Hi,
Transforming from MGA94 (using the GSB GDA94 NTv2 to GDA2020 option) on known points does not give the published values. It's in the <10mm range, but I'm puzzled as to why there is any difference?
See below screen shots;
The published values are here;
PDF
Calculated comparisons are here;
Geodetic Survey Marks.xlsx
Any ideas?
Thanks.
Hello Christopher,
I believe this was explained in another Forum Post from last year, except the transform was going the other direction. Take a look and see if the comments will help in your case:
New GDA2020 Geocoordinate Systems in CE
https://communities.bentley.com/products/geospatial/desktop/f/bentley-map-forum/174192/new-gda2020-geocoordinate-systems-in-ce---how-does-it-work
What version of OpenCities Map are you using?
Hi Dan,
I posted here as suggested by Mark Shamoun;
https://communities.bentley.com/products/microstation/f/microstation-forum/168350/new-datum-for-gcs-in-australia-gda2020-is-comming/573385
Christopher,
I have verified the differences observed but am not surprised by them. The transformation from GDA94 to GDA2020 probably relies upon a complex system and transformation through 7 parameter transformation or NTv2 grid shift files is an approximation. The NTv2 file format allows for arbitrary densification of the data, and more precise grid shift files could have been designed by the Australian Government, yet, denser grid shift files result in files of greater size. The size of the Australian file is 83 megabytes which is already relatively large. The governement probably limited the density of the grid to keep the file size acceptable. A difference of 1 cm is not too bad. A bit large for cadastral purposes in dense urban area but acceptable for most purposes otherwise.
I have encountered NTv2 file as large as 1 gigabytes for provinces of Germany and probably these can result in millimetric accuracy yet the size is unmanageable and Bentley System does not distribute those files which must be installed manually by clients.
If the Australian government wants to they can design denser files and distribute them.
Answer Verified By: Christopher Albany
Hi Alain,
Thank you for verifying this and for the information provided.
We will liaise with Landgate and/or GeoScience Australia if indeed higher accuracy conversions are required.
Thank you,
Chris.
Alain Robert said:A difference of 1 cm is not too bad. A bit large for cadastral purposes in dense urban area but acceptable for most purposes otherwise. I have encountered NTv2 file as large as 1 gigabytes for provinces of Germany
A difference of 1 cm is not too bad. A bit large for cadastral purposes in dense urban area but acceptable for most purposes otherwise.
I have encountered NTv2 file as large as 1 gigabytes for provinces of Germany
1cm for cadastral purposes seems really good even for us crazy Germans
Regards
Frank
since 1985: GIS, CAD, Engineering (Civil) Senior Consultant : [Autodesk Civil 3D , Esri ArcGIS, VertiGIS: in previous days : Bentley MS V4 - V8i, GeoGraphics, Bentley Map V8i, InRoads, HHK Geograf, IBr DAVID] : Dev: [C, C++, .NET, Java, SQL, FORTRAN, UML][direct quote by: http://en.wikipedia.org/wiki/Helmut_Schmidt]: "Wer Kritik übel nimmt, hat etwas zu verbergen"Wer Grammatik- und/oder Rechtschreibfehler findet, der darf sie behalten :-)