An attempt was made to detect changes in spectral signals due to prop scarring and restoration efforts.
Area of Interest
A polygon for the Area of Interest (AoI) were hand-drawn to approximate polygons identified in the “CCA24 Phase 1 Installation Map Package_Nov2025.pdf” document [ref email for Tylar only].
A “reference region” of nearby area with similar optical characteristics was drawn.
Imagery, PreProcessing, Time Series Extraction
Landsat-Harmonized Sentinel-2 imagery was sampled across these polygons using this GEE script.
CloudScore+ cloud filtering was applied. BandSum normalization was performed using the sum of all bands except for band 9.
The resulting time series data were uploaded to github/7yl4r/HabEvent/aoi-extractions.
Indices
Four spectral indices are calculated from the raw band values to target seagrass.
- WAVI:
(B8-B2)/(B8+B2) - inverse NDSVI (~NDSVI)
(B3-B2)/(B3+B2) - red_edge1/red
B5/B4 - Normalized Difference red_edge1-red
(B5-B4)/(B5+B4)
Seasonal Decomposition, Reference Region Adjustment, & Results
A seasonal decomposition with an additive model was applied to separate the seasonal signal from long term trends and residuals.
The reference region is used as a baseline against which to compare the AoI. This can be done with a subtraction or a division operation.
The resulting differenced values are converted to z-scores for display on a common y-axis.
Subtraction
A sine-shaped seasonal signal is observed in each of the indices. Long-term trends for WAVI, red_edge1_red_ratio, and ND_red_edge1_red follow each other closely.
Division
The expected sine-wave-shaped seasonal signal is apparent in the red_edge1_red_ratio index. The other three indices have less well-defined seasonal signals.
Spikes in the residuals are observed; these may be the result of divide-by-almost-zero when the reference region has very low values.