The chart’s rules of grammar

A chart is not a photograph of the sea — it is a legal document with strict grammar. Every number on it is measured from a stated reference, and the first thing a navigator checks is which reference. Get the grammar and any chart in the world reads the same way.

Depths (soundingssoundingsThe charted depth figures, measured below chart datum.full glossary →) are measured below chart datumchart datumThe reference level charted depths are measured from — roughly the lowest the tide ever falls.full glossary →, the level the tide almost never falls under — so a charted depth is close to the least water you will ever find there. Soundings print as a large figure with a small one after it for decimetres: a big 12 with a small 8 is 12.8 m. Heights of land features — and the clearances under bridges and cables — are measured the other way, from high references: land heights from MHWS, overhead clearances from HAT, the meanest cases in each direction.

Drying features get a third treatment: an underlined figure on the green intertidal tint is a drying heightdrying heightHow far a bank or rock that covers and uncovers stands ABOVE chart datum (underlined on the chart).full glossary →, standing that far above chart datum and covered only when the tide exceeds it.

Scale — what the chart is for

Scale is a ratio: on a 1:25,000 chart, one unit on paper is 25,000 at sea. Counter-intuitively, the smaller the number the larger the scale — 1:25,000 shows a harbour in glorious detail; 1:150,000 shows a coastline for passage-making. The working rule: plan on the small-scale chart, navigate on the largest-scale chart you have for the waters you are actually in. Detail you cannot see is detail that can hurt you.

Charts also state their horizontal datum — effectively which mathematical model of the earth the positions sit on. Modern charts use WGS84, the same datum GPS speaks natively, so plotter positions transfer straight onto the paper. On an old chart with a different datum, a GPS position can be hundreds of metres adrift; the chart’s title block says what, if anything, to apply.

Corrections — a chart is only as good as its date

Buoys are moved, wrecks are found, depths shoal — and the chart only knows if somebody tells it. Official charts are kept honest by NOTICES TO MARINERS, the published correction stream; a corrected chart records each applied notice in small print at the bottom-left corner, and that line of tiny numbers is the chart’s service history. The working questions for any chart aboard: what edition, and corrected to when? An uncorrected chart of a changing area is a confident liar.

The same discipline wears different clothes electronically. Official hydrographic-office charts feed both RASTER products (exact pictures of the paper, warts and all) and VECTOR products (databases redrawn at every zoom). Vector charts update easily and query nicely, but their zoom behaviour — detail appearing and DISAPPEARING by level, as the GNSSGNSSSatellite navigation in general — GPS and its siblings.full glossary → lesson warns — and the age of the survey underneath are both invisible at a glance. Leisure chart brands repackage the same official data on their own cycles. Whatever the format: know its date, keep it updated, and treat “the screen looks modern” as evidence of nothing.