Form of the earth/Lat and Long Flashcards
Isogonal line
Lines joining places with equal variation.
Isogonals converge at both the geographic and (˚M) N and S poles
Agonic line
Lines joining areas of zero variation
Isoclinic line
Lines of equal dip
Aclinic line
Lines joining areas of zero dip
Variation is?
The angular difference between ºT and ºM
Deviation is…?
The angular difference between ˚C and ˚M
The max difference is normal 3˚ E/W however EASA accuracy limit for the DRC is 10˚
What is a graticule?
All lines of parallels of latitude and longitudes are collectively known as graticule.
What is a rhumb line?
Is a regularly curved line that cuts all the meridians of longitude at the same angle.
Rhumb lines are ALWAYS convex to the EQ side while great circles are always convex to the polar side.
What is a great circle?
A circular track which has the centre of the earth as it’s centre of radius. Thus the radius of GC is equal to the earth’s radius.
All GC pass through the the EQ while small circles do not.
What are parallels of latitude?
Latitudes stretch W to E but are measured N or S of the EQ.
Latitudes have a max angle of 90˚
What are meridians of longitude?
Longitudes stretch N and S but measured E and W.
Longitudes max at 180˚ therefore a meridian and it’s anti meridian make a great circle (360˚).
What is a prime meridian?
It is the meridian which goes through London and it’s angle is at 0˚
What is the problem with flying GC?
Due to increasing convergency as you fly further north or south, to fly a GC you must continually change track.
What is a vertex?
A point on the GC where you are at the highest latitude.
So there will be a northern and southern vertex; 2 points, and are antipodal
A GC crosses the EQ at 2 points, what are these 2 points called?
Crossing point or the node.
For calculations, to check if you have correctly calculated the nodes, the 2 points has to add up to 180˚
At either vertex the track direction is either 090˚ or 270˚, why?
This is because vertices are antipodal, exact opposite ends from each other. That is why the 2 points add up to 180˚.
As vertices are GC. A GC is made up of a meridian and anti meridian. Since meridians are measured E to W, the points on a compass would be 090˚ and 270˚. Thus vertices must have a track direction of either 090˚ and 270˚.
Deviation west, magnetic best
Deviation east, magnetic least
Deviation west, compass best
Deviation east, compass least
What is a dip angle?
This is when a compass points directly to the strongest horizontal magnetic field being emitted by the earth’s core.
Due the magnetic field emitting from the South Pole to the North Pole, the point of:
- max dip occurs at the (˚M) North Pole i.e. 90˚ dip
- min dip occurs at the (˚M) South Pole i.e. 0˚ dip
What is a compass swing?
A compass swing is the checking of the direct reading compass (DRC) and whether it is accurately aligned to ˚M north.
When is a compass swing required?
- Change of base
- A/C hit by lightening
- major change in A/C equip
- A/C parked in same direction for a long period of time
- when new compass is fitted
- when carrying magnetic freight or freight with large ferrous content
- when suspect deviation
- when scheduled for A/C maintenance
+/- 10˚ is the acceptable error (according to EASA)
What causes deviation?
Interference of the A/C as both A/C and it’s instruments carry large quantities of ferrous metals e.g. Magnets in instruments
And
Electrical equip also create electromagnetic fields also causing erroneous reading
What is a compass card?
A card advising adjustments that should be made for deviation errors.
Deviation values change for different HDGs.
What is a latitude?
A latitude is a point on the surface in relation to the EQ.
This angular difference between the EQ and the point can be indicated as a latitude.
Latitude max 90˚
When using the term departure, what do we mean by this?
The distance along the parallel of latitude.
Define geocentric latitude?
The angle between a line drawn on the surface of the earth to the centre of the earth and the plane of the equator.
Geocentric latitudes assume a perfect sphere where the line drawn from any point on the surface will always end at the centre of the earth.
Define Geodesic.
The shortest distance between 2 points will always be a straight line. A straight line on a curved surface is called geodesic.
Define geodesic (or geographic) latitude
We have learnt that on a perfect sphere any geocentric latitude has earth’s centre as it’s centre of radius.
But because the earth is an oblate spheroid, not all points on the surface are perpendicular to the earth’s centre. With the exception of the points along the equator and at the N/S poles. This is therefore called the geographic latitude.
Where is the difference between the geodetic and geocentric the greatest?
Mid Lat at exactly 45˚ N/S
The min is therefore at the EQ and at the poles. In fact geographic and geocentric latitudes are the same here.
Due to earth’s ellipsoid, what is 1˚ in NM near the Eq compared to the poles?
1˚ NM at Eq/nearer Eq/low altitudes < 60nm
1˚ NM at the poles/nearer poles/high altitudes > 60nm
What is the maximum difference in mins between the geocentric and geodesic (or geographical) latitude?
11.6’ (mins)
Magnetic variation is applied at the AC or at the VOR?
Always at the VOR because you are working in radials.
Radials are from the VOR so variation must be applied there.
