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Author Topic: Wrong output in export point cloud in WGS84 from GDA2020  (Read 776 times)

Phogi

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Wrong output in export point cloud in WGS84 from GDA2020
« on: May 27, 2022, 09:16:47 AM »
Hi Alexey,

I'm using Metashape v1.8 and I found this is a bug in exporting dense point cloud (laz for example) in GDA2020 / MGA to WGS84.

As in Metashape WKT:
'GEOGCS["GDA2020",DATUM["Geocentric Datum of Australia 2020",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","1168"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9102"]],AUTHORITY["EPSG","7844"]]'

GDA2020 should be same as WGS84 which makes sense for the moden applications, and it does so when converting in Metashape, but when exporting, the WGS84 or WGS84/UTM zone 55 file will have a shift compared to GDA2020, and it is not the same as GDA94 as well.

Please have a look, thank you!!!

Best,

Paulo

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Re: Wrong output in export point cloud in WGS84 from GDA2020
« Reply #1 on: May 27, 2022, 05:04:32 PM »
Hello Phogi,

on my side I have exported a point cloud:
- first in GDA2020/MGA 56
- second  in WGS84 / UTM 56 S

When I open them in Global mapper, i do see a shift of about 1.5 m between the 2 files. In attached screen copy, I have selected same 3 points in both files and shift can be seen in XY. Both sets have same elevation...File WGS84/UTM 56S is colored by elevation...
« Last Edit: May 27, 2022, 05:33:57 PM by Paulo »
Best Regards,

Paul Pelletier,
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Phogi

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Re: Wrong output in export point cloud in WGS84 from GDA2020
« Reply #2 on: May 30, 2022, 03:22:22 AM »
Thank you Paulo, yeah I think Metashape adjusted the datum toWGS84 parameters in the chunk correctly as GDA2020 is the new "WGS84" version so should be all 0 transformation, but the point cloud export step is using the old logic that treating the WGS84 is GDA94 so it is adding the 7 parameter datum transform there. I'm not sure from which version it has this bug but at I had a try v1.74 to v1.82 all having this error. Hope this can be fixed as soon as possible.

Phogi

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Re: Wrong output in export point cloud in WGS84 from GDA2020
« Reply #3 on: May 30, 2022, 03:47:37 AM »
Hi Alexey and Paulo,

I figured out what happens and Metashape seems export the GDA2020 correctly. The reason why I see the point cloud shifted in Global mapper is in Global mapper it is default treating GDA2020 needs a NTv2 transform, so when it is loading WGS84/UTM output, in Metashape's metadata, it is toWGS84(0,0,0,0,0,0,0) but in Global mapper, it applied the datum transform!

So this can be ignored now, it's not a bug, thanks, mates.

Cheers,

Paulo

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Re: Wrong output in export point cloud in WGS84 from GDA2020
« Reply #4 on: May 30, 2022, 09:11:05 AM »
Phogi,

good point! However, I am wondering what NTv2 grid Global mapper is using as the observed shit of some 1.5 m is large for a datum GDA2020 practically aligned with WGS84. Actually with Ausdatum tool I converted a point from GDA2020 / MGA 56 to WGS84, I am getting a few cm shift and not meter level...see

PS. I think the NTv2 grid used by GM for GDA2020, actually is the NTv2 grid shift for GDA94 to GDA2020 or WGS84... a Global mapper bug
« Last Edit: May 30, 2022, 10:54:27 AM by Paulo »
Best Regards,

Paul Pelletier,
Surveyor