Public Atlas Editorial · Published 2026-09-29 · Updated 2026-09-29. Independent human review pending. Next scheduled review: 2026-10-29. AI-assisted resource.
The short answer
Use the glossary to identify what a measurement means, then follow its guide for the reference, worked example and source. Similar-looking units can describe different things.
- Conservation storage
The amount of water stored within a reservoir’s conservation pool. It is not necessarily the same as total storage.
Read the explanation and sources →- Percent full
A storage figure divided by the relevant capacity and expressed as a percentage. Check which storage and capacity a source uses.
Read the explanation and sources →- Datum
The reference level from which a height is measured. Heights with different reference levels cannot be compared as though they shared a zero.
Read the explanation and sources →- MLLW
Mean Lower Low Water: a tidal datum based on the average lower low water over a specified tidal epoch. It is a reference level, not a promise of the lowest possible water.
Read the explanation and sources →- Tide prediction
A calculated tide value for a station and time. It differs from an observed water-level measurement and does not include every weather effect.
Read the explanation and sources →- Gauge height (stage)
Water-surface height relative to a gauge’s reference. It is not the depth across the river and is not directly comparable between unrelated gauge references.
Read the explanation and sources →- Discharge
The volume of water passing a location per unit of time. This differs from a height measurement.
Read the explanation and sources →- Radon zone
EPA’s geographic classification of indoor radon potential. It does not establish the radon level in a particular home.
Read the explanation and sources →- ZCTA
A Census ZIP Code Tabulation Area: a geographic approximation of ZIP-code areas. It is not the same as a postal delivery route and may overlap counties.
Read the explanation and sources →- Locality pay
A geographic adjustment reflected in the applicable federal pay table. Table selection matters; special rates and other pay rules may apply.
Read the explanation and sources →- Burn ban
A restriction issued by the relevant authority. Its scope, timing and exceptions must be checked in the applicable order. A fire-danger indicator alone is not that order.
Read the explanation and sources →- Preliminary storm report
An early report that can change as information is reviewed. A later archive may use different event records and classifications.
Read the explanation and sources →
Common questions
Why does the reference datum matter?
A height needs a reference level. Comparing values with different zero references can create a misleading difference.
Where can I find an example for a glossary term?
Each definition links to a field guide. Use that guide’s example and sources before applying the term to a result.