Datum Transformations

Datum transformations are required when a coordinate conversion involves a change from one geodetic datum to another. This typically happens when legacy survey data based on an older datum have to be converted to reference a new datum.

OCRIS uses a seven parameter transformation when converting geographic coordinates (latitude and longitude) from one geodetic datum to another. This process includes an intermediate step involving a three-dimensional (X,Y,Z) coordinate space that is defined by the position and orientation of the datum relative to the centre of the ellipsoid that the datum is based on. Using a process of translation, rotation and scaling, the coordinates of the survey point can be recalculated in terms of the target reference system.

Field	Description
Delta X	The X axis component of the translation vector
Delta Y	The Y axis component of the translation vector
Delta Z	The Z axis component of the translation vector
Rotate X	The X axis component of the rotation matrix
Rotate Y	The Y axis component of the rotation matrix
Rotate Z	The Z axis component of the rotation matrix
Scale change	The scale factor
Height difference	The average height difference between the source and target height datums in the area of study (if known). This parameter is only useful in a very limited sense and is generally set to zero. (See the discussion below.)

OCRIS uses the Coordinate Frame rotation convention (as used in Australia). This means that rotation values are defined as being positive counterclockwise (looking towards the origin of the axis). If your parameters are defined for the Position Vector method, this implies that the rotation values have opposite signs. In that case, simply change the signs of the three rotation parameters when entering the values for the transformation.

If the convention on which the rotation values are based is not explicitly stated, it may be necessary to test the results in order to determine which convention is the correct one. This can be done by using the OCRIS coordinate calculator, given a control point for which the coordinates are known in both systems.

If parameters are available for transforming from datum A to datum B, but not from datum B to datum A, then it might be possible to define the inverse calculation by simply changing the sign of all seven parameters. However, the accuracy of this method cannot be guaranteed. It is always better to search for an exact solution for the transformation, or to use one that is known to be reversible.

The height difference is only meaningful if the actual height datums used for the orthometric heights in the source and destination coordinate reference systems are different. In Australia, for example, the move from AGD84 to GDA94 did not affect orthometric heights, because the height datum remained unchanged. However, when converting legacy survey data in a relatively small study area where there has been a known shift in the height datum, it may be possible to use the height difference parameter to automatically adjust the orthometric height accordingly. The practicability of this method will depend mostly on the size and location of the study area, as well as the availability of accurate information regarding the datum shift.

A seven parameter transformation typically produces results with an accuracy of  ±1 metre, although this may vary depending on local factors. While this level of accuracy is sufficient for most purposes, it may not always meet the statutory requirements of a particular application. In such cases, the use of high accuracy transformation grids (usually available at country or state level) is recommended. These transformations will have to be performed outside of OCRIS. The resulting survey data could then be imported into the database and ranked appropriately.