Chapter 2: Geodesy and Datums
Positions mean nothing without a datum. Geodesy models the Earth as an ellipsoid approximating the geoid, and charts are referenced to a horizontal datum that may differ from the GNSS datum by hundreds of metres.
Geoid, ellipsoid and spheroid
The geoid is the equipotential surface of Earth's gravity field that mean sea level would follow; it is irregular. For computation an ellipsoid of revolution is fitted to it, defined by semi-major axis a and flattening f. WGS 84 uses a = 6 378 137 m and 1/f = 298.257223563. Heights above the ellipsoid (GNSS) differ from heights above the geoid (charted) by the geoid separation N.
Horizontal datums and datum shift
A horizontal datum ties the ellipsoid to physical monuments. WGS 84 and NAD 83 are effectively coincident for practical navigation, but older local datums — Tokyo, European 1950, Old Hawaiian, many island datums — can displace a charted position by several hundred metres. Charts carry a note giving the shift to apply to satellite positions. Never plot a GNSS position on a chart of a different datum without applying the stated correction.
- Check the chart's datum note before plotting any GNSS fix
- Set the GNSS receiver datum to match the chart when possible
- Treat unsurveyed or lead-line surveyed areas with wide margins
Vertical datums
Soundings are referenced to a chart datum (sounding datum) chosen so that the water is rarely lower — commonly Mean Lower Low Water (US) or Lowest Astronomical Tide (IHO recommendation). Charted heights of lights and bridges are referenced to a high-water datum such as Mean High Water Springs, so clearances shrink at low water only for heights, never for depths.
Key takeaways
- • WGS 84 is the GNSS datum; the chart datum may not be WGS 84.
- • Datum mismatch is a silent error — it does not degrade GNSS accuracy indicators.
- • Depths are below chart datum; vertical clearances are above a high-water datum.
Knowledge check
Immediate feedback with rationales. Answer each question to see the explanation.
Chapter 2 quiz
0 of 3 answered
1.A chart on Old Hawaiian Datum notes that satellite positions should be shifted. If the shift is ignored, the plotted position is:
2.Charted depths are normally referenced to:
3.The geoid is best described as: