Temporal and conceptual variability of chart datum based on harmonic constants: a case study for Ilha Fiscal

Abstract This study evaluates chart datum (CD) variability considering sea-level record length and the datum concept adopted. A 1989–2021 tide-gauge record from Ilha Fiscal, Rio de Janeiro, was analysed using multiple series and three CD concepts: approximately Mean Low Water Springs (MLWS), Lower Low Water Large Tide (LLWLT) and Lowest Astronomical Tide (LAT). For six-month series, CD variability for MLWS reached 13 cm. Mean Sea Level (MSL) variability was the main driver of CD changes, exceeding the influence of Z 0 variability. When CD was defined as MLWS from short series, sea level remained below the datum more often than for datums derived from longer records or lower datum concepts. The results indicate that the reference epoch may be as important as the datum concept itself. Thus, CD should be understood not only as a conceptual surface but also as a time-dependent realisation, with implications for hydrographic practice and navigational safety.