Unformatted text preview: Data Model If a coordinate system is not explicitly associated with a geometry, a Cartesian
coordinate system is assumed.  Geodetic coordinates (sometimes called geographic coordinates) are angular
coordinates (longitude and latitude), closely related to spherical polar coordinates,
and are defined relative to a particular Earth geodetic datum. (A geodetic datum is
a means of representing the figure of the Earth and is the reference for the system
of geodetic coordinates.)  Projected coordinates are planar Cartesian coordinates that result from
performing a mathematical mapping from a point on the Earth‘s surface to a
plane. There are many such mathematical mappings, each used for a particular
purpose.  Local coordinates are Cartesian coordinates in a non—Earth (nongeoreferenced)
coordinate system. Local coordinate systems are often used for CAD applications
and local surveys. When performing operations on geometries, Spatial uses either a Cartesian or
curvilinear computational model, as appropriate for the coordinate system associated
with the spatial data. For more information about coordinate system support in Spatial, including geodetic,
projected, and local coordinates and coordinate system transformation, see Chapter 6. 1.5.5 Tolerance Tolerance is used to associate a level of precision with spatial data. Tolerance reflects
the dis ance that two points can be apart and still be considered the same _(for exampTe' to
accommodate rounding errors). The tolerance value must be a positive number greater
than zero. The significance of the value depends on whether or not the spatial data is
associated with a geodetic coordinate system. (Geodetic and other types of coordinate systems are described in Section 1.5.4.) I For geodetic data (such as data identified by longitude and latitude coordinates),
the tolerance value is a number of meters. For example, a tolerance value of 100
indicates a tolerance of 100 meters. The tolerance value for geodetic data should
not be smaller than 0.05 (5 centimeters), and in most cases it should be larger.
Spatial uses 0.05 as the tolerance value for geodetic data if you specify a smaller
value. I For nonvgeodetic data, the tolerance value is a number of the units that are
associated with the coordinate system associated with the data. For example, if the
unit of measurement is miles, a tolerance value of 0.005 indicates a tolerance of
0.005 (that is, 1 / 200) mile (approximately 26 feet), and a tolerance value of 2
indicates a tolerance of 2 miles. In both cases, the smaller the tolerance value, the more precision is to be associated
with the data. A tolerance value is specified in two cases:
I In the geometry metadata definition for a layer (see Section 1.5.5.1)
 As an input parameter to certain functions (see Section 1.5.5.2) For additional information about tolerance with linear referencing system (LRS) data,
see Section 7.6. 16 Oracle Spatial User's Guide and Reference L r3.
F ...
View
Full Document
 Spring '09
 eckberg
 Databases

Click to edit the document details