AIR NAVIGATION PART 5: MAP PROJECTIONS [Main Title]
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- Title: AIR NAVIGATION PART 5: MAP PROJECTIONS [Main Title]
- Film Number: AMY 413
- Other titles:
- Summary: A technical instructional film which explains how the world may be regarded as a sphere overlain with a pattern of circles (latitude and longitude) enabling any location to be identified. It is more practical to represent this sphere (or globe) as a flat map or chart. This is achieved by map projection, of which there are several types, the choice dependant upon the purpose of the map. The film uses good animated graphical illustrations accompanied by a clear commentary.
- Description: The film opens with a rotating globe. For practical navigation purposes the world may be regarded as a sphere imposed with a series of vertical circles of equal diameter, or meridians which all pass through the north and south poles and represent lines of longitude. A series of horizontal circles commencing with a maximum diameter equal to the equator, decreasing to zero at the north and south poles, represent lines of latitude. The diameter of the equator is equal to that of the meridians, and the meridian that passes through Greenwich, the prime meridian, is the starting point for dividing the equator into 360 degrees. The meridians are likewise divided into 360 degrees. It is now possible to establish accurately any point in the world. The globe has two properties of note, the Great Circle and Rhumb lines. The shortest distance between any two points on the surface of the earth is the short arc of the Great Circle passing through these points. A flight from London to Hong Kong would start at 57 degrees to the meridian, making a different angle at each crossing of a meridian and finish up at 145 degrees. This is the Great circle track involving a constant change of compass course. To fly the same route on a fixed bearing of 109 degrees the track crosses each meridian at this angle and is called the Rhunb track. The difference between the two tracks is about 500 miles. It is not possible to represent all the features of a globe on a flat sheet of paper. Some elements must be sacrificed, shapes, sizes, distances cannot all be correct on a flat map, and a series of map projections are available to the map maker, who chooses a projection that best meets his requirements. “The Cylindrical Projection”: An animated diagram shows a globe rolling along as it leaves a record of what the eye would see through a narrow slit at a right angle to the motion of the globe. The effect is to transform the globe into a cylinder. On the flat projection the parallels of latitude are stretched out as the poles are approached. The reverse effect is observed with the meridians which have been compressed from half a circumference to a diameter, and the shape of the land masses very much distorted but their areas are still accurately represented. In terms of area in square miles, England is on the same scale as Sumatra. The Mercators Projection is a development of the Cylindrical projection . Mercator charts and maps show the shape of the land masses correctly by stretching the meridian lines to the same degree as the parallel lines of latitude, so the scale at any latitude is the same in any direction. Areas and distances are now not uniform, and England appears as the same size as Sumatra but is in fact half the size of Sumatra. A straight line on a Mercator chart is a Rhumb line which cuts all the Meridians at the same angle. The film returns to the London Hong Kong flight track of 109 degrees and the Great Circle track is superimposed on the chart and as already noted exceeds the Rhumb line track by 500 miles. The advantages of flying on a constant compass bearing are considerable and for most flights the difference is negligible. Practically all charts (but not maps) used by the Royal Air Force are on Mercator projections. In the Gnomonic Projection any straight line represents a Great Circle. This is illustrated in the film by a lamp inside a translucent globe which projects the spherical surface of the globe onto a flat screen. A straight flight line from London to Hong Kong is drawn on this map to see the difference in direction it takes from the Mercator Projection. In this case the scale of the chart will increase towards the edges, that is the further the light is away from the point of contact - known as the point of tangency - on the flat screen, sizes and distances will not be in proportion. Shapes are also distorted but any straight line drawn on this map represents a Great Circle. The Gnomonic is one of the group of projections known as perspective projections. “Conical Projection”: Another animated diagram depicts a cone siting on a globe in contact with a parallel of latitude, where the surface of the globe north of this parallel is stretched and raised to the cone surface. Removing and opening up this cone shows that the along the parallel, where the cone touches the globe the scale is true, but from this point outwards, the base and apex of the cone, the scale increases. The scale of latitude and longitude do not vary in the same proportion, but a transformation can be made such that the scale of the north-south direction conform to that of the east-west direction, and so preserve correct shape. This development of the simple conical projection is known as the Conical Orthomorhic. Cartographers who require their maps to maintain the same scale throughout the map sheet do not stretch the map far away from the origin, but start again with a new origin as soon as the map becomes distorted. For maps supplied to RAF navigators, distortion can for practical purposes be disregarded, and are on some form of conical projection, for example the Catini projection for the RAF Edition ¼ inch maps. Charts are nearly always constructed on the Mercator projection where the distortion although not negligible, is regular and distances can be obtained by using the appropriate scale of latitude, in which a minute of latitude equals one nautical mile.
- Alternative Title:
- Colour: B&W
- Digitised:
- Object_Number: AMY 413
- Sound: Sound
- Access Conditions:
- Featured Period: 1946-1975
- Production Date: 1939
- Production Country: GB
- Production Details: Air Ministry (Production sponsor) Gee Films (Production company)
- Personalities, Units and Organisations:
- Keywords:
- Physical Characteristics: Colour format: B&W Sound format: Sound
- Technical Details: Format: 16mm Number of items/reels/tapes: 1 Footage: 880ft; Running time: 25 mins
- HD Media:
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