Nav Revision Flashcards
What are the features of a conformal projection?
Produce charts that represent bearings accurately
Projection on which angles are preserved
Bearing on Earth’s surface should be represented by the same angle on the chart
For a given point on the chart, scale expansion is the same in all directions
At what angle do meridians and parallels intersect on conformal charts?
90 degrees
same as on Earth’s surface
What is another name for a conformal chart?
Orthomorphic
How can the figure/shape of the Earth be modelled for navigation and charts?
Geoid
Ellipsoid/spheroid
Sphere
What are the two categories of projection?
Perspective (true/geometric|)
Non-perspective (mathematical)
What are perspective projections?
Drawings of shadows cast by meridians and parallels on a transparent model Earth onto a plane surface.
Aka True (geometric)
What is a non-perspective projection?
A perspective projection which has been adjusted mathematically by relocation of the graticule to attain a more accurate representation so that some distortions are removed.
What are the 3 types of perspective projection?
Azimuthal - Shadows projected onto a flat surface
Cylindrical - Shadows projected onto a cylinder surrounding the model
Conical - Shadows projected from a light source in the centre of the reduced Earth onto a cone tangential to the model that can then be developed
How can scale be represented on maps/charts?
Graduated scale (i.e. ruler on chart)
Representative fraction (e.g. 1:500,000)
Plain Statement (e.g. 2cm is 1 mile)
What is scale?
The ratio of chart length to Earth length.
It is impossible for scale to be …. over an entire projection.
Constant
How many times larger is the apex angle than the point of tangency in a conical projection?
2
E.g. tangential point = 45 degrees, apex angle = 90 degrees
What is a standard parallel?
A line of latitude in a conical or cylindrical projection in normal aspect, where the projection surface touches the globe.
What are the 4 types of standard parallel?
Conical one Standard Parallel projection
Conical two Standard Parallel projection
Cylindrical one Standard Parallel projection
Cylindrical two Standard Parallel projection
Note - Be able to recognise from pictures in course manual
What are the types of conical projection?
Perspective projection
Conical One Standard Parallel projection (1 red line)
Conical Two Standard Parallel projection (2 red lines)
Non-perspective projections
Note - Be able to recognise all from pictures.
What are conical projections used for?
Producing maps of SMALL AREAS on the Earth’s spherical surface onto a cone that is subsequently developed
What is the constant of the cone/Convergence Factor?
The ratio of the convergence between any two meridians on a map or chart to the change of longitude between them.
What is the Lambert Conformal Conical Projection?
NON-PERSPECTIVE projection.
Assumes that the reduced Earth is a PERFECT SPHERE.
Where are Lambert Conformal Conical Projections used?
LFC (1:500,000)
Note - Unsuitable for use at the poles
What azimuthal projections can we produce?
Polar azimuthal
Equatorial azimuthal
Oblique azimuthal
(Be able to recognise all of the above from the pictures in the course manual)
What are the TYPES of Azimuthal Projection?
Stereographic projection
Polar graticule meridians
Gnomonic projection
Azimuthal equidistant
Explain stereographic projection.
Azimuthal projection
Perspective and conformal
Mainly used for polar based navigation
Explain Polar graticule meridians.
Azimuthal projection
Radial straight lines from the point of tangency
Scale expands away from the point of tangency
Explain Gnomonic projection.
Azimuthal projection
Perspective and NON-conformal
Meridians and parallels are projected onto a surface from the centre of the reduced Earth
Displays great circles as straight lines
Scale increases away from the point of tangency
Explain Azimuthal equidistant.
Azimuthal projection
NON-Perspective
NON-Conformal
Distances from the point of tangency are correct to scale
Bearing and distance can be measured accurately from the point of tangency
What is Mercator’s Projection?
A type of cylindrical conformal or orthomorphic projection.
Uses straight meridians and parallels that intersect at right angles.
Straight lines on the map are lines of CONSTANT AZIMUTH (RHUMB lines)
What are the different types of Mercator’s Projection?
General Mercator’s projection
Plane/Oblique
Transverse
Note - See course manual for extra details + be able to recognise from pictures.
What are the different types of year?
Year
Sidereal Year
Tropical Year
Civil Year
What is a year?
Length of time taken for the Earth to orbit the Sun
What is a sidereal year?
Time taken by the Earth to orbit the Sun as measured against a fixed star
What is a tropical year?
A Tropical Year is one complete orbit of the Sun measured as the PERIOD between TWO successive VERNAL EQUINOXES relative to the First point of Aries.
Approx. 365 days, 6 hrs
What is a civil year?
Civil year = Calendar year using the Gregorian calendar
Assumes 1 yr = 365 days, with every 4th being a leap year (366 days)
What are the 2 types of day?
Apparent Solar Day
Mean Solar Day
What is an Apparent Solar Day?
The interval between two successive returns of the sun to the local meridian.
What is a Mean Solar Day?
Average length of Apparent Solar Days observed over a period of years.
Measured relative to a fictional Mean Sun moving at a constant rate.
24 hrs
What is Apparent Solar Time?
Kept according to the actual position of the Sun in the sky, measured at successive crossings of the Sun at a local meridian at noon.
Varies throughout the year, not exactly 24hrs.
What is Local Mean Time (LMT)?
Uses a theoretical Mean Sun which orbits Earth every 24hrs in the plane of the equator.
Errors averaged out to give LMT.
How is LMT calculated at different longitudes?
1 degree of longitude = 4 mins
15 degrees = 1hr
Going East = +
Going West = -
OR
Air Almanac Conversion of Arc to Time Table
What is UTC?
Coordinated Universal Time
World standard reference time for aviation
LMT at the Greenwich Meridian
How can UTC be converted to LMT?
LMT = UTC + Long E
or
LMT = UTC - Long W
Depends which way you’re going
What is TAI?
International Atomic Time
Based on the measurement of a particular vibration of the Caesium-133 atom.
Forms the basis of UTC.
How are Radio Time Signals transmitted?
Transmitted by numerous NATIONAL and COMMERCIAL broadcast stations throughout the world.
Normal time check 6 pips prior to the hour
What is the Prime Meridian?
0 degrees longitude
Greenwich Meridian
ZULU time
What is the International Date Line?
180 degrees
Anti-Meridian
Zone boundary separating +12 and -12 hours
Crossing West adds one day, crossing East subtracts one day
Not a straight line (learn picture in book)
What is Standard Time?
A particular LMT that a national authority has decreed will be kept throughout its country.
What is Daylight Saving Time (DST)?
Clocks advance 1 hour in Spring to maximise evening daylight.
British Summer Time (BST) starts on the last Sunday in March and 01:00 GMT.
BST ends on the last Sunday in October at 02:00 BST.
What is Zone Time?
Countries with significant East/West extent have further subdivisions of time within the, e.g. USA, Australia.
How can we convert from zone time to UTC?
UTC = Zone Time - East change in longitude
What are the dimensions of the Twilight Zone?
.
What are the types of Twilight?
Civil twilight - Up to 6 degrees
Nautical twilight - Up to 12 degrees
Astronomical twilight - Up to 18 degrees
Morning Civil Twilight (MCT) and Evening Civil Twilight (ECT)
What factors affect the duration and time of twilight for a stationary observer?
Latitude
Sun’s declination
Alitude
How does twilight vary with latitude?
Duration increases with increasing latitude.
Sun’s declination in the SAME hemisphere = Earlier sunrise, later sunset
Sun’s declination and observer’s latitude are CONTRARY, sunrise occurs later and sunset earlier with increasing latitude.
How does twilight vary with altitude/height?
At height, sunrise becomes earlier, sunset later.
Duration of twilight DECREASES.
(atmosphere is less deep, reduction in refracted light and diffused light to do a decrease in scattered particles)
How does flying West affect sunset and twilight?
GS > Earth’s rotational speed = Sun’s position in sky remains constant or appears to move in reverse
Sun takes longer to set and Twilight becomes longer.
Opposite is true if flying East
What does the UK Air Almanac provide?
Convenient form of astronomical data:
Time conversion tables, sunrise, sunset, twilight, moonrise, moonset, duration and time of astronomical phenomena, etc.
Explain TDAs.
Tactical Decision Aids
Uses Numeric Weather Prediction (NWP) and Met Office Night Illumination Model (MONIM) to forecast light levels 24/7 for anywhere in the world.
Request to Met Office TDA allows decisions to be made re use of electro-optical night vision devices/timing for operations, etc.
What is the Earth’s rate of rotation?
15 degrees per hour
What does every projection have?
Central Meridian - Its middle longitude
Central Parallel - Middle latitude/latitude of origin
Natural Origin - Where the Central Meridian and Central Parallel intersect. Starting point of the projected coordinates
What is sometime done to the Natural Origin to avoid negative coordinate values or extremely large values?
It is shifted, giving a COORDINATE SYSTEM ORIGIN.
Shifts are described using False Eastings and False Northings
Note - Be able to recognise picture in course manual
What is the National Grid at its largest extent?
Starts as a 5x5 series of 500km x 500km grids, labelled A to Z (but omitting I).
What is the National Grid base on and where is the natural origin?
Transverse Mercator Projection
49 N 2 W
Where is the BNG false origin?
100km North and 400km West of grid origin
The UK is covered by 6 of the 500km x 500km grid squares of the BNG, how are these split up and labelled?
Each is split into 100km squares, lettered A to Z (omitting I).
A is in the TOP LEFT.
What is UTM?
Universal Transverse Mercator
World map
NUMBERS along the bottom
60 longitude zones, 20 latitude bands
What is UPS?
Universal Polar Stereographic
Circular (azimuthal projection)
One for N pole, one for S pole, origin for each is at the pole.
How is the North Pole divided for UPS?
Divided by the Greenwich meridian and the 180 degree meridian.
West longitudes = Y
East longitudes = Z
How is the South Pole divided for UPS?
Divided by the Greenwich meridian and the 180 degree meridian.
West longitudes = A
East longitudes = B
What is the MGRS?
Military Grid Reference System
Used with the UTM and UPS girds.
What is GEOREF?
Geographic Reference System
LETTERS along the bottom
24 longitudinal zones
12 bands of latitude
Applied in UK coastal and air defence
What Datum is used for most charts/maps/GPS?
WGS 84 - World Geodetic System
What are some common datums?
WGS 84
ED 50 - European Datum
Local Datum - Best fits a local area of interest
