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3 Non-geometry standards
graphie coordinates. Because ellipsoidal heights are sometimes difficult to relate to
the topographie earth's surface, the third coordinate is usually not related to the ellipsoid. Instead it could be given in an independent elevation reference system. This
method is acceptable, because in small-scale and medium-scale applications, the decrease of the height accuracy caused by introducing a second reference system is insignificant compared to the accuracy of the height values.
The origin of a Cartesian System lies close to the centre of mass of the earth with
the equator-plane of the earth usually defining the xy-plane of the system. The positive x-axis points to the interseetion of the Greenwich meridian with the equator - a
point off the African Atlantic coast. The positive z-axis points to the North Pole and
the xyz-system realises a right hand system. Cartesian systems have become popular
since they are used for describing the orbits ofnavigation satellites such as GPS.
A projected co ordinate system is usually a two-dimensional system referred to a
geometrie developable surface like a cylinder, a cone, or a plane. These surfaces
realise a local approximation of the earth's surface. In most cases, one axis of the
system points north or south, the other east or west.
Datum and coordinate systems
A geodetic datum gives the relationship of a coordinate system to the earth. It is
geodetic, vertical, or engineering. In many cases, it requires an ellipsoid definition. A
vertical datum gives the relationship of gravity-related heights to a surface known as
the geoid. The geoid is a surface close to mean sea level. According to ISO 19111, a
datum shall be engineering if it is neither geodetic nor vertical.
A prime meridian defines the origin from which longitude values are specified.
Most geodetic datums use Greenwich as their prime meridian.
A Coordinate Reference System is called fully defined if it has a datum and a coordinate system. Historically, the datum was often the position of an observatory
near the centre of a country where the position and the orientation of the coordinate
system are defined in relation to the physical reality of the earth. The prime meridian
defines the origin of the coordinate values of the first ellipsoidal axis.
A reference that is defined by two different Coordinate Reference Systems - one
of these is an elevation system - is called a compound Coordinate Reference System.
A compound Coordinate Reference System is usually used to provide an independent reference for horizontal and vertical coordinates.
The standard does not address any specific coordinate system such as UTM (Universal Transverse Mercator) or WGS84 (World Geodetic System 1984). Also, the
standard excludes the details of vertical systems bound to the physical structure and
the real geometrie shape of the earth being known as the geoid.
3 Non-geometry standards
graphie coordinates. Because ellipsoidal heights are sometimes difficult to relate to
the topographie earth's surface, the third coordinate is usually not related to the ellipsoid. Instead it could be given in an independent elevation reference system. This
method is acceptable, because in small-scale and medium-scale applications, the decrease of the height accuracy caused by introducing a second reference system is insignificant compared to the accuracy of the height values.
The origin of a Cartesian System lies close to the centre of mass of the earth with
the equator-plane of the earth usually defining the xy-plane of the system. The positive x-axis points to the interseetion of the Greenwich meridian with the equator - a
point off the African Atlantic coast. The positive z-axis points to the North Pole and
the xyz-system realises a right hand system. Cartesian systems have become popular
since they are used for describing the orbits ofnavigation satellites such as GPS.
A projected co ordinate system is usually a two-dimensional system referred to a
geometrie developable surface like a cylinder, a cone, or a plane. These surfaces
realise a local approximation of the earth's surface. In most cases, one axis of the
system points north or south, the other east or west.
Datum and coordinate systems
A geodetic datum gives the relationship of a coordinate system to the earth. It is
geodetic, vertical, or engineering. In many cases, it requires an ellipsoid definition. A
vertical datum gives the relationship of gravity-related heights to a surface known as
the geoid. The geoid is a surface close to mean sea level. According to ISO 19111, a
datum shall be engineering if it is neither geodetic nor vertical.
A prime meridian defines the origin from which longitude values are specified.
Most geodetic datums use Greenwich as their prime meridian.
A Coordinate Reference System is called fully defined if it has a datum and a coordinate system. Historically, the datum was often the position of an observatory
near the centre of a country where the position and the orientation of the coordinate
system are defined in relation to the physical reality of the earth. The prime meridian
defines the origin of the coordinate values of the first ellipsoidal axis.
A reference that is defined by two different Coordinate Reference Systems - one
of these is an elevation system - is called a compound Coordinate Reference System.
A compound Coordinate Reference System is usually used to provide an independent reference for horizontal and vertical coordinates.
The standard does not address any specific coordinate system such as UTM (Universal Transverse Mercator) or WGS84 (World Geodetic System 1984). Also, the
standard excludes the details of vertical systems bound to the physical structure and
the real geometrie shape of the earth being known as the geoid.