Terrain Editing Tutorial: Terrain Objects Import for Farming Simulator 17 (FS17) by PMC.
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HOWTO Import Objects into my FS17 terrain using Giants Editor FS17.
Never ever import an object outside of your own MyTerrain directory! Never import objects you don't have permissions from the original author. Never place object 3D models and textures into your terrain project dir if there is a required mods option available.
Open the other terrain where you want to get the object from. Choose the object, make sure you get all the transform group if there several things, then use file -> export selection with files, then browse to your own MyTerrain\maps\objects\ directory, create new directory which easily identified this object, then finally save the i3d file. It asks do you want to get the parent directory structure, you must answer NO to this.
Place the object directory into your MyTerrain\maps\objects\ directory. Use file -> import and choose the file name.