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This is the same concept as a datum. While a datum is a mathematical
and geometric concept, it acts like the concrete foundation of a
skyscraper.
Geodetic datums can be classified in two ways.

For classical geodesy, a local geodetic datum (e.g. HK80
geodetic datum, figure 5.2.) is customary defined by:
• latitude and longitude of initial point;

• azimuth of a line from this point;
• semi-major axis and flattening of the reference ellipsoid;
• the deflection of vertical at the initial point (optional).

Figure 5.2. Local geodetic datum

For satellite geodesy, a global geodetic datum (e.g. WGS 84,

figure 5.2.) is defined by:
• three constants to specify the origin of the coordinate system (x,
y, z);

• the constants to specify the orientation of the coordinate system;
• two constants to specify the dimension of the reference ellipsoid
(a and f).

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