Spatial Data Warehouses - Data Warehouse Systems: Design and Implementation - page 434

last revised in 2004, usually referred to as WGS 84. This ellipsoid is used by
the Global Positioning System (GPS). Nevertheless, different regions of the
world use different reference ellipsoids, minimizing the differences between
the geoid and the ellipsoid.
The ellipsoid is used to measure the location of points of interest using
latitude and longitude. These are measures of the angles (in degrees) from
the center of the Earth to a point on the Earth's surface. Latitude measures
angles in the North-South direction, while longitude measures angles in the
East-West direction. While an ellipsoid approximates the shape of the Earth,
a datum defines where on the Earth to anchor the ellipsoid.
To produce a map, the curved reference surface of the Earth, approximated
by an ellipsoid, must be transformed into the flat plane of the map by means of
a map projection . Thus, a point on the reference surface of the Earth with
geographic coordinates expressed by latitude and longitude is transformed
into Cartesian (or map) coordinates ( x, y ) representing positions on the map
plane. However, as a map projection necessarily causes deformations, different
projections are used for different purposes, depending on which information
is preserved, namely, shapes and angles, area, distance, or directions. These
four features are conflicting (e.g., it is not possible to preserve both shapes
and angles as well as area), and thus, the importance placed on each of these
features dictates the choice of a particular projection.
A spatial reference system (SRS) assigns coordinates in a mathematical
space to a location in real-world space. An SRS defines at least the units of
measure of the underlying coordinate system (such as degrees or meters),
the maximum and minimum coordinates (also referred to as the bounds),
the default linear unit of measure, whether data are planar or spheroid, and
projection information for transforming the data to other SRSs. SRSs are
in general good for only a specific region of the globe. If two geometries are
in the same SRS, they can be overlaid without distortion. If this is not the
case, they must be transformed. As there are thousands of SRSs, each one is
identified by a spatial reference system identifier (SRID).
11.3.2 Vector Model
The spatial data types described in Sect. 11.1.1 were defined at the conceptual
level, describing spatial features from an abstract perspective, without taking
into consideration how these will be implemented into actual systems.
The vector model provides a collection of data types for representing
spatial objects into the computer. Thus, for example, while at an abstract
level a linear object is defined as an infinite collection of points, at the
implementation level such a line must be approximated using points, lines,
and curves as primitives. A point is represented by coordinates such as ( x, y )
or ( x, y, z ) depending on the number of dimensions of the underlying space.