Measurement methodology

We preserve the source’s units, dates, measurement type, quality and water-surface reference. A daily observation and a daily mean are different measurements. The latest available value may be delayed; it is not a live reading.

Freshness and completeness

For dynamic pages, measurement age is recalculated on each request. A daily record is considered stale after seven provider calendar days. Timestamps keep their original offset, and freshness is unknown if the time zone cannot be resolved. If a refresh fails, the last available measurement and its original date remain visible. Static pages show freshness at the time the page was generated.

A provider-reported daily mean does not establish that all expected samples exist. Sampling-complete means are suppressed when completeness is unverified. Exact-date differences between eligible provider-reported daily means remain available with an explicit sampling-completeness qualification. Approved and provisional describe provider quality, not sampling completeness.

Changes and historical context

One-day and seven-day changes require eligible observations on both exact dates, from the same series, water datum and known operating regime. We do not fill in missing dates or substitute nearby measurements.

Prior-year comparisons use the exact month and day within the fixed January 1, 2006–December 31, 2025 reference interval. The actual eligible years and sample count appear on each page. The captured series begins in 2024 for this release; it is not a complete twenty-year record or a seasonal norm.

Percentiles are disabled in the frozen methodology. The declared quantile method is Hyndman–Fan type 7; at least ten distinct eligible years would be required under an approved future qualification. February 29 percentiles remain suppressed.

Extrema and charts

The lowest and highest values come from eligible daily records in the displayed date range. We list every tied date and label incomplete years. Missing days may hide more extreme levels. Daily-mean highs and lows are not instantaneous or all-time records, and corrections may change them.

Charts retain first, minimum, maximum and last values in display buckets. Missing, invalid or incompatible observations break the line. This reduction is for display only; calculations and the accessible history table use original values.

Datum, units and corrections

Incompatible datum or series segments are excluded from combined comparisons. Unit conversion does not transform a vertical datum. Where the provider’s foot subtype is unestablished, native feet are displayed without conversion. Decimal places follow source reporting and do not imply measurement accuracy.

Refreshes reconcile provider corrections into a new version of the lake dataset before replacing the active version. The downloaded CSV includes statistic, units, series, quality and source provenance.