105.4 Bearings and Fixes Flashcards
A line drawn from current location to a known, visible object is call either a
“Magnetic Bearing” or a “Line of Position”
The point at which two Magnetic Bearings (or Lines of Position) cross is known as a
fix
Name X ways to take a bearing
- Hand held bearing compass
- Other hand held compasses (iPhone)
- Approximate line taken over boats magnetic compass
- Steer the boat toward the object
If object of bearing is too small, use binoculars to
identify something larger behind it or even cloud above
At night, if difficult to get a bearing on a flashing light, use…
A solid light behind it or even a star directly above it
To check amount of leeway using compasses…
- Face aft and take hand-held reading directly up middle of wake
- Add 180 degrees
- Compare to compass heading
- Difference is leeway
For a fix to be accurate, two bearing lines should be done
at the same time, or no more than a minute apart
To further refine a fix, can be helpful to take a ____ bearing
third
Plot fixes on chart and compare with DR estimated position. Should be within
range – i.e., 10% of distance run on that bearing
In more dangerous areas, fixes should be done….
with greater frequency
Best bearing points in coastal cruising
- Things noted on the chart (towers, tanks, bridges)
- Prominent geographical features (mountain, right or left edge of an island?!?)
- Buoys, but…
Using buoys for bearings should consider…
- certainty of identifying the buoy correctly
2. buoy shifted out of position
An Estimated Position is based on a single bearing line compared to…
DR position estimate
An Estimated Position is at the intersection of the single bearing line and a line
perpendicular to it through the DR position estimate
An Estimated Position is _____ accurate than a two, three or running fix
less
A running fix is a position determined by the crossing of two bearing lines taken
at two different times on the same fixed object
To take a running fix
- Take bearing to known object
- Record bearing and the time of that bearing
- Continue along DR course for arithmetically convenient amount of time (10, 15, 30 min)
- Take a second bearing on the same landmark and plot the line. Note the time.
- Determine distance run between bearings and plot position on DR line.
- Redrawn the angle of the first bearing line through that DR position to intersect the second bearing.
- Intersection is actual point.
To calculate distance off shore (three steps). This is also known as _____ the angle on the bow, and can work for multiple angles.
- Record time of taking a bearing of object 45% off of bow.
- Record time when object is 90% off beam
- Distance between recording times is the distance off shore.
Doubling
What is a relative bearing?
A bearing that is not magnetic but instead relative to a hypothetical 0% at the bow of the boat.
What is a turn bearing?
Using landmark, identify angle reached from it when it is time to turn
What is a Danger Bearing
Line from landmark over the danger area used to determine a “not-less-than” (NLT) heading line once sailing
What is a Range or Transit
That point at which two landmarks line up
How can glow of distant city be used?
Take a bearing to the center of the glow and plot
When a major light is “dipping”, how can it be used
Just appearing on the horizon, but “blinking” due to motion of the boat up and down. Right at the edge of its published visible range PLUS range to horizon of views height above water level. See Geographic Range Table on 4-13 of Coastal Navigation.
If there is no third landmark available, a third point of input could be the
soundings (comparing depth to published info)
Get in the habit of verifying even three line fixes with
soundings
Formula for calculating distance of a dipping light
(Sq Root of Eye Height above water) x 1.15)) + (Sq Root of Light Height above water) x 1.15))
DR Position
is based on Direction and Distance
Single bearing or LOP does ____ give a fix
Not
Two bearings when crossed
indicate fix
Three crossed bearing provide the most ____ ___.
accurate fix
Running fix determines position by
crossing two bearings taken at different times. Advance the original bearing to create a trapazoid.
Estimate Position is not a
fix
To determine your most probable Estimated Position along a bearing line, draw a line _______ to the bearing line that intersects your __ _____ at a known time.
probable; DR position; perpendicular
Plotting Symbols
- M
- MB
- S
- DR with
- Fix with dot and full circle
- RFix with dot and full circle
- EP with dot and square
- GPS with dot and triangle
- Notation above / below DR line
Meaning
- Magnetic or could be Mile
- Magnetic Bearing
- Speed
- DR Position with dot and HALF circle
- Two or three-line fix with dot and FULL circle
- Running fix with dot and FULL circle
- Estimated Position with dot in square
- GPS Position with dot in triangle
- Ex. C045M // S7.5 (D=10)
The best landmark bearings fall between __ and __ degrees. Why?
30 and 90. Crisp crossing of lines
Which are better for taking bearings – distant objects or nearby ones? Why?
Nearby ones. Greater chance of bearing error being magnified on distant ones.
Always take a fix whenever: (4 most important times)
- Night approach
- Fog begins to appear or expected
- Danger areas
- Approaching new coastal area
Never plot a new DR from a ____ or an ____ ____. Use only a three-line fix or a two line fix verified by (three options)
two-line fix or Estimated Position
- double checking two line fix
- soundings
- GPS fix
A DR position and a fix after a certain time should be within 10% of distance run on that DR assuming no ____ or _____.
current or leeway
If unsure of position, assume you are….
… in a position of highest risk and plot a course away from danger
