Service objects: Coordinate system

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Domain Representation Georeference Coordinate system
Coordinate system

A Coordinate system is a service object for point, segment and polygon maps, and also used by georeferences of raster maps. A coordinate system contains information on the kind of coordinates used by maps; for instance a map can use a user-defined coordinates, coordinates defined by a national standard or coordinates of a certain UTM zone.
There are five main types of coordinate systems:

  • coordinate system boundary only: to define XY-coordinates for maps by only specifying the boundaries of the area in the map. This type of coordinate system should only be used when no projections need to be linked. Note that, maps with a coordinate system boundary only, cannot be transformed into any other coordinate system.
  • coordinate system projection: to define XY-coordinates for maps by specifying the boundaries of the area and its projection information such as ellipsoid information and/or datum information. Note that maps with different coordinate systems having different projections can be transformed into one another.
  • coordinate system latlon: to define XY-coordinate of a map in Latitudes and Longitudes in combination with ellipsoid and/or datum information. Coordinate systems of type latlon are suitable to store, display and print spatial data.
  • coordinate system formula: it is used when maps of same area having different coordinate systems and projections. To be able to transform a map to a desire coordinate system, users need to know the relation between the two coordinate systems.
  • coordinate system tiepoints: It is used to transfer maps to different coordinate system when users do not know the relation between the two coordinate systems. Users need to specify the tiepoints and transformation method in order to transform the map to a desired coordinate system.

In addition to above, there are two more coordinate systems available in the ILWIS \SYSTEM directory:

  • coordinate system Unknown: in general is used when the coordinate system of maps are unknown to ILWIS. Users can also assign the Unknown coordinate system to their maps if the position of the spatial entities does not matter.
  • coordinate system LatLon: used with raster or vector data that uses LatLon-coordinates on a sphere (world-wide).
Projections

In ILWIS Projections are used by Coordinate systems to defines the relation between the map coordinates (X,Y) and the geographic coordinates latitude and longitude. The Earth's surface is curved, however in maps it is presented as a flat surface. Therefore, the display of an area on a map will always lead to some deformation or distortion; there is no 'perfect' projection. If you show only a small part of the Earth, like a town, the distortion will be almost insignificant. If, on the other hand, a map shows a continent, deformations and distortions will be a major problem. To correctly represent the curved Earth's surface on a flat map, you need a special projection. The geographic coordinates are converted to a metric coordinate system, measuring the X- and Y-directions in meters. Each projection has unique equations for the transformation from geographic to metric coordinates and vice versa. Because of the earth's rotation, the shape of the earth is not a perfect sphere. The earth is flattened towards the poles: the equatorial axis (diameter of the equator circle) is longer than the polar axis. The shape of the earth can be represented by an ellipsoid, or as it is sometimes called, a spheroid (shapes that are generated by revolving an ellipsis around its minor axis). The choice of the ellipsoid which fits best a certain region of the earth surface to be mapped depends on the surface curvature and geoid undulations in that region. Hence every country has its own 'best fit' ellipsoid.

Ellipsoids

Due to the fact that Earth does not have a perfect sphere shape but slightly flattened towards the poles and because of the variation in surface reliefs; an ellipsoid is used in combination with a projection to present the Earth's shape more accurately.
Over the years, various different ellipsoids are calculated. Variations in calculated ellipsoids are due to the irregularities in the surface of the Earth.
The choice of the ellipsoid which fits best a certain region of the Earth surface to be mapped, depends on the surface curvature and undulations in that region.
Hence every country has its own 'best fit' ellipsoid.