This report screens Coral Health (Landsat, long-record) across a 354.4 km² study area from satellite imagery composited over 2024-06-01 to 2024-07-01.
The Allen Coral Atlas maps 72.457 km² (71.4%) of the area as coral/algae habitat, over which the coral, landscape and trend results were computed.
These are satellite-derived screening results, not field-validated; see Limitations.
How this map was built. The pipeline ran these steps in order, each handing its region/mask to the next:
What you are looking at. The colours show Coral Health (Landsat, long-record). On the legend, values run from about 0.22 to 2.46:
Higher = more coral-like cover, lower = more bare substrate. Note the index cannot separate live coral from macroalgae (both are dark) — a high value can mean healthy coral OR algal overgrowth, so pair it with turbidity/chlorophyll before concluding. Deep open water has been removed, so every coloured pixel is genuine shallow reef/seabed.
Source & caveat. USGS Landsat Collection 2 Level-2 Science Products (LC08 / LC09 / LE07 / LT05). This is a satellite-derived screening index — treat it as a guide and confirm on the ground before drawing firm conclusions.
| Class | Value range | Area (km2) | Area (ha) | Share (%) |
|---|---|---|---|---|
| Very Low | 0.22 to 0.67 | 31.816 | 3181.6 | 70 |
| Low | 0.67 to 1.12 | 8.862 | 886.2 | 19.5 |
| Medium | 1.12 to 1.57 | 2.249 | 224.9 | 4.9 |
| High | 1.57 to 2.02 | 1.047 | 104.7 | 2.3 |
| Very High | 2.02 to 2.46 | 1.469 | 146.9 | 3.2 |
| Richness (classes present) | 5 | Shannon diversity (SHDI) | 0.915 |
| Shannon evenness (SHEI) | 0.568 | Simpson diversity | 0.468 |
| Dominant class share | 70.0% |
| Number of patches | 1,013 | Patch density (/100 ha) | 22.29 |
| Mean patch size (ha) | 4.49 | Area-weighted mean patch (ha) | 123.97 |
| Largest Patch Index (%) | 4.6 | Edge density (m/ha) | 66.5 |
| Benthic class | Area (km2) | Area (ha) | Share (%) |
|---|---|---|---|
| Coral/Algae | 72.457 | 7245.7 | 71.4 |
| Rubble | 19.717 | 1971.7 | 19.4 |
| Sand | 7.319 | 731.9 | 7.2 |
| Rock | 1.617 | 161.7 | 1.6 |
| Seagrass | 0.347 | 34.7 | 0.3 |
| Microalgal Mats | 0.068 | 6.8 | 0.1 |
Allen Coral Atlas mapped 72.457 km² (71.4%) of this area as Coral/Algae. Because the ACA step ran before Coral Health, the Coral Health index, landscape metrics and trend above were all computed over this ACA-confirmed coral only.
| Step | Layer | Role | Status | Detail |
|---|---|---|---|---|
| 1 | Land Mask (exclude ocean) | mask | computed | Land-only mask published (ocean excluded via the SRTM land footprint). Dry-land indicators placed after this are now restricted to land. |
| 2 | Coral Habitat (Allen Coral Atlas) | mask | computed | Allen Coral Atlas: Coral/Algae covers 72.457 km2 (71.4%). Its coral mask is now published — any Coral Health / Trend / index step AFTER this runs over ACA coral only. |
| 3 | Coral Health (Landsat, long-record) | index | computed | Coral Health on Landsat (30 m): ln(green) - ln(blue) on optically-shallow reef; oriented so HIGHER = more coral/algae cover (darker bottom), LOWER = bright sand/rubble; stretched 0.22 to 2.46. Built from the harmonized Landsat 5/7/8/9 archive (1984+) so the record spans the FULL timeline, incl. pre-2018. Coarser than the Sentinel-2 version and NOT sun-glint corrected -> treat as a screening long-record index, and note small cross-sensor (TM/ETM+/OLI) offsets. |
All layers were computed on Google Earth Engine over the study boundary. Point-in-time layers were composited over 2024-06-01 to 2024-07-01 using the Median reducer at an adaptive spatial scale matched to the boundary size (Sentinel-2 SR Harmonized with per-pixel SCL cloud masking; Landsat Collection-2 for land indices / pre-2015). Marine optical layers used a blue-green bottom index, ln(green)−ln(blue) (ln B3 − ln B2), over optically-shallow reef (deep water excluded), oriented so higher values indicate darker, light-absorbing bottoms (coral / algae / seagrass) and lower values bright sand / rubble; it reflects benthic cover/brightness and cannot on its own distinguish live coral from macroalgae. The Sentinel-2 index applies sun-glint correction (Hedley et al., 2005); the Landsat long-record index (30 m, 1984+) is not sun-glint corrected and carries small cross-sensor (TM/ETM+/OLI) offsets, so it is treated as a screening long-record index. Processing chain: Land Mask (exclude ocean) → Coral Habitat (Allen Coral Atlas) → Coral Health (Landsat, long-record).
| Sensor / platform | Dataset (Earth Engine ID) | Temporal coverage | Native resolution | Bands used | Role in this study |
|---|---|---|---|---|---|
| Landsat 5/7/8/9 (Collection 2, Level-2 SR) | LANDSAT/LC09|LC08|LE07|LT05/C02/T1_L2 | 1984 → present (multi-mission) | 30 m (optical); 100 m thermal, resampled 30 m | SR blue/green/red/NIR/SWIR1/SWIR2 + ST thermal | Urban Sprawl (NDBI), Urban Heat / LST, long-record indices |
| NASA SRTM digital elevation | USGS/SRTMGL1_003 | Static (acquired Feb 2000) | 30 m | elevation | Land Mask (land/ocean footprint), terrain layers |
| Allen Coral Atlas — benthic habitat | ACA/reef_habitat/v2_0 | Static ~2018–2020 baseline | ~5 m mapped classes | benthic class (Coral/Algae, Sand, Rubble, Rock, Seagrass, Microalgal mats) | Coral-habitat mask — restricts marine steps to mapped coral |
| Parameter | Value |
|---|---|
| Boundary area (km²) | 354.365 |
| Date range | 2024-06-01 to 2024-07-01 |
| Compositing reducer | Median |
| Adaptive spatial scale (m) | 30 |
| Composite thumbnail (px) | 500 |
| Processing chain | Land Mask (exclude ocean) → Coral Habitat (Allen Coral Atlas) → Coral Health (Landsat, long-record) |
| Datasets | NASA SRTM Digital Elevation 30m (USGS/SRTMGL1_003); Allen Coral Atlas (Earth Engine: ACA/reef_habitat/v2_0).; USGS Landsat Collection 2 Level-2 Science Products (LC08 / LC09 / LE07 / LT05) |
| Cloud handling | Sentinel-2: per-pixel SCL mask; image-level filter relaxed (<=90%); clear pixels reduced by the reducer above |
| Coordinate reference system | EPSG:4326 (WGS84) |
| Analysis parameters | Shallow-water mask: green reflectance > 0.015; display stretch: 2-98th percentile; OISST anomaly raw x0.01 °C vs 1971-2000 climatology; Sentinel-2 image cloud filter <=90% + per-pixel SCL; benthic composite max cloud <=90%. |
| Bounding box (WGS84) | 99.5979 W, 6.4934 S, 99.7141 E, 6.7417 N |
| Software versions | R 4.6.0; rgee 1.1.8; earthengine-api 1.6.11; app db7d10a |
| Platform | Google Earth Engine (rgee) - Spatial Research Suite pipeline |
| Generated | 2026-08-24 10:38 IST |