Class DatumShiftTransform

All Implemented Interfaces:
Serializable, Parameterized, Lenient­Comparable, Math­Transform
Direct Known Subclasses:
Interpolated­Geocentric­Transform, Interpolated­Molodensky­Transform, Interpolated­Transform, Molodensky­Transform

public abstract class DatumShiftTransform extends AbstractMathTransform implements Serializable
Transforms between two CRS (usually geographic) based on different datum. A datum shift may be needed when two CRS use different ellipsoids as approximation of the shape of the Earth. Sometime two CRS use the same ellipsoid but with different anchor point (i.e. their coordinate systems have their origin in different locations).

There is many different datum shift methods, ranging from transformations as simple as adding a constant offset to geographic coordinates, to more complex transformations involving conversions to geocentric coordinates and/or interpolations in a datum shift grid. The simple cases like adding a constant offset are handled by other Math­Transform implementations like Linear­Transform. More complex methods are subclasses of this Datum­Shift­Transform base class, but users should not assume that this is the case of every transforms performing a datum shift.

Datum shift methods overview

The two CRS's ellipsoids have slightly different scale and rotation in space, and their center are located in a slightly different position. Consequently geodetic datum shifts are often approximated by a constant scale, rotation and translation applied on geocentric coordinates. Those approximations are handled in SIS by concatenations of Ellipsoid­To­Centric­Transform with Linear­Transform instead of a specific Datum­Shift­Transform subclass.

If the geodetic datum shifts is approximated only by a geocentric translation without any scale or rotation, and if an error of a few centimetres it acceptable, then the Molodensky­Transform subclass can be used as an approximation of the above method. The Molodensky method requires less floating point operations since it applies directly on geographic coordinates, without conversions to geocentric coordinates.

Some countries go one step further and allow the above geocentric translations to be non-constant. Instead, a different geocentric translation is interpolated for each geographic input coordinates. This case is handled by the Interpolated­Geocentric­Transform subclass, or its Interpolated­Molodensky­Transform variant if a few centimetres accuracy lost can be afforded.

A simpler alternative to the above is to interpolate translations to apply directly on geographic coordinates. This is the approach taken by NADCON and NTv2 grids. SIS handles those datum shifts with the Interpolated­Transform subclass.

See Also:

Defined in the sis-referencing module