# 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

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 183Modified AiryAndrae 1876 (Den., Iclnd.)Appl. Physics. 1965Australian Natl & S. Amer. 1969Bessel 1841Bessel 1841 (Namibia)Clarke 1866Clarke 1880Comm. des Poids et Mesures 1799Delambre 1810 (Belgium)Engelis 1985Everest 1830Everest 1948Everest 1956Everest 1969Everest (Sabah & Sarawak)Fischer (Mercury Datum) 1960Modified Fischer 1960Fischer 1968GRS 67 (IUGG 1967) GRS 1980 (IUGG, 1980Helmert 1906HoughIAU 1976International 1909 (Hayford)Kaula 1961Krassovsky 1942Lerch 1979MERIT 1983Maupertius 1738New International 1967Naval Weapons Lab., 1965Plessis 1817 (France)South American 1969Southeast AsiaSoviet Geodetic System 85 WalbeckWGS 60WGS 66WGS 72WGS 84

The Datum models supported by TerrainViz are the following:

 World Geographic System 1984World Geographic System 1972North American 1983North American 1927 Eastern USNorth American 1927 Western USNorth American 1927 Conus North American 1927 Alaska North American 1927 Canada North American 1927 Alberta/BCNorth 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. AmericaNorth American 1927 Canal ZoneNorth American 1927 CaribbeanNorth American 1927 Bahamas North American 1927 San Salv. North American 1927 Cuba North American 1927 Greenland North American 1927 Aleutian ENorth 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 AreaAzimuthal EquidistantCassini SoldnerCylindrical Equal AreaEquidistant ConicEckert IVEckert VIGall StereographicGnomonicLambert Azimuthal Equal Area Lambert Conformal ConicMercatorMollweideOrthographicPolyconicRobinsonSinusoidalTransverse MercatorUTMVandergrinten