Charts Flashcards
Mercator Projections
A line which cuts all the meridians at the same angle is called a Rhumb line. Rhumb line are straight and cut the meridians at the same angle.
Gnomic Charts
Cannot be used as a navigational chart.
How to make Great circle
Use a gnomic chart draw straight line as desired great circle. Pick way points equally spaced in degrees. Take coordinates of waypoints and then transfer to Mercator projection chart. Will appear arced. Distance between way points is equally spaced in degrees on the hour over the full passage (e.g 12hours - 15 degrees)
Define Great circle
On any Sphere, the shortest distance between any 2 points is the circumference of the circle which joins them and whose center is at the center of the sphere.
How to find a range using a sextant?
Take a bearing of object to get position line.
Use sextant to get an observed angle.
Correct for index error.
Find its height of object from the chart measured MHWS.
Go into Norris’s to the VSA table and get a range off.
Range and Bearing work out a positional fix.
What about Height of Tide with VSA?
Reading Norries in prep for exam I read because it is MHWS you are on safe side. It will give closest possible range.
If Height of Tide is given?
Make allowance for height of tide from MHWS.
Add difference in tide to MHWS to the height of the object.
Adjust the sextant for index error
Make an allowance for Height of Eye
If you height of eye is less than 12m an allowance would need to be made.
Where is a light measured from?
The center of the lense.
What is a PI?
It is a way of continuously monitoring our position. Not used for fixing our position.
It can back up and confirm a gps position.
Need a sextant.
Need to know the time.
Need a nautical almanac.
Air tables.
Sun run sum.
Murpassage.
Noon sight will give latitude.
Sun run sum will give you a murpassage which can than be used as a running fix.
Can then calculate your position.
What causes tide?
The gravitational pull of the sun and the moon.
Where would you get today information?
The admiralty tide tables and admirably tidal stream atlases.
Spring and neaps?
Springs higher highs and lower lows.
Neaps lower highs and higher lows.
