TERNAV MIDTERM FR Flashcards
To measure courses, use the
chart’s
compass rose .
may give true and
magnetic directions.
Compass roses
are on the outside of the rose;
True directions
are on the
inside
magnetic directions
is a DR position corrected for the
effects of leeway, steering error, and current.
( E S T I M A T E D
P O S I T I O N )
It involves calculating the set and drift and
applying these values to the DR to obtain an
EP(estimated position)
is enclosed with a square and labeled
horizontally with the time.
EP(estimated position)
– the periodic
horizontal movement of the
water’s surface by the tide-
affecting gravitational forces of the
Moon and Sun
Tidal Current
– the horizontal movement
of the sea surface caused by
meteorological, oceanographic, or
topographical effects
Current
– refers to the current’s
direction
Set
– refers to the current’s speed
Drift
– the leeward motion of
vessel due to that component of the
wind vector perpendicular to the
vessel’s track
Leeway
– the
direction of a straight line from the
last fix to the EP
Estimated course made good
– the
length of the course made good
divided by the time between the fix
and the EP
Estimated speed made good
Measures the angle between
two reference points observed
from the observer’s position
P O S I T I O N B Y
H O R I Z O N T A L
A N G L E S
Using a sextant, the navigator
measures the _
between the two selected
reference points
horizontal angle
If the HA is <90o, the angles
are measured from the
baseline _ the ship
TOWARDS
If the HA is >90o, the angles
are measured from the
baseline _ from the ship
AWAY
will be where the 2
position circles intersect
FIX
–
measuring the angle
between the Top of an Object
and the horizon
Vertical Sextant Angle
are
said to be in transit when
both are in a straight line, as
seen from the ship
Two conspicuous objects
It provides a good
opportunity to obtain the
compass error
P O S I T I O N L I N E B Y
T R A N S I T B E A R I N G
is said to be abeam of the ship, when it comes in line with perpendicular to the ship’s fore and aft centreline on any side of the vessel
landmark or lighthouse
The distance of that lighthouse at this moment (when abeam) is called _ and bearing is known as _
beam distance, ‘beam bearing’.
Bearing of that lighthouse when abeam will be either _ of ship’s heading (ship’s course) at that moment
90° to the portside or 90° to the starboard side
BC —
Second bearing of light
AD —
Course steered between observations
AB = BC =
Distance run between first and second observations
BC —
Distance from the light at the time of second observation
The procedure is similar to doubling the angle on bow.
Four-point bearing on the bow
This method also suffers from the disadvantages that distance from the object is only known when ship is already off that position.
Four-point bearing on the bow
We use this process for estimating the distance abeam, at which the ship is going to pass any observed object.
Special angles on the bow
gives the distance abeam from the object in advance, enabling us to take a decision regarding safe passing distance etc.
Special angle method of estimating distance
AD =
ship’s course steered