Guide

What chart datum means

Every height on this site is measured from it, and it is not sea level.

The zero line

Chart datum is the level all tide heights and all charted depths are measured from. In New Zealand it is set at approximately the lowest astronomical tide — the lowest level expected from sun and moon alone. It is chosen so that the water is almost never lower than the depth printed on a chart, which is exactly the assurance a mariner needs.

Why heights are almost never negative

Because the zero sits near the lowest expected tide, nearly every predicted height is positive. A negative height is possible when weather pushes water below the astronomical low, and it is the condition where boats touch bottom in places that "should" have been fine.

It is not mean sea level

Mean sea level sits well above chart datum — around 1.9 m at Auckland. Land surveying in New Zealand uses local vertical datums instead, so a tide height and a land elevation are measured from different zeros and cannot be compared directly.