Newer versions of Autodesk Civil3D may require resizing before opening large orthomosaics. If you’re having issues, you can try these steps:
Download and install QGIS 2.14.9 (Free)
– Open the .tiff files in QGIS. (You can hide the Ortho by unchecking the box in the layer menu on the left for faster processing)
– On the top bar, navigate to Raster –> Conversion –. Translate (Convert Format)
– Select input layer and output file location and name
– Check “outsize” and select the percentage to which you want to reduce the Ortho resolution.
– Click OK

– To open in Civil3D, use the command “mapiinstert” and select your resized .Tif file.