Terrain Surface importing from Civil 3D incorrectly (shifted location)

[Anonymous]

When exporting a surface from Civil 3D to import as a terrain file for 2D modeling, there is approximately a 3.6′ shift that is occurring at all points along the surface upon import into HEC-RAS. When the surface is exported (from HEC-RAS) as a GeoTIF and brought back into Civil 3D, it appears that the shift is doubled. However, breaklines can be exported back and forth as shapefiles without any apparent distortion. If anyone else has run into this problem or has any ideas how to attempt a solution, feedback would be appreciated. Thanks in advance.

[Anonymous]

This is probably the issue

https://forums.autodesk.com/t5/autocad-civil-3d-forum/civil-3d-causes-horizontal-shift-in-dem-geotif-surfaces/td-p/5351613

[Anonymous]

I’ve looked into this, and while that shift does occur when importing a GEOTIFF into civil 3d, it does not appear that this is the root of the problem in HECRAS. The issue that I am having is when a TIN surface is exported to a GEOTIFF. Surfaces are being generated from survey data, corridors, etc. that are in the correct geospatial position. The shift only occurs after the exported TIFF is used to generate a terrain in RAS Mapper. This is evident when importing objects such as breaklines from shapefiles. They appear to come in in a shifted location, when in reality the surface has shifted to the southwest (this can be checked by exporting these objects and re-importing to Civil 3D). Not sure if I did a great job explaining that, but any further insight would be greatly appreciated. I can’t imagine that we are the only ones having this issue.

[Anonymous]

have you found a solution to this problem?

[BB]

This issues still exists with C3D 2020. The problem is with Civil 3d (and they’ve known about it for a while). You can check it yourself by simply exporting a surface from C3D as a DEM and then re-import it. You’ll notice that the re-imported surface has a horizontal shift…or translation error, which seems to vary in magnitude. Don’t know a work around other than exporting the surface as a LandXML –> importing into ArcGIS as a TIN –> exporting the TIN as a DEM –> create terrain surface in HEC from the DEM. Hope that helps. If you know of a better solution, please post.

[BB]

Actually, just found a decent explanation for the error and a C3D workaround. Here’s what the following handout says:

“NOTE: For this project, the projection has a base unit of US Survey FEET. The GeoTIFF metadata also has abase unit of US Survey FEET. When a surface is created from a DEM, it appears that Civil3D erroneously
assumes that a DEM with a unit of feet is International FEET.

As the tutorial project is working with a state plane reference system, this causes the imported DEM grid to be shifted both south and west of where it should actually land.

One U.S. survey foot is equal to 1.000002 International Feet.

For this tutorial, Civil3D places the surfaces the surface with an error that is scaled from the origin of the stateplane system.

If the project is in meters then this shift will not appear. Note that this error appears to only appear when DEMs are IMPORTED into Civil3D and the units are in US Survey Feet.

Solution: Correct the DEM shift error by moving the surface 6 feet east and 20 feet north.”

[Author]

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