Vertical Datum Problem for export with different Project and Output CRS

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Hi all,

I have a project with GCPs imported using a given CRS (an old EPSG:27562 Lambert Centre France CRS), and therefore also set my project CRS to this same EPSG:27562 CRS.The Z coordinate of these GCPs is expressed not as height above ellipsoid, but using the altitude expressed in the NGF-IGN69 datum, official for France.

When I export the pointcloud using this same CRS, then the Z coordinate of the points is correctly expressed in the NGF-IGN69 french datum.

However, when I export the dense cloud to the more recent EPSG:3947 CRS (Lambert-93 RGF93 CC47), then the Z coordinate is modified - although it should also be expressed in the NGF-IGN69 vertical datum and not the height above ellipsoid. The whole pointcloud is then offset roughly 44.60m along +Z. The strange thing is that this offset is not exactly the difference between the height above ellipsoid and NGF-IGN69 which has a mean of 46.177 across all my topo points.

Do you know if this is a known bug? Is there a workaround to prevent RC from modifying the Z coordinate durnig the export with an output CRS different from the project CRS but which share the same vertical datum?

Thanks in advance!

2

Dear Iconem, thank you for noticing this.

RealityCapture uses EPSG definitions to transform coordinate systems. The Z values are also transformed.

If you know the Z offset, you can set it in Export settings to preserve wanted heights.

What we recommend is transform your coordinates in some third party transformation service and then use these coordinates in your project.

What will happen, when you set the project coordinate system as EPSG:3947, import the GCPs in old coordinate system, but don’t change project coordinate system and compute the model?