vegetation removal (mowing, harvest, clearing).
A project holds the objects you monitor plus their change-detection settings and recorded events.
Choose one from the project dropdown. Its info line shows the polarisation, change direction, and source type; the objects count appears once objects are loaded.
The objects-loaded line confirms how many were added.
| Button | Opens |
|---|---|
| ↑ Upload Objects | Load monitoring polygons from KML/KMZ. |
| ⚙ Settings | Change-detection parameters — see Settings & thresholds. |
| 🔍 Inspect | The pixel-level Change Inspector. |
Tip: Configure Settings before your first scan — polarisation and direction are locked in once a scan has run, so they're cheapest to get right up front.
The ⚙ Settings modal defines how change is detected. The processing-mode fields lock after the first scan (they define the recorded events), so set them before scanning.
| Setting | Meaning |
|---|---|
| Polarisation | VH or VV — the band compared between dates. |
| Direction | Decrease, Increase, or Both — which way the backscatter must move to count. |
| Time offset (days) | 0 = compare with the nearest previous acquisition; N = compare with one ~N days earlier. |
| Speckle filter | None / Frost / Lee / Median — smoothing applied before differencing. |
| Filter size | 3×3, 5×5, or 7×7 kernel for the speckle filter. |
A pixel is flagged if either threshold is exceeded:
Use the ASC/DSC (separate per orbit) or ALL (combined) mode to set thresholds per orbit track.
| Filter | Meaning |
|---|---|
| Min pixels (MMU) | Minimum changed pixels to record an event. |
| Min ratio | Minimum fraction of changed vs valid pixels. |
| Confirmations | Persistence checks after detection (0 = ongoing; N = lock after N checks). |
| Entry threshold | Minimum reference signal (DN) for an object to qualify. |
| Persistence threshold | Signal level the object must hold for confirmation. |
Tip: Each setting has an ⓘ info button with a fuller explanation. Start from the project defaults and tighten Min pixels / Min ratio if you get too many small false detections.
There are two ways to run change detection, both in the Scan section.
A full pass over a date range — use it the first time, or to re-analyse history.
An incremental scan from the last scan date to now — use it routinely to catch new change.
| Situation | Use |
|---|---|
| First run, or changed settings | Retrospective Scan |
| Routine monitoring of new acquisitions | Check & Update |
Note: Scan progress and the Stop control live on the shared task card, not in the panel — so you can navigate while a scan runs.
After a scan, the Results section summarises what was found and the map colours every object by how much it changed.
| Count | Meaning |
|---|---|
| Confirmed | Change detected and persisted through the confirmation checks. |
| Detected | Change flagged but not yet confirmed. |
| Dismissed | Flagged then ruled out (shown faded on the map). |
| Unchanged | Monitored but no significant change. |
| Button | What it does |
|---|---|
| VIEW | Opens the results window — a sortable table beside a map, with a per-object history chart when you click a row. |
| SAVE | Exports results as CSV, GeoJSON, or shapefile. |
| DROP | Deletes the current scan results (asks first). |
Tip: Sort the results table by changed pixels or change ratio to triage the strongest detections first; click a row to see that object's history and locate it on the map.
The 🔍 Inspect window is for pixel-level analysis — it shows you exactly what changed between two acquisitions for the objects on screen.
Along the top:
| Control | Meaning |
|---|---|
| Previous date / Current date | The two acquisitions to compare. |
| Difference mode | Relative % or Absolute DN. |
| Change direction | Decrease, Increase, or Both. |
| Ground track | Restrict to one orbit track for a same-geometry comparison. |
| Speckle filter / size | Smoothing before differencing. |
| Threshold slider | Raise to show only stronger change. |
| Load | Fetch and render the difference imagery. |
Toggle Outlines, Change pixels, Intensity gradient (blue = decrease → red = increase), SAR current, SAR previous, and Satellite backdrop. Outline colour and width are adjustable.
Tip: Pick the same ground track for both dates — comparing across orbit geometries introduces apparent "change" that's really just a different viewing angle.