Package org.apache.sis.referencing.operation

package org.apache.sis.referencing.operation
Relationship between any two Coordinate Reference Systems (CRS). An explanation for this package is provided in the OpenGIS® javadoc. The remaining discussion on this page is specific to the Apache SIS implementation.

This package provides an ISO 19111 Coordinate Operation implementation and support classes. The actual transform work is performed by the sub-packages, but most users will not need to deal with them directly.

Apache SIS extensions

Some SIS implementations provide additional methods that are not part of OGC/ISO specifications:

Apache SIS specific behavior

The following operations have a behavior in Apache SIS which may be different than the behavior found in other software products. Those particularities apply only when the math transform is created directly. Users do not need to care about them when the coordinate operation is inferred by Apache SIS for a given pair of CRS.
  • Longitude rotation (EPSG:9601) — the longitude offset may be specified in any units, but SIS unconditionally converts the value to degrees. Consequently, the user is responsible for converting the longitude axis of source and target CRS to degrees before this operation is applied.

Early binding versus late binding implementations

There is sometimes multiple ways of transforming coordinates for a given pair of source and target CRS. For example, the Bursa-Wolf parameters may vary depending on the area of interest, like in the transformations from NAD27 to WGS84. Even for a fixed set of Bursa-Wolf parameter, there is various ways to use them (Molodensky, Abridged Molodensky, Geocentric translation, etc.).

EPSG identifies two approaches for addressing this multiplicity problem. Quoting the GIGS guideline:

  • Early binding: A priori association of a coordinate transformation with a geodetic CRS. The association is usually made at start-up of the session or project, as that is defined in the software, but always before any data is associated with the ‘CRS’. In general the ‘coordinate transformation’ specified uses the ‘CRS’ of the data as the source ‘CRS’ and WGS 84 as the target ‘CRS’.
  • Late binding: Association at run time of a coordinate transformation with a CRS. Late binding allows the user to select the appropriate transformation upon import of ‘geospatial data’ or merge of two geospatial datasets. This means that, in cases where there are multiple existing transformations, the user can choose the appropriate one, possibly aided by additional information.

Source: Geospatial Integrity of Geoscience Software Part 1 – GIGS guidelines. OGP publication, Report No. 430-1, September 2011

Apache SIS is a late binding implementation, while a little trace for early binding exists in the form of the Default­Geodetic­Datum​.get­Bursa­Wolf­Parameters() method for those who really need it. This means that when searching for a coordinate operation between a given pair of CRS, Apache SIS will query EPSGFactory before to try to infer the operation path by itelf. The Coordinate­Operation­Context can be used for further refinements, for example by specifying the area of interest.