Subscribe to this thread
Home - General / All posts - M8 Custom Projection GDA2020
ColinD

2,050 post(s)
#14-Jul-22 07:35

M8 I have set up custom projection for GDA2020 in C:\Program Files\Manifold System\Grids which contains GDA94_GDA2020_conformal_and_distortion.gsb and GDA94_GDA2020_conformal_and_distortion.xml. A folder Australian Standard (MBS) is in the projection table containing Australia MGA2020 (56).

Australia MGA2020 (56) shows as the projection of a drawing assigned to same.

Problem is that applying change projection (to GDA2020) to a GDA94 drawing results in no change even though projection is shown as GDA2020; there is no Project Component? popup.

If any number like 0.0000001 is added to Local Scale and accepted in Assign Projection, then Change Projection is run the Project Component? popup appears and on acceptance the projection is changed with the correct shift albeit out by a negligible amount.

What am I missing?

Thanks

PS I know I can do this in M9 but need it to work in M8.


Aussie Nature Shots

adamw


10,175 post(s)
#14-Jul-22 16:32

What is the original projection (GDA94, but which one? you can attach a MAP file with a drawing in that projection with all objects deleted), what is the definition of Australia MGA2020 (56) projection (this looks like a custom projection, need the XML for that) and what is in the GDA94_GDA2020_xxx.XML file (need the XML again, don't need the .GSB)?

ColinD

2,050 post(s)
#15-Jul-22 09:15

Thanks Adam, I mostly work in Australia MGA94 (56) and Australia MGA94 (55), see the attached as requested.

Attachments:
Australia Geocentric 2020 (GDA2020).xml
GDA2020 Technical Manual V1.6.pdf
GDA94.map
GDA94_GDA2020_conformal_and_distortion.xml


Aussie Nature Shots

adamw


10,175 post(s)
#15-Jul-22 11:55

OK. That's a defect in Manifold 8.

Two coordinate systems with equal parameters are considered to be the same even if the datums have different names and there is a custom conversion between them. There is no easy way to fix it, that's how Manifold 8 thinks about the coordinate systems - if all parameters are the same, then there is nothing to convert. Custom conversions between datums come later.

Here's what you can do: alter the major axis of the GDA2020 datum (its ellipsoid) by a tiny amount so that the results of conversions are altered by a proportionally small amount (chances are, they won't be altered at all because computation errors are higher), but the system would consider the datums - and thus coordinate systems based on these datums to be different.

Like this (the beginning of Australia Geocentric 2020 (GDA2020).xml):

<xml>

<ellipsoid>

    <name>Australian Geocentric 2020 Ellipsoid</name>

    <majorAxis>6378137.000000001</majorAxis>

    <eccentricity>0.08181919104281579</eccentricity>

</ellipsoid>

...

I bolded the change. We added 1 nanometer to the major axis. That's right on the edge of the accuracy of a 64-bit floating-point value.

Hope this helps.

ColinD

2,050 post(s)
#15-Jul-22 22:51

Indeed it does help, thanks a lot!!


Aussie Nature Shots

Manifold User Community Use Agreement Copyright (C) 2007-2021 Manifold Software Limited. All rights reserved.