remote-sensing-analysis

Remote Sensing Analysis

Purpose: turn raw Earth observation imagery into defensible analytical products. The failure modes here are subtle — uncorrected DNs treated as reflectance, clouds counted as land cover change, indices computed on the wrong bands — so this skill front-loads the checks.

Data access (STAC-first)

Search via STAC APIs rather than per-provider portals; the workflow is uniform and scriptable:

import pystac_client
import odc.stac

catalog = pystac_client.Client.open("https://earth-search.aws.element84.com/v1")
items = catalog.search(
    collections=["sentinel-2-l2a"],
    bbox=[27.0, 38.3, 27.4, 38.6],
    datetime="2025-05-01/2025-09-30",
    query={"eo:cloud_cover": {"lt": 20}},
).item_collection()
ds = odc.stac.load(items, bands=["red", "nir", "scl"], resolution=10, chunks={})

Key collections: sentinel-2-l2a (10 m optical, surface reflectance), landsat-c2-l2 (30 m, 1982→), sentinel-1-grd (SAR, weather-independent). Microsoft Planetary Computer mirrors most (needs planetary_computer signing). For continental/global extents or decades-long stacks, route to google-earth-engine instead of downloading. Record collection + item IDs + search parameters for reproducibility.

Processing-level discipline

Always state which level you used. Never mix TOA and BOA scenes in one composite or time series. Landsat Collection 2 L2 needs its scale factors applied (reflectance = DN * 0.0000275 - 0.2).

The Sentinel-2 baseline discontinuity — passes the level check above

Processing Baseline 04.00, applied from 25 January 2022, added a constant BOA_ADD_OFFSET (currently −1000) to L2A digital numbers so that negative surface reflectance can be encoded. Two scenes on opposite sides of that date are both L2A: the level check above sees nothing wrong while their DNs sit 1000 apart. Differencing them yields a systematic reflectance shift that reads as real change and survives every mask, threshold and accuracy report you apply afterwards.