6.7.2. Preprocessing, Geodetic System, Datums, and Projections

The preprocessing classes of TerrainViz, deriving from SoTVizPreprocessor , import GIS file formats which can be DEM (Digital Elevation Model) file format ( DTED, STDS,…), geo-referenced raster file format ( GeoTiff…), or raster file format with geo-referencing information (JPEG, TIFF, GIF,...).

All these GIS file formats store elevation or localization data using the Geodetic Reference System. Thus, the preprocessing classes must convert from a geodetic coordinate system (longitude, latitude or projected coordinates in the geodetic reference system) to the Open Inventor world coordinate system (a Cartesian coordinate system).

Geodetic reference system

This reference system approximates the earth dimensions with an ellipsoid and a patch called Datum which gives the orientation and the position of this ellipsoid relative to the real center of the earth.

The ellipsoid is defined by two values: the equatorial radius (ER) and either polar radius (PR), inverse flattening (1/F), or eccentricity (E). Those two last values can be computed from radius values through the following formulas:

  • Flattening F = 1 – PR/ER

  • Squared Eccentricity E ² = 2F – F ²

Geodetic system

Figure 6.9. Geodetic system



Different universities, laboratories, and topography organizations have computed different ellipsoid models and Datum models which correspond to different regions of the Earth. In other words, in order to reduce the gap between the approximation model and the reality, each country has one or more Ellipsoid and Datum models.

The positions are commonly expressed as UTM, x/y/z or latitude, longitude, elevation triplets.

The Ellipsoid models supported by TerrainViz are the following:

  • Normal Sphere (radius = 6370997m)

  • Airy 183

  • Modified Airy

  • Andrae 1876 (Den., Iclnd.)

  • Appl. Physics. 1965

  • Australian Natl & S. Amer. 1969

  • Bessel 1841

  • Bessel 1841 (Namibia)

  • Clarke 1866

  • Clarke 1880

  • Comm. des Poids et Mesures 1799

  • Delambre 1810 (Belgium)

  • Engelis 1985

  • Everest 1830

  • Everest 1948

  • Everest 1956

  • Everest 1969

  • Everest (Sabah & Sarawak)

  • Fischer (Mercury Datum) 1960

  • Modified Fischer 1960

  • Fischer 1968

  • GRS 67 (IUGG 1967)

  • GRS 1980 (IUGG, 1980

  • Helmert 1906

  • Hough

  • IAU 1976

  • International 1909 (Hayford)

  • Kaula 1961

  • Krassovsky 1942

  • Lerch 1979

  • MERIT 1983

  • Maupertius 1738

  • New International 1967

  • Naval Weapons Lab., 1965

  • Plessis 1817 (France)

  • South American 1969

  • Southeast Asia

  • Soviet Geodetic System 85

  • Walbeck

  • WGS 60

  • WGS 66

  • WGS 72

  • WGS 84

The Datum models supported by TerrainViz are the following:

  • World Geographic System 1984

  • World Geographic System 1972

  • North American 1983

  • North American 1927 Eastern US

  • North American 1927 Western US

  • North American 1927 Conus

  • North American 1927 Alaska

  • North American 1927 Canada

  • North American 1927 Alberta/BC

  • North American 1927 E. Canada

  • North American 1927 Man/Ont

  • North American 1927 NW Terr.

  • North American 1927 Yukon

  • North American 1927 Mexico

  • North American 1927 C. America

  • North American 1927 Canal Zone

  • North American 1927 Caribbean

  • North American 1927 Bahamas

  • North American 1927 San Salv.

  • North American 1927 Cuba

  • North American 1927 Greenland

  • North American 1927 Aleutian E

  • North American 1927 Aleutian W

Projections

Each portion of the surface of the Earth stored within GIS files is a planar map. Obviously this means that the curvilinear Earth surface is projected onto a plane.

As you can imagine, such a projection involves some distortion in the planar representation. There are different kinds of projections from the curvilinear earth surface to a plane. Each of these projections has properties which reduce the distortion issues according to the position on the earth of the projected area.

The following are the projections supported by TerrainViz.

  • Albers Conic Equal Area

  • Azimuthal Equidistant

  • Cassini Soldner

  • Cylindrical Equal Area

  • Equidistant Conic

  • Eckert IV

  • Eckert VI

  • Gall Stereographic

  • Gnomonic

  • Lambert Azimuthal Equal Area

  • Lambert Conformal Conic

  • Mercator

  • Mollweide

  • Orthographic

  • Polyconic

  • Robinson

  • Sinusoidal

  • Transverse Mercator

  • UTM

  • Vandergrinten

Azimuthal projection

Figure 6.10. Azimuthal projection



Conic projection

Figure 6.11. Conic projection