You can set the following parameters for a projection:
The ellipsoid that specifies the figure
of the Earth.
For more information on ellipsoids,
refer to section
Ellipsoids.
Each projection is associated with an ellipsoid. By default,
most of the projections use the ellipsoid
SPHERE. Only some specific projections, such as the Universal
Transverse Mercator or the Universal Polar Stereographic, use a nonspherical
ellipsoid by default.
You will obtain more accurate projections using an appropriate
ellipsoid, especially with large scale maps. Note, however, that computations
are more complex and slower than when using a sphere.
To specify the ellipsoid you want to use for a projection,
use the method
setEllipsoid.
IlvProjection projection = new IlvMercatorProjection();
projection.setEllipsoid(IlvEllipsoid.WGS84);
You can either use a static member of the class
IlvEllipsoid
,
which defines a number of commonly used ellipsoids, or create your
own ellipsoid as explained in the section
Defining new ellipsoids.
You can also use one of the predefined ellipsoids listed in the section
Predefined ellipsoids.
The unit converter that specifies the
measurement unit in which Cartesian coordinates should be expressed.
The central meridian and the central
parallel of the projection.
These parameters
can be set with the
setLLCenter method. Projections produce less distortion near
their center.
The offset applied to the Cartesian
coordinates, also called false easting and false northing. These parameters
can be set with the method
setXYOffset. The offset can be used in conjunction with the
unit converters to control the range of projected coordinates for
a region. For example, the range of the region may be set so that
the region fits into a square of size 200 x 200. In
JViews Maps applications, the range of the data
is not an issue, since a transformer can be automatically applied
to fit all the graphics contained in an
IlvManager
into
a window. Therefore, false easting and false northing are mainly used
to adapt a projection to geographic data that has already been projected
using a Cartesian offset.
You can also:
Specify whether the coordinates are
geodetic (the default value) or geocentric using the
setGeocentric method.
The geocentric
latitude of a point is defined by the angle formed by a line joining
the point to the center of the Earth and the equatorial plane, whereas
the geodetic (or geographic) latitude of a point is defined by the
angle formed by the vertical line passing through this point and the
equatorial plane. The two values differ since the Earth is not exactly
a sphere but rather an ellipsoid. Both latitudes are related through
the relation tan phiG = (1 - e ^ 2) tan phi
where e
is
the eccentricity of the ellipsoid used to model the shape of the Earth.
If an application handles geocentric data, this parameter
must be set. Most of the available cartographic data available is
expressed with geographic latitudes.
Specify whether the projection uses
longitude reduction, that is, forces longitude to be in the range
[-PI;PI]
or
accepts any longitude, using the method
setUsingLongitudeReduction.
The above parameters are common to all the projections.
They can be set with the API of the class IlvProjection
,
which is the base class of all the projections in the package. Some
projections have additional specific parameters. For example, secant
latitudes can be specified for a conic projection, or the latitude
of the true scale can be specified for most cylindrical projections.
For more information, refer to the documentation of the API for each
projection.