Spatial Reference Systems (SRS) are a way to
link coordinates to a reference, so that objects whose coordinates
are expressed in different systems can be displayed in the same
manager.
JViews Maps features,
or generally speaking graphic objects on maps, are representation
of real objects. These map features have to be linked to real-life
objects, and this is performed by attaching the attributes to the
map features. These attributes can be a location, a time, or any
descriptive quality or quantity. For example, you can describe the
position of a restaurant either using its coordinates (The
restaurant coordinates are 2D28’30’’E, 48D59’05’’N), or a
description (The restaurant is at the crossing of X and Y streets,
on the same walkway as the cafe). This link between objects and
their real life counterpart is called a reference
system.
Coordinate system base class
The JViews Maps
supports Spatial Reference Systems where the
reference attributes are coordinates. The abstract class IlvCoordinateSystem serves as base class
for all the coordinate systems.
An
IlvCoordinateSystem
is defined by:
- A optional name.
- An array of
IlvUnitdefining the units to be used on each axis. - An array of
Stringdefining the name of each axis. - The dimension of the coordinates in use in the system are defined by the number of axes.
There are three major classes of
coordinate systems useful for mapping software:
- Geocentric Coordinate SystemRepresents coordinates in a three-axes Cartesian system, whose origin is the center of Earth. This coordinate system is mainly used in datum conversion.
- Geographic Coordinate SystemRepresents the coordinates specified by angles on an ellipsoid (longitude and latitude). An optional height above the ellipsoid can be used here. For example, this is the standard longitude and latitude given by GPS.
- Projected Coordinate SystemRepresents the coordinates of the Earth on a 2-D surface. There are as many projected coordinate systems as the number of existing projections. Projecting coordinates on a 2-D surface is mandatory to display data on a map. As the projection process introduces some errors, not all projections are well suited to represent an area on Earth. For more information, see Map projections.
Geocentric Coordinate System
The IlvGeocentricCoordinateSystem class defines
a geocentric coordinate system, that is, a three-dimensional
Cartesian system. The origin point of this Cartesian system is
the center of the Earth.
The axes are perpendicular and defined as
follows:
- the x-axis lies in the plane containing the equator, and has positive values towards the Greenwich meridian
- the y-axis lies also in the plane containing the equator, and is positive towards the longitude 90 degrees east of Greenwich
- the z-axis corresponds to the polar axis, and is positive northwards
The geocentric coordinate system
Coordinates in a geocentric coordinate
system are expressed in linear units along the axis.
The geocentric coordinate system is mostly
used as the base reference system from which geographic and
projected coordinates are derived. For example, geodetic datums
(horizontal datum) are defined by the shift, rotate and scaling
parameters to convert a geocentric coordinate system to a
reference geocentric coordinate system (in most cases, the WGS84
datum).
Geographic Coordinate System
The IlvGeographicCoordinateSystem class defines
an ellipsoidal coordinate system where coordinates are specified
as latitude and longitude on an ellipsoid, with an optional third
coordinate which represents the ellipsoidal height (altitude
above the ellipsoid).
- Longitude is specified in angular units from the prime meridian of the coordinate system. By convention, coordinates less than 180 degrees east of the prime meridian are positive, coordinates more than -180 degrees west are negative.
- Latitude is specified in angular units from the equator. By convention, northward latitudes are positive and southward coordinates are negative.
- The convention for representing the poles are longitude set to 0 and latitude set to 90 degrees for the north pole or -90 degrees for the south pole.
The geographic coordinate system
Projected Coordinate System
The geographic and geocentric coordinate systems are not well
suited to display maps, since these coordinate systems define map
features in a three dimensional world. Before displaying objects,
the dimension of coordinates must be reduced to 2. This is
performed using map projections (see Map
projections).
A projected coordinate system includes all
the parameters that allow you to describe a coordinate system in
which each planar coordinate is computed from geographic
coordinates using a mathematical function. These include:
- The reference geographic coordinate system (the surface modeling the Earth).
- The mathematical transformation itself (the projection).