Problem with georeference system in ArcGIS

[Marinka]

Hey,

I have a point cloud for a part of a beach. I added the ground control points and exported as a DEM to ArcGIS. For the same area I also got extra coordinate points, which are quite clearly recognizable structures. These structures I can even recognize from a DEM. When I project these points in ArcGIS and load the DEM, the points do not match with the structures. I have added a picture and the line in the picture indicates the difference between the points and the DEM.

Then I added these points to Agisoft program, along with the ground control points and then these points do match quite well with the structures. (See picture 2 the points I am talking about are z1, z2 etc.) So it seems that the georeference system is not quite well projected in ArcGIS. I use the Dutch RDnew (EPSG) 28992 georeference system, and I wonder if someone knows why the DEM is differently projected in ArcGIS?

[photogrammetrix]

Hi,

Did you generate your point cloud originally from GPS-tagged images (WGSS84 CRS and spheroid) and applied the Amersfort ground control points in Photoscan in a second step using the "convert coordinate  system" from  ground control pane?

As far as I interpret the 28992 EPSG, Amersfort is a projected coordinate system,  using bessel speroid.
In case your original point cloud was  in WGS84  and you would like to convert to Amersfort a geodetic datum transformation will be necessary in order to transform between the different  spheroids. The projection engine in ArcGIS should recognize that and offer a suitable transformation.

I am not sure if Photoscan is able to do a correct datum transformation for Amersfort  28992 in a similar way.

May be the alignment error is caused by such a kind of  problem.

cheers

[Marinka]

Hey,

No we did not use GPS-tagged images, so we only had local coordinates and then we applied the Amersfoort ground control points. We did not use a conversion.

[stihl]

Marinka,

Did you export the DEM as a TIFF or ARC ASCII?

I had serious issues with the GeoTIFF header in the exported TIFF file from Photoscan. This caused a significant shift in the ortho and DEM from the actual GCP locations.

If you have access to the software Global Mapper, could you try and and import the DEM there and export it again as a TIFF or ASCII but with the option "Don't write GeoTIFF header" enabled. This should remove the shift. It has for me at least.
Instead you can also import the Orthophoto in Adobe photoshop and saving it again as a TIFF. This should also remove the GeoTIFF header. Make sure to make a copy of the original .tfw and/or .prj so that the TIFF will have a CRS. Just rename them to the same file name as the newly saved TIFF from Photoshop.

It's an ArcGIS thing though. As we don't use ArcGIS, we never noticed the shift. A client of ours notified us of the issue.

[gorin]

Hi Marinka,
Have you choose ESPG 28992 georeference system in photoscan ?
If not i think that is the problem. You have to choose your georeference system in photoscan, and don't use local coordinates. (i think)

[stihl]

Gorin,

I believe he or she did. If that wasn't the case, the DEM wouldn't even show up near the GCP locations in a GIS at all.
This also sounds very similar to the issue I had. I also worked in Amersfoort RD New with the same ESPG code.