How is a wildfire scar identified in satellite imagery?
Burn scars can be investigated through changes in vegetation and surface reflectance between satellite observations before and after the event. Normalized Burn Ratio combines near infrared and shortwave infrared bands; the difference across dates is called dNBR. Neither a colorful change map nor one threshold definitively establishes a fire perimeter or soil damage without acquisition dates, masks and field checks.
Choosing comparable satellite scenes
1. Why do image dates matter? The prefire image should represent vegetation at a comparable stage of its growing season. Contrasting a green spring scene with a dry late summer scene can show a large difference even without fire. The postfire scene is best acquired when smoke and cloud have cleared, but a much later scene may show regrowth that masks the initial impact. Harvest, snow and land-use changes need separate interpretation. Processing levels, spectral bands and geometric positions must be compatible. A one-pixel offset can create apparent change along a narrow edge. Always report acquisition times and areas that could not be observed. 2. How is NBR calculated? Normalized Burn Ratio is near infrared reflectance minus shortwave infrared reflectance, divided by their sum. Living foliage and burned surfaces can produce different signals in these bands. USGS derives Landsat NBR from processed surface reflectance data. Subtracting postfire NBR from prefire NBR gives a dNBR change layer. A large difference may indicate stronger spectral change, but one threshold does not have the same ecological meaning in every vegetation type. Bare soil, water and shadow influence values too. This ratio measures reflected light, not flame temperature or the depth to which soil burned. 3. Is the mapped scar identical to the incident perimeter? An operational fire perimeter, a thermal hotspot and a later vegetation change outline are different datasets. Unburned islands can lie inside the operational boundary. Pixel size mixes narrow roads and sparse shrubs. Cloud, smoke, snow and shadow must be masked with product-specific quality flags. The USGS burn-severity glossary acknowledges places that cannot be mapped reliably because of these interferences. Unobserved ground must not be labeled unburned. Compare the satellite outline with independent event reports before explaining any difference as a mapping error.
Severity, area and field validation
4. What does burn severity mean? Satellite spectral difference is sensitive to vegetation condition; it is not identical to the amount of organic soil consumed or survival of individual trees. Vegetation type, initial canopy density and rainfall-driven regrowth all affect the signal. High dNBR alone does not establish severe soil effects everywhere. Calibrate classes against field plots and relevant local agency assessments. Whether a fire occurred and whether green cover changed strongly are separate questions, and a landscape can contain a mosaic of outcomes inside one boundary. 5. How is area estimated? Add up valid pixels assigned to a burned class using an appropriate map projection. Report cloudy ground, water, harvested fields and missing observations separately. Different pixel sizes, thresholds and denominators prevent meaningful comparison from screenshots alone. Even if a small fire occupies several pixels, its exact edge remains uncertain. Compare sampled boundaries with independent field photographs, aerial imagery and official incident data. The apparent precision of a numerical area must reflect uncertainty from classification, geometry and observation gaps. 6. What can the map support? Spectral change can guide recovery surveys and studies of postfire erosion or flooding. It cannot by itself direct active evacuations, establish smoke exposure or certify damage at one building. State the sensor, bands, image dates, formula, class thresholds, masked fraction and validation samples. Follow current emergency and forest authority instructions for safety decisions. An archive image of last month's burn scar is not a live active-fire map, even when displayed on an interactive website.