Good way to get from kml to BTB is to use a lot of nodes in Google Earth. The copy the track mesh you imported to BTB and then delete a lot of nodes. Reduce them by 50-75%, you need only 1/4 to make basic the shape. It helps by making the mesh a lot smoother, while still keeping the original road shape and elevation. You might need to fiddle with them, remember that you can copy any node with M key so if you accidentally delete too much, click ctrl and then M and copy the node from the original imported mesh. This is why you used a lot of point sin GE, to get more GPS reference points..
At the end check what the 3D view is showing, you can paint the original with different texture to really see the two road meshes apart. Don't be afraid to use cross-section and camber tools to better manipulate the shape, nodes don't work in 3D view, you can manipulate height better than other axes (use the 2D side views, zoom in close before editing the control arms, see what is does from 3D view. This will be the most time consuming but there's no easier way to get accuracy.....) Before you export the test, remember to set the original imported mesh as No Render, not driveable, no collision (no everything..) so it' doesn't show up in game.
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