There are reasons that moving your project to GDA 2020 will be useful (or move data back to MGA 94), but you should approach the transformation of your data with caution.
We will tell you which buttons in 12d Model will do what, but the decision about whether to push the button is with you.
You will not be able to transform tins and trimeshes, or Super Alignments.
These elements of the design will need to be re-created from the translated data.
This is not a deficiency in 12d Model, it is because the data will be distorted as a result of the transformation.
Translated, rotated and scaled by different amounts across the site.
Alignments in 12d Model can be translated, but caution is needed. 12d simply translates each I.P. and then reattaches the same radii curve and transition lengths. As each I.P. is translated by a slightly different scale factor, it is possible (and highly likely on long alignments) that T.P.’s may overlap, T.P.’s on the alignment may not match exactly with the transformed position of the tangents in the strings models. We may only be talking about 0.5mm or less, but when we talk about transferring alignments we use 16bit precision.
You should watch the video available at this link to gain a basic understanding of the Geodetic principles involved.
More information and fact sheets can be found on the Intergovernmental Committee on Surveying and Mapping (ICSM) website:
Transport for NSW has a Technical Direction SurveyingSVTD 2019/01 which may be of benefit:
The following is an extract from the Technical Direction:
“PN 144 scope and coverageThere is no change to the implementation method, which will be an ‘incremental’ approach. The implementation date will change to 1 January 2020, where all infrastructure projects currently underway are to maintain the previously set construction datum (i.e. GDA94 or in some instanced in Sydney Trains, ISG 56), unless it is cost effective to convert them to GDA2020.“
The Buttons To Press Within 12d Model
So you are going to transform your data, unfortunately it is not the end of the decisions. There are 2 transformation methods to go between MGA 94 and MGA 2020.
- a 7 parameter similarity (Helmert) transformation (in 12d Model language – Medium Accuracy)
- a 2D transformation and distortion grid files (in 12d Model language – High Accuracy)
More information on these transformations can be found here:
The files for Medium Accuracy are shipped with 12d Model, for High Accuracy an extra download is needed.
WARNING: When performing a Medium Accuracy Transformation the panel is expecting ELLIPSOID heights, not AHD heights. If you select points with Z values based on AHD they will be translated as a normal part of the 7 parameter process (Ie they will be treated as Ellipsoidal heights which they are not). For this reason if you transferring design/survey data based on AHD, the High Accuracy Transformation should be considered.
Grid Shift Files (*.GSB)
Unfortunately it is still not the end of the decisions, a choice between two 2D transformation grids is needed. More information on these files can be found here:
The following is an extract from this page:
GDA94 ⇔ GDA2020
ICSM is currently developing two 2D national transformation grids:
- Conformal: simply a grid version of the 7 parameter similarity (Helmert) transformation: the basic difference between GDA94 and GDA2020 coordinates primarily due to plate tectonic movement (approximately 1.7m NNE over 26 years); and
- Conformal + Distortion: incorporates the conformal 7 parameter transformation plus localised and regional distortion revealed by incorporating coordinates on ground survey control networks across the nation.
For most of our conversions it will be the the Conformal + Distortion grids that will be used.
The grids can be downloaded here:
The best place to keep the files after downloading is the User_Lib directory. An extra variable needs to be added to your env.4d or env_config.4d file
Some final notes on GSB’s
There are no GSB’s to convert from ISG/AMG to MGA 2020. An intermediate conversion to MGA 94 is needed.
There are two types of GSB files. One is the 2D transformation as discussed here. The other is a similar file but instead of being a 2D transformation in contains information about the separation of the Geiod to the ellipsiod https://www.icsm.gov.au/australian-height-datum. These will generally have GEOID in their file name. They will work within the panel if selected, but will give erroneous results.