The unit of the chart. The unit of the chart is the length of an equatorial mile expressed in millimetres. The basic element used to build a map in Mercator projection is the unit-of-the-chart.

# Category: Basic Knowledge

Rhumb line in the Mercator projection The Mercator projection is a plane conformal projection and is not perspective. This projection is the most often use in navigation.

The variation of the chart scale in the Mercator projection In the Mercator projection, the chart scale varies, increasing proportionally with (sec ϕ) from the Equator to the poles.

Difference of Meridional Parts or Meridional Difference. The Difference of meridional parts or Meridional difference (Δϕc): It’s the distance measured on the meridian between two parallels of a Mercator projection. Or, the difference between the meridional parts of any two given parallels.

Meridional Parts Meridional Parts (or Increased Latitude): It is the length of the arc of meridian in the Mercator projection, comprised between the Equator and the considered latitude and expressed in minutes of Equator. Noted with (ϕc) or (M).

Rhumb Line The Rhumb Line: A conventional line on the Earth’s surface that intersects all geographic meridians at the same angle. It has the shape of a spiral that approaches the poles without going through them.

Nautical Charts. Types of Scales. The Nautical Charts showing the Earth’s surface or a portion thereof on a plan, in a certain proportion (scale) and with deformations subject to mathematical laws.

Echo Sounder. The correction of depth. Echo Sounder: It is an electric device used to determine the water depths by measuring the time interval for sound waves to go to the sea bottom and back again. It is also called Depth Finder or Acoustic Depth Finder.

Estimated Time of Arrival. ETA ETA: Conventional abbreviation for ‘Estimated time of arrival’ or ‘Expected time of arrival’. The estimated time of arrival or ETA is a measure of when a vessel is expected to arrive at a certain place.

The relationship between distance, speed and time. The relationship between distance, speed and time is based on the well-known formula: Distance = Speed * Time