In upgrading from ArcGIS 10.4.1 to 10.5.1 I've noticed that ArcMap seems to handle raster coordinate system info a little differently now.
We have an older set of 10m NED elevation rasters on our data server. In 10.4.1 these files could be re projected from their native GCS NAD 83 to any other common projected system. No errors.
In 10.5.1 the same input raster will not project because Arc interprets the vertical coordinate system as "unknown" and the project raster tool hangs.
If I check the reference system in the image properties in 10.4.1 I can't see any lines for the VCS. In 10.5.1 there are VCS properties for the same image and they are set to "unknown" (unknown VCS datum...). If I redefine the input raster and set the VCS on the Z coordinate system tab all works ok in 10.5.1 even if the output of project raster is set to have no VCS!
The issue is, I don't understand why ArcMap suddenly cares if the VCS is set? The input file is the same in both 10.4.1 and 10.5.1. In 10.4.1 Arc seems to interpret an unknown VCS as "no VCS", in 10.5.1 it seems like I have to explicitly tell it it's no VCS or set a real VCS. What has changed about VCS handling in ArcMap at 10.5.1?