Marine Navigation: Chart Projections, Coordinates, and Corrections

latitude

Angular distance north or south of the equator.

longitude

Angular distance east or west of the prime meridian.

nautical mile (NM)

1 minute of latitude; simplified as 2,000 yards.

great circle

The shortest path between two points on Earth.

rhumb line

A line crossing all meridians at the same angle; maintains constant heading.

chart projection

The method of flattening Earth's surface onto a chart.

Advantages of Mercator projection

Straight rhumb lines, easy plotting and measuring.

Disadvantages of Mercator projection

Curved great circles, distorted high-latitude areas.

Advantages of Gnomonic projection

Straight great circle routes.

Disadvantages of Gnomonic projection

Curved rhumb lines, hard to measure distance/direction.

Chart 1

Official guide to chart symbols by NOAA and NGA.

chart scale

Ratio of chart distance to real-world distance.

shoal water on NOAA charts

Blue.

Notice to Mariners (NTM)

National chart updates for ocean-going vessels.

Local Notice to Mariners (LNM)

Regional updates for U.S. waters.

how are paper charts corrected

With black ink and mariner·s cross, starting from newest NTM/LNM.

how are electronic charts corrected

Via CDs, downloads, or manual edits; must be updated within 45 days.

datum

Reference plane for chart measurements.

datum shift

Adjustment when GPS datum doesn·t match chart datum.

navigational draft

Ship·s draft plus safety buffer.

shoal water

Any depth equal to or less than the navigational draft.

waypoint

Specific location on a chart, marked with open circle and dot.

trackline

Straight line connecting waypoints; intended path.

cross track error

Perpendicular distance off the intended trackline.

trackline label

Includes true course, magnetic course, speed in knots, and distance.