Marine Navigation

100 flashcards by MentisLibrary

Finding your position and keeping the ship safe: charts and compass, dead reckoning and fixes, tides and currents, aids to navigation, the collision regulations, electronic and celestial navigation, and passage planning.

What is a nautical mile, and how does it relate to latitude?
1,852 metres by international definition, which is very close to the length of one minute of arc of latitude.
What is a knot?
One nautical mile per hour. It is a speed, so the phrase knots per hour is wrong.
How is latitude measured, and how is longitude measured?
Latitude runs 0 to 90 degrees north or south of the equator. Longitude runs 0 to 180 degrees east or west of the prime meridian at Greenwich.
What makes the Mercator projection useful for navigation?
A straight line drawn on it is a rhumb line, a line of constant true course, so a course can be laid off with parallel rules and steered directly.
How is distance measured on a Mercator chart?
From the latitude scale at the side of the chart, using the part of the scale at the same latitude as the area being measured.
What is the gnomonic projection used for?
Great circle planning. On a gnomonic chart a great circle, the shortest path between two points, plots as a straight line.
What does chart scale mean, and which scale shows more detail?
Scale is the ratio of chart distance to real distance. A large scale chart such as 1 to 20,000 covers a small area in great detail. A small scale chart such as 1 to 500,000 covers a large area with less detail.
What datum do charted depths use, and what datum do charted heights use?
On United States charts, soundings are referred to mean lower low water, and heights of lights, land features, and bridge clearances are referred to mean high water.
Why must the units of soundings be checked on every chart?
Depths may be printed in metres, in fathoms, or in feet depending on the chart, and the units are stated in the title block and often in the chart margins.
Why can a GPS position disagree with an older chart?
The chart may be drawn on a different horizontal datum than the satellite system uses. Modern charts use WGS 84, but older surveys may be offset by hundreds of metres.
What is Chart No. 1?
The publication that explains every symbol, abbreviation, and term used on nautical charts.
How are charts and publications kept current?
Through the Notice to Mariners, which lists corrections to charts, light lists, and sailing directions, along with temporary and preliminary changes.
What do the two rings of a chart compass rose show?
The outer ring is oriented to true north. The inner ring is oriented to magnetic north and is annotated with the variation, the year it was measured, and the annual rate of change.
What is the difference between IALA Region A and Region B?
They assign opposite colours to lateral marks. In Region B, which includes the Americas, Japan, and Korea, red marks are kept to starboard when returning from seaward. In Region A, which includes Europe, Africa, and most of Asia and Australasia, red is kept to port.
What shapes and numbers do lateral buoys carry in IALA Region B?
Red marks are conical nun buoys with even numbers. Green marks are cylindrical can buoys with odd numbers. Numbers increase proceeding from seaward.
How is a preferred channel mark read?
It is banded red and green horizontally. The colour of the topmost band tells which channel is preferred: a red top band means treat the mark as red, and a green top band means treat it as green.
What is a safe water mark, and how is it passed?
A mark showing red and white vertical stripes, sometimes with a red spherical topmark and a white Morse code A light. It indicates navigable water all around and may be passed on either side.
How is an isolated danger mark identified?
Black with one or more broad red horizontal bands, topped by two black spheres, and lit with a white flashing group of two.
What does a yellow mark indicate?
A special mark, used for anchorages, cable and pipeline areas, spoil grounds, military exercise zones, and similar features. Its light, if any, is yellow.
What do the light abbreviations F, Fl, Oc, Iso, Q, and Mo(A) mean?
Fixed, flashing, occulting, isophase, quick flashing, and a light flashing the Morse letter A. Occulting means the light is on longer than it is off, and isophase means equal light and dark.
How is a light described on a chart?
Characteristic, colour, period, height, and nominal range, in that order.
What is the difference between nominal range, geographic range, and luminous range?
Nominal range is the range in an atmosphere with 10 nautical miles of meteorological visibility. Geographic range is limited by the curvature of the earth and the heights of the light and the observer. Luminous range accounts for the actual visibility at the time.
How is the distance to the visible horizon calculated?
Multiply the square root of the height of eye in feet by 1.17 to get nautical miles.
What is a range, or leading line?
Two fixed marks at different heights and distances. When they appear in line, the vessel is on the safe leading line; if the nearer mark drifts to one side, steer toward it to regain the line.
What does a sectored light do?
It shows different colours over different arcs of bearing, commonly a red sector covering a shoal or an obstruction and a white sector covering safe water.
Why must a buoy never be trusted as a sole position reference?
Buoys float on a mooring and can be dragged off station by ice, storm, or collision, and they can be extinguished or lost entirely.
What is a racon?
A radar beacon that responds to a vessel's radar and paints a Morse character on the display, radiating outward from the beacon's position.
What is the difference between true, magnetic, and compass direction?
True direction is measured from true north. Magnetic direction is measured from magnetic north and differs from true by variation. Compass direction is what the vessel's compass reads and differs from magnetic by deviation.
What is variation and where is it found?
The angular difference between true north and magnetic north at a given place. It is printed inside the chart's compass rose along with the year of measurement and the annual rate of change.
What is deviation and where is it found?
The error caused by the vessel's own magnetic influences. It changes with the vessel's heading and is recorded on a deviation card or curve made when the compass was swung.