FIXING Flashcards
Methods of fixing
Range and bearing (Radar)
3 Point fix (Compass)
Transit and range
Transit and contour line
Running Fix
Azimuth of heavenly body (Sextant on ocean passage)
Horizontal Sextant Angle
Vertical Sextant Angle
GPS
Fixing from a single distant object
VSA
To do a Vertical Sextant Angle we need two things
The height of a object like a lighthouse which is given from MHWS and shown on a chart. We need to allow for height of tide
The other thing we need is the angle between the horizon and the middle of the light on the lighthouse
Once we have these two figures we can use Norries Tables and go to the“Distance by vertical sextant angle” table and look for the hight column followed by and then match the angle to find the distance
Ways to fix position
Range and bearing (Radar)
3 Point fix (Compass)
Transit and range
Transit and contour line
Running Fix
Azimuth of heavenly body (Sextant on ocean passage)
Horizontal Sextant Angle
Vertical Sextant Angle
GPS
Explain running fix
Take a bearing (line of position) of a conspicuous object ashore.
Run on your course and speed for a time period of your choosing.
Apply the set and drift for the selected time period.
Transfer the first bearing to the end of the set and drift line.
Take a second bearing of the same object. Where it crosses the transferred line of position is the fix.
Fixing position with Radar methods
Range and bearing
3 Range fixes
Radar overlay onto ECDIS
How do you fix your position on board using gps as secondary means?
Range and bearing (Radar)
3 Point fix (Compass)
Transit and range
Transit and contour line
Running Fix
Azimuth of heavenly body (Sextant on ocean passage)
Horizontal Sextant Angle
Vertical Sextant Angle
Position fixing coastal
Range and bearing (Radar)
3 Point fix (Compass)
Transit and range
Transit and contour line
Running Fix
Horizontal Sextant Angle
Vertical Sextant Angle
How often do we fix position
We fix position at a frequency that does not put the vessel into danger
Considerations: To fix more often
Traffic Density
Proximity to land or navigational dangers
In areas of strong currents or wind
How to fix position in open sea?
Azimuth of a heavenly body
GPS
How would you fix your position using contours?
You can fix you position using contour lines when you can combine crossing a contour line with a line of position, or even better, by combining a line of position, range and contour
How would you fix position in the monitoring stage?
Azimuth of a heavenly body, sun, moon, star
GPS
Mid Atlantic, notice a big difference between GPS and fix, what do you do?
Call the Master because you are now unsure of your position
Look at the HDOP value
Try secondary GPS and compare the two
Azimuth of a heavenly body
What is DR?
A position derived from a pre calculated fix, taking into account our true course steered and speed through the water
How do you plot a DR?
From the vessel’s last position fix, draw a line representing the vessel’s true course over the time period. In this case, the vessel has steered 090° T at 16 knots for one hour.
Using a drawing compass and the latitude scale on the side of the chart, draw an arc across the previous line at the appropriate distance. In this case, one hour at 16kts is 16nm.
Where the arc intercepts the line, label it with the time. There is no other symbology applied to DR positions.
What is an EP?
An estimate of the position of the ship at a certain time, worked out by adding the effect of tidal streams and currents to the DR position.
How to do an EP?
Plot the vessel’s heading & speed from the vessel’s last position fix to find the DR position. In this case, the vessel has steered 090° T at 10 knots for one hour.
From the DR position, plot the tidal vector. By reference to the Admiralty Tidal Stream Atlas, we can determine that the vessel has experienced a tidal stream of 045°T at 3kts for the past hour.
Find the Estimated Position. Mark it with a triangle and add the time.
