Hi Bentley Communities,
I am reaching out to see if anyone can help with the below problem.
Essentially we are having problems transforming our data sets from WGS84 to MGA2020.
We are capturing photos with a P4 RTK Drone connected to the CORS network for RTK corrections. The P4 RTK Drone collects all the data in WGS84, there is no setting to change projections.
The workflow below is based on previous advice received from Bentley.
2 Run first AT - No control points, on WGS 84, photo positioning metadata ticked on.
When applying the control at this stage in MGA94 + Ausgeoid09 the model comes in very well, and the initial positions before applying the ground control are normally very close to start with.
However, when we change over the gtx file (Geoid) and vert overrides excel sheets in the gdal folder (as advised by Bentley) , restart CC and then try this exact method, we select MDA2020 + Ausgeoid2020. At this stage I ensure that there is only one geoid in the gdal folder as well, as they have the same EPSG code.
The initial coordinate differences before applying control are around 1.5m different (the difference between GDA94 and GDA2020) before we’ve even started.
My understanding is that this should be a straight 7 parameter transformation between coordinate systems, so why is CC not applying this shift?
Essentially, it appears that CC isn’t recognising the GDA2020 transformation from WGS84 to GDA2020 . Is there a parameter file we need to update maybe?
I would be interested to see what other people are doing for MGA2020 and what their workflow is?
Any help would be appreciated,
Jimmy
James Livesey said: The P4 RTK Drone collects all the data in WGS84, there is no setting to change projections.
Not entirely true. After importing photos you need to set that photos altitude is from WGS84 ellipsoid and not sea level. It can be only set once after import and then this lost . This setting is still undocumented.
James Livesey said:CC isn’t recognising the GDA2020 transformation from WGS84 to GDA2020
Seems 7 parameter affine transformation is not supported. I also tried affine ABRIDGEDTRANSFORMATION parameter but no luck. Even that CC uses GDAL libraries same WKT definition which works in QGIS doesn't work in CC. But PROJ4 Helmert towgs84 parameter worked.
There is also this idea which now has status planned - Add custom affine transformation to projected | Reality Modeling Ideas (aha.io)
James Livesey said:vert overrides excel sheets in the gdal folder (as advised by Bentley)
Advised but I would not reccomend. Better add each GTX in separate WKT definition as this way at least you will have more control and each Contextcapture update will not overwrite the edited csv and less alarming error if model is opened on other computer which doesn't have correct GTX.
Sounds suspicous why you are exchanging GTX files? You need to create additional vertical systems if needed. If you jut overwrite one GTX with other then of course it won't work correcty as CC doesn't care about geoid names.
Hi Oto, thanks for the reply.
Oto said:James Livesey said: The P4 RTK Drone collects all the data in WGS84, there is no setting to change projections. Not entirely true. After importing photos you need to set that photos altitude is from WGS84 ellipsoid and not sea level. It can be only set once after import and then this lost . This setting is still undocumented.
Yes, I am aware of this setting in realtion to altitude. I was refering to the actual DJI setting on the controller, there is no option to change projections (for position)
Oto said:James Livesey said:CC isn’t recognising the GDA2020 transformation from WGS84 to GDA2020 Seems 7 parameter affine transformation is not supported. I also tried affine ABRIDGEDTRANSFORMATION parameter but no luck. Even that CC uses GDAL libraries same WKT definition which works in QGIS doesn't work in CC. But PROJ4 Helmert towgs84 parameter worked.
My question here is, if I can get results with MGA94, adopt the exact workflow, except with MGA2020, why isn't CC applying the shift?
How can the transformation work for one projection and not the other?
James Livesey said:there is no option to change projections (for position)
Of course there is no other option as it uses JPG tags and that only has options for altitude reference.
James Livesey said:My question here is, if I can get results with MGA94, adopt the exact workflow, except with MGA2020, why isn't CC applying the shift?
To apply geoid difference Contextcapture needs both geoids.
James Livesey said:How can the transformation work for one projection and not the other?
7 parameter transformation is low accuracy(~1 meter) usable in GIS softwares. Try to set WGS84 in QGIS and import same control points in both systmes in QGIS and see if it does transformation correctly.You need to provide grid shift files for better accuracy.
In what coordinate system you input control points?