Water Mask / NDWI
Separates water from many land surfaces using Green and NIR contrast.
NDWI = (Green − NIR) / (Green + NIR)
Best used when: a first water/non-water separation is required.
Hydro Preprocessing
Prepare optical imagery for aquatic analysis by generating a water mask, reducing sunglint, and optionally deriving depth-invariant spectral information for shallow-water studies.
Remote Sensing CoreEnd-User DocumentationTheory + Formula + Parameters1. What This Feature Does
Prepare optical imagery for aquatic analysis by generating a water mask, reducing sunglint, and optionally deriving depth-invariant spectral information for shallow-water studies.
Cloud workflow: choose data → configure the scientific method → run → review the resulting layer/report. No programming is required.
2. Recommended Workflow
Identify Green/NIR or other required bands.
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Generate or review the water mask.
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Apply sunglint correction if needed.
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Optionally derive water-column/depth-invariant bands.
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Use the result for water quality, benthic, shoreline, or coastal analysis.
3. Theory, Methods & Equations
Hedley Sunglint Correction
Uses the NIR relationship with visible bands to remove a component of specular surface reflection.
R′i = Ri − bi(RNIR − RNIR,min)bi is the regression slope between band i and NIR over water.
Best used when: sun glint visibly contaminates water pixels.
Depth-Invariant / Water Column Correction
Transforms shallow-water reflectance so that bottom/material differences can be separated more effectively from water-depth effects.
Xi = ln(Ri − Ri,∞)
Dij = Xi − (ki/kj)XjA Lyzenga-style depth-invariant relationship.
Best used when: mapping shallow benthic/substrate classes where depth strongly changes brightness.
4. Input Data
| Input | Type | Requirement | Description |
|---|---|---|---|
| Input Reflectance Raster | raster | Required | Surface Reflectance is recommended. Landsat/Sentinel scaled Level-2 products are normalized automatically. |
5. Parameters Available in the Application
| Parameter | Default | Choices / Range | How to Use It |
|---|---|---|---|
| Water Mask | On | — | Use NDWI (Green/NIR) to isolate water pixels in the output. |
| Sunglint Correction | On | — | Apply Hedley-style NIR regression correction to visible bands. |
| Water Column Correction | Off | — | Generate Lyzenga depth-invariant indices from corrected visible reflectance. |
| Blue Band | — | 1 to — | |
| Green Band | — | 1 to — | |
| Red Band | — | 1 to — | |
| NIR Band | — | 1 to — | |
| NDWI Water Threshold | 0.0 | -1.0 to 1.0 | Pixels with NDWI greater than this threshold are treated as water. |
| Deep Water NIR Percentile | 20.0 | 1.0 to 50.0 | Lowest-NIR water pixels used to estimate the sunglint regression. Shown for: True |
| Water Column Output | depth_invariant_indices | depth_invariant_indices, corrected_plus_indices | Shown for: True |
| Output Nodata | -9999 | — |
6. Output & Interpretation
The result should be interpreted according to the selected method and the physical meaning of the input data. Preserve source units, coordinate reference information, NoData meaning, acquisition date, and preprocessing level when comparing results.
Scientific interpretation: an algorithm can produce a numerically valid result even when the input data are unsuitable. Always check masks, units, sensor characteristics, spatial resolution and reference data.
Important: Water-column methods are assumptions about shallow-water optics; they do not replace field calibration or bathymetric validation.
7. Best Practices
- Use analysis-ready inputs and remove invalid/cloud/noise artifacts that are not part of the target phenomenon.
- Choose parameters from the physical scale of the data, not only from visual appearance.
- Keep categorical and continuous rasters conceptually separate when selecting interpolation/resampling methods.
- Compare the result with the source image and independent reference information.
- Document the settings used when results will be compared across dates, sensors or study areas.