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A missing observation stays unknown. Satellite measurements alone do not confirm damage or danger.
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A radar backscatter difference measures a change in returned radar power. Moisture, vegetation, surface roughness, viewing conditions and speckle can affect the signal. Compatible acquisition metadata and native aligned grids support comparison, but the sign of a difference alone does not identify water loss, damage or its cause.
ASF NISAR GCOV guideA small radar patch cannot establish whole-lake conditions.
OPERA DSWx water classifications map surface-water presence at their documented spatial support. Cloud, shadow, snow, vegetation and sensor limitations affect usable coverage. Water area, water-surface elevation, discharge and volume are different quantities; converting among them requires additional compatible measurements and geometry.
NASA OPERA DSWx-S1 productOpen-water detection has spatial and revisit limits; no classified footprint is an automatic flood-damage map.
Explore thresholds in a measured gauge elevation or water-area history when suitable accounting inputs are absent. Supply-and-demand accounting needs compatible flow or volume units, time intervals and stated assumptions. Inspect missing observations and the vertical datum before comparing gauge elevations.
USGS Water Data APIsElevation alone does not establish reservoir volume or downstream discharge.
Discharge describes how quickly water passes a measuring station. A daily mean reading, a difference between two days, and a season-matched descriptive slope answer different questions. Rainfall, season, upstream operations and withdrawals may contribute; the flow record alone does not identify their effects. Station flow does not measure reservoir volume, area-wide retained water or flood danger.
USGS Water Science School: StreamflowInspect station identity, dates, approval status and gaps. A descriptive historical slope is not a forecast or a causal estimate.
On the same valid pixels of two compatible class-1 indicator maps, net mapped change is gained footprint minus lost footprint. The gain-plus-loss footprint describes how much mapped footprint changed locations, even when those areas offset. Class 1 at both dates does not establish continuous water between observations. Valid zero includes other classes, including partial water, and does not establish dry ground. Masked pixels remain unknown comparisons rather than zero-valued changes.
Earthsignal class-1 accounting and NASA OPERA DSWx-HLS definitionsThis is mapped class-1 footprint accounting, not accuracy-adjusted water area, a flood-damage map or causal attribution. · Use the documented product, layer and processing release; unknown release semantics remain withheld.