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Map projections

Map projections. Reference points. In cartography, it is imperative to take into account the spherical shape of the earth With geodesy we are able to transform the curvature of the earth into a plain surface and place on it geodetic points (references) that

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Map projections

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  1. Map projections

  2. Reference points • In cartography, it is imperative to take into account the spherical shape of the earth • With geodesywe are able to transform the curvature of the earth into a plain surface and place on it geodetic points (references) that serve as a foundation of the map Pan-west coast - France(Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center).

  3. The shape of the earth • The ellipsoid • Newton introduced the concept that the sphere was flattened at the poles and could be represented as an ellipsoid

  4. The shape of the earth • The ellipsoid (continuation) • 20 different ellipsoids were defined, adapted every time to a given region (local ellipsoids) • With the arrival of the satellites in geodesy, it was possible to establish global ellipsoids that could be applied to the entire surface of the earth

  5. The shape of the earth • The geoid • Ellipsoid does not take into account the heterogeneity inherent on the earth’s surface (assessment problem of the altitude) • Altitude measurements must be based on the mean sea level • identified along the shorelines (extended under the continents) • based on an equal force of gravity = the representation is called geoid

  6. The shape of the earth • Coordinates • The global positioning system is based on geographical coordinates: • Longitudes - angles formed from a starting meridian (Greenwich meridian or a national meridian) • Latitudes - angles measured from the equator • Both are expressed in degrees: Degree, Minutes, Seconds(DMS); DM or DD

  7. The shape of the earth • Altitude • Altitude is the height of a point on the relief in relation to the geoid

  8. Projections • The projection is based on the principle of projecting positions on the earth surface to a given geometric surface: • a cylinder • a cone • or a plain surface • later the geometric surface can be cut and straightened to assume a flat shape (like the map)

  9. Projections • Type of projection • The geometric figures can be tangent or secant to the ellipsoid (the average deformation is less for a secant projection as the average point is closer to one of the two reference lines) • The choice of the projection system is done in order to minimise changes. To adapt the type of projection according to: • the location of a country • the position on the earth surface (the poles, the equator...) • extent of the country (small islands or a large continent)

  10. Projections • Examples of projection • Cylindrical projections (e.g.: Mercator projections) • done on a cylindrical shape • is tangent to the ellipsoid and the tangency can be on the equator or on a given meridian • The cylindrical transverse projection (UTM projection used for example with GPS). Here the central meridian and the equator are two orthogonal straight lines, while the other meridians and parallels beside them become curved orthogonal lines.

  11. Projections • Examples of projection(continuation) • The map projection UTM in France • geodetic reference system WGS84 or ED50 (European Datum 1950) • reference ellipsoid IAG GRS80 with WGS84, Hayford 1909 with ED50 • type of map projection cylindrical transverse • the ground is cut out in 60 zones of 6° each one (3 on France - 30, 31 and 32) • unit: m

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