TERNAV (MIDTERM). Flashcards
When plotting courses of vessels and the set of a current/tidal stream, the direction of travel is indicated by _ pointing in the direction of travel
arrow heads
One arrow head =
the course steered
Two arrow heads =
the course made good
Three arrow heads =
the set of the current
Whilst currents/tidal streams have the greatest effect on the movement of vessels, the _can also affect their movement
wind
The resultant speed through the water will be recorded by
certain logs and the speed over the ground or effective speed will only be recorded by a
Doppler Log in shallow water or GPS or by calculating the speed between the fixes.
Secondly, the wind produces a sailing effect caused by the vessel’s
superstructure. In general, the vessel will be moved bodily to _
the amount will vary with trim, design of the vessel and whether it is
in a loaded or light ship condition
leeward,
The difference between the fore and aft line and the direction made
Leeway
It can be assessed by visually estimating the angle between the
fore and aft line and the ship’s wake.
Leeway
it is always important
that leeway is always applied to the
true course.
When the course steered is given, to find the leeway track, leeway is
applied
downwind.
When it is required to find the course to steer, to counteract the wind, leeway
is applied
upwind.
The Earth is an
irregular oblate spheroid (a sphere flattened at the poles).
Measurements of its dimensions and the amount of its flattening are subjects of
geodesy.
is a near enough approximation to a geodesic for most problems of navigation.
great circle
is the line of intersection of a sphere and a plane which does not pass through the center.
small circle
is usually applied to the upper branch of the half-circle from pole to pole which passes through a given point. The opposite half is called the _
meridian, lower branch.
is a circle on the surface of the Earth parallel to the plane of the equator.
parallel or parallel of latitude
It connects all points of equal latitude. The poles are single points at latitude 90°. All other parallels are
parallel or parallel of latitude, small circles
is a great circle at latitude 0°
equator
can define any position on Earth.
Coordinates of latitude and longitude
is the angular distance from the equator, measured northward or southward along a meridian from 0° at the equator to 90° at the poles.
Latitude (L, lat.)
is the shorter arc of the parallel or the smaller angle at the pole between the meridians of the two places.
The difference of longitude (DLo) between two places
The distance between two meridians at any parallel of latitude, expressed in distance units, usually nautical miles, is called
departure (p, Dep.).
It represents distance made good east or west as a craft proceeds from one point to another. Its numerical value between any two meridians decreases with increased latitude,
departure (p, Dep.).
is numerically the same at any latitude.
DLo
may be designated east (E) or west (W) when appropriate.
Either DLo or p
is the length of the rhumb line connecting two places. This is a line making the same angle with all meridians.
Distance
Meridians and parallels which also maintain constant true directions may be considered special cases of the
rhumb line
. Any other rhumb line spirals toward the pole, forming a
loxodromic curve or loxodrome.
it is designated _ and should be properly labeled to indicate the origin (prefix)
course angle (C)
and direction of measurement
(suffix).
is the intended horizontal direction of travel with respect to the Earth.
Track (TR)
The terms _ are used to indicate the path of intended travel
intended track and trackline
consists of one or a series of course lines, from the point of departure to the destination, along which one intends to proceed.
track
A great circle which a vessel intends to follow is called a _,
great-circle track
it consists of a series of straight lines approximating a great circle
great-circle track
is the direction in which a vessel is pointed at any given moment, expressed as angular distance from 000° clockwise througlA360°.
Heading (Hdg.)
constantly changes as a vessel yaws back and forth across the course due to sea, wind, and steering error.
Heading
is the direction of one terrestrial point from another, expressed as angular distance from 000° (North) clockwise through 360°.
Bearing (B, Brg.)
When measured through 90° or 180° from either north or south, it is called
bearing angle (B)
are sometimes used interchangeably, but the latter more accurately refers to the horizontal direction of a point on the celestial sphere from a point on the Earth
Bearing and azimuth
is measured relative to the ship’s heading from 000° (dead ahead) clockwise through 360°.
relative bearing
However, it is sometimes conveniently measured right or left from 000° at the ship’s head through 180° This is particularly true when using the table for
Distance of an Object by Two Bearings.
is the position of one point relative to another.
Direction
Navigators express _ as the angular difference in degrees from a reference direction, usually north or the ship’s head.
direction
is the horizontal direction in which a vessel is intended to be steered, expressed as angular distance from north clockwise through 360° Strictly used, the term applies to direction through the water, not the direction intended to be made good over the ground.
Course (C, Cn)
is often designated as true, magnetic, compass, or grid according to the reference direction.
course
is the single resultant direction from the point of departure to point of arrival at any given time.
Track made good (TMG)
is the direction intended to be made good over the ground
Course of advance (CO)
is the direction between a vessel’s last fix and an EP
course over ground (COG)
is a line drawn on a chart extending in the direction of a course.
course line
True Bearing =
Relative Bearing + True Heading.
Relative Bearing =
True Bearing - True Heading.
Accurate declination tables for the Sun have been published for centuries, enabling ancient seamen to compute
latitude to within 1 or 2 degrees.
Those who today determine their latitude by measuring the Sun at their meridian and the altitude of _ are using methods well known to _
Polaris, 15th century navigators
A method of finding longitude eluded mariners for
centuries.
which determines GMT by observing the Moon’s position among the stars, became popular in the 1800s.
lunar distance method,
Navigators have made latitude observations for
thousands of years.
was formed, offering a small fortune in reward to anyone who could provide a solution to the problem.
In 1714, the British Board of Longitude
- [ ] An _ responded to the challenge, developing four chronometers between 1735 and 1760
Englishman, John Harrison,
The most accurate of these timepieces lost only 15 seconds on a 156 day round trip between London and Barbados. The Board, however, paid him only half the promised reward. The King finally intervened on navigator had set his watch or checked its error and rate with the local mean time determined by celestial observations. The local mean time of the watch, properly corrected, applied to the Greenwich mean time obtained from the lunar distance observation, gave the longitude.
ewan
- [ ] The calculations involved were .
tedious
Few mariners could solve the triangle until _ published his simplified method in
Nathaniel Bowditch, 1802 in The New American Practical Navigator
Reliable_ were available by 1800, but their high cost precluded their general use aboard most ships
chronometers
- [ ] However, most navigators could determine their longitude using .
Bowditch’s method
This eliminated the need for parallel sailing and the lost time associated with it.
Bowditch’s method
were carried in the American nautical almanac into the 20th century
Tables for the lunar distance solution
- [ ] The time sight was mathematically sound, but the navigator was not always aware that the longitude determined was only as accurate as the latitude, and together they merely formed a point on what is known today as a _
line of position.
- [ ] is the angular distance between the prime meridian and the meridian of a point on the Earth, measured eastward or westward from the prime meridian through 180°.
Longitude (I, long.)
- [ ] is the angular length of arc of any meridian between their parallels. It may be designated north (N) or south (S) when appropriate.
The difference of latitude (I, DLat.) between two places
It is the numerical difference of the latitudes if the places are on the same side of the equator; it is the sum of the latitudes if the places are on opposite sides of the equator.
The difference of latitude (I, DLat.) between two places
The _on the same side of the equator is half the sum of their latitudes. it is labeled N or S to indicate whether it is north or south of the equator.
middle or mid-latitude (Lm) between two places
- [ ] is the line of intersection of a sphere and a plane through its center. The shortest line on the surface of a sphere between two points on the surface is part of a great circle.
great circle
This is the largest circle that can be drawn on a sphere.
great circle
On the spheroidal Earth the shortest line is called a
geodesic.
- [ ] is the process of determining one’s present position by projecting course(s) and speeds) from a known past position, and predicting a future position by projecting course(s) and speed(s) from a known present position. T
- [ ]
Dead reckoning
is the primary plotting method will vary with the type of system. allows the display of the ship’s heading projected out to some future position as a function of time, the display of waypoint information, and progress toward each waypoint in turn.
ECDIS
is only an approximate position because it does not allow for the effect of leeway, current, helmsman error, or compass error.
DR position
- [ ] Until ECDIS is proven to provide the level of safety and accuracy required, the use of a traditional DR plot on paper charts is a _, especially in restricted waters. The following procedures apply to DR plotting on the traditional paper chart.
prudent backup
- [ ] Maintain the DR plot directly on the chart in use. DR at least .
two fix intervals ahead while piloting
If transiting in the open ocean, maintain the DR at least
four hours ahead of the last fix position.
allows the navigator to evaluate a vessel’s future position in relation to charted navigation hazards. It also allows the conning officer and captain to plan course and speed changes required to meet any operational commitments.
Maintaining the DR plot directly on the chart
- [ ] To measure courses, use the chart’s _nearest to the chart area currently in use. Transfer course lines to and from the compass rose using parallel rulers, rolling rulers, or triangles. If using a , simply set the plotter at the desired course and plot that course directly on the chart. Transparent plastic navigation plotters that align with the latitude/longitude grid may also be used.
compass rose
(PMP)
parallel motion plotter
- [ ] may give both true and magnetic directions.;
Compass roses
are on the outside of the rose
True directions
are on the inside
magnetic directions
. For most purposes, use
true directions.
The only common nonconformal projection used is the a
gnomonic;
usually contains instructions for measuring direction.
gnomonic chart
- [ ] Draw a whenever restarting the DR.
new course line
- [ ] Express the time using, according to procedure. Label the plot neatly, succinctly, and clearly.
four digits without punctuation, using either zone time or Greenwich Mean Time (GMT)
- [ ] helps in determining sunrise and sunset; in predicting landfall, sighting lights and predicting arrival times; and in evaluating the accuracy of electronic positioning information. It also helps in predicting which celestial bodies will be available for future observation. But its most important use is in projecting the position of the ship into the immediate future and avoiding hazards to navigation.
Dead reckoning
- [ ] is the horizontal movement of the sea surface caused by meteoro-logical, oceanographic, or topographical effects. From whatever its source, the horizontal motion of the sea’s surface is an important dynamic force acting on a vessel.
Current
- [ ] is the periodic horizontal movement of the water’s surface caused by the tide-affecting gravitational forces of the Moon and Sun.
- [ ]
Tidal current
refers to the current’s direction,
Set
refers to the current’s speed.
drift
is the leeward motion of a vessel due to that component of the wind vector perpendicular to the vessel’s track.
. Leeway
- [ ] especially affects sailing vessels and high-sided vessels
- [ ]
Leeway
The direction of a straight line from the last fix to the EP is the
estimated track made good.
The length of this line divided by the time between the fix and the EP is the
- [ ] estimated speed made good.
- [ ]
- [ ] Clearing the harbor at 0900, the navigator obtains a last visual fix. This is called
taking departure, and the position determined is called the departure.
