# Satellite Change Explorer

> Compare NASA satellite images of any place on two dates: true colour, NDVI, night lights. Pick a place (latitude/longitude + radius, or a bounding box) and two dates. The tool builds side-by-side NASA GIBS images for each chosen layer — MODIS/VIIRS true colour, 7-2-1 false colour, 30 m Harmonized Landsat Sentinel-2, NDVI composites and VIIRS night lights — with the resolution, revisit and cloud caveats you need to read them correctly.

Part of usefulweb.site (https://usefulweb.site/). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://satellitechangeexplorer.usefulweb.site/ (interactive tool at https://satellitechangeexplorer.usefulweb.site/tool)
- JSON API: POST https://satellitechangeexplorer.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://satellitechangeexplorer.usefulweb.site/api/v1/tool)
- OpenAPI: https://satellitechangeexplorer.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://satellitechangeexplorer.usefulweb.site/mcp - tool `satellite_change_explorer`

## Inputs
- lat (number): Latitude
- lon (number): Longitude
- radiusKm (number): Half-width of the area
- bbox (text): Or bounding box (west,south,east,north) - Overrides latitude/longitude when set.
- dateBefore (date): Before date - Default: one year before the "after" date.
- dateAfter (date): After date - Default: yesterday (UTC).
- layers (multiselect): Layers
- width (integer): Image width

## Method
- The area is a lat/lon box: from a centre, half-width r km gives ±r/111.32° latitude and ±r/(111.32·cos φ)° longitude. Image height follows the box aspect in degrees (plate carrée, EPSG:4326).
- Images come from NASA GIBS via the Worldview Snapshot API (https://wvs.earthdata.nasa.gov/api/v1/snapshot, REQUEST=GetSnapshot, CRS=EPSG:4326, BBOX=south,west,north,east, TIME=YYYY-MM-DD); an equivalent GIBS WMS 1.3.0 GetMap URL is returned too.
- Layer identifiers, maximum resolutions (tile-matrix sets), formats and first available dates were taken from the GIBS EPSG:4326 "best" WMTS capabilities. Composite layers are requested at the start of their period (16-day periods begin on day-of-year 1, 17, 33, …; monthly on day 1).
- If one image pixel is smaller than half the layer resolution the result flags "upsampled": detail finer than the native resolution is interpolation, not information.
- No automated change detection: the tool cannot download image pixels, so it does not compute change statistics. Compare visually and mind clouds, season, sun angle and sensor differences.

## Limits
- Imagery courtesy of NASA EOSDIS GIBS. Visual comparison only; not suitable for legal or insurance evidence without the source data.