Interrogating the EPSG database

From the 6.1 version (July 2001) for the EPSG database a conformal strategy was adopted according to which the recordings that contain errors will never be deleted. It has been put into practice by introducing an indicator (deprecated indicator) that can take the values true or false (yes or no) as the respective recording contains or not significant errors. All interrogations made by the TransLT in the EPSG database use a filter that eliminates the recording that have the deprecated field set as yes.

When reading from the table Coordinate_Operation the values regarding the accuracy of the operations, if a transformation type operation hasn’t got a registered value, TransLT considers the value of the accuracy as being 999.0 meters.

Note: Supplementary information about the meaning of the terms and the organization mode of the EPSG database can be found in the Guidance Note Number 7, part 1: Using the EPSG Geodetic Parameter Dataset published by OGP and available for download and viewing at the address www.epsg.org.

Interpreting the parameters taken from the EPSG database

In some cases the parameters taken from the database must be adjusted to respect the calculation mode from TransLT. The parameters are modified in the following cases:

1. For 2D transformations

The operations with coordinates with the methods: Affine parametric transformation, Affine geometric transformation and UKOOA P6 seismic bin grid transformation are used in TransLT as non-orthogonal affine 2D transformations with 6 parameters and in this case the coefficients of these methods are changed in: Tx, Ty, R, Sx, Sy and ε (non-orthogonal coefficient). If the non-orthogonal coefficient ε is 0.0 then the method used is the one with 5 parameters and also here, if the scale factors Sx and Sy are identical, the method used is the one with 4 parameters.

For the operation with coordinates with the Similarity transformation method the rotation is inversed to keep the rotation sense used by TransLT.

2. For 3D transformations

For the operations with coordinates with the methods: Position Vector transformation (geocentric domain), Position Vector transformation (geog3D domain) and Position Vector transformation (geog2D domain) the rotations are reversed to keep the rotation direction used by TransLT. The direction of the rotations for these methods defined in EPSG database is inverse, the rule of 3D rotations is different from the one used in TransLT.

Recommendation: The parameters taken from the database for projections, 2D transformations, 3D transformations or polynomial transformations must be verified because it is possible that in the current version of the database they are not up to date. For example, a case was found for a projection for which the scale factor from the projection origin in the database is 1.0 and in reality for the respective projection a change factor from units to meters of 0.999998261 must be used (CRS: NZGD49 / Amuri Circuit).

Operations that are not implemented in this version of this program