The conversion between two Spatial Reference
Systems (SRS) is performed by a coordinate transformation, that is,
a mathematical transformation or a chain of mathematical
transformations.
For example, when superimposing Global Positioning System (GPS)
points (expressed in longitude and latitude) on a state map (where
coordinates are expressed in meters, in UTM
projection), the GPS coordinates are expressed in a geographic
coordinate system, while the state map coordinates are in a
projected coordinate system. Once the SRS for each data source is
known, the JViews Maps
package is able to construct a mathematical transformation to
convert from one coordinate system to another.
SRS and coordinate transformations are
mainly used for two purposes.
The first one is to render the pipeline of
the reader framework, and the main steps are:
- Defining the source SRSThe SRS of source data is often stored in the reader or in the feature of the iterator that is used. If not, the reader provides the
setSourceCoordinateSystem()method. The SRS of source data is stored in map features returned by the iterator. - Defining the target SRSThe SRS of target data is optional. In fact, the rendering mechanism only needs a coordinate transformation. Before rendering your graphic objects, you have to create the coordinate transformation once.
- Creating the coordinate transformationIf no coordinate transformation is needed, the rendering can be performed using an identity transformation.
- Iterating on map features provided by the iterator, and rendering them.
The second one is to position free data,
(without the renderers, you create the coordinate of the graphic
object yourself), and the main steps are nearly the same:
- Defining the source SRS.
- Defining the target SRS.
- Creating the coordinate transformation.
- Transforming each coordinate manually.
The following packages define some useful
SRS to be used with maps:
-
Contains coordinate systems and related classes.
-
Defines the transformations between coordinate systems.
-
Defines factories and utility classes to convert coordinate systems to and from Open GIS Well-Known Text (WKT) specifications.
-
Defines projections used in projected coordinate systems.NoteThe
projectionpackage also contains the definition of ellipsoids and geodetic datum (horizontal datum), even when these are not specific to projections and are used by thecoordsyspackage. This is to keep the compatibility with earlier JViews Maps versions prior to 5.0, where only projected coordinate systems were handled through the use of projections.