Hi Pilou, I'm sorry but it is very difficult for me to understand what steps you are using just looking at your one single image there.

> OK I have found the good solution but I am totally perplex
> My object is on the ground

So there are 3 objects in your file, are you talking about this one here:



If so then can you show me what spot on the object you are clicking on to make the source frame?


> The curve is on the ground

Are you talking now about the independent curve object, or the surface edge curve, because you will get different behavior if you use those different ones.


Again, could you please show me the final result that you are looking for, because that is not clear to me yet.

Is this the result that you want to get here? :



If both objects you want to position are in the same plane, you may find it easier to use the Orient Line to Line tool instead of the full 3D orient. The orient line to line version is on this button here:



That tool is more simple because it doesn't involve orienting the full x,y,z axes between things, you instead draw 2 lines to do the orientation.

This works best when the lines are in the same plane, like in this case where you have both things you want to align on the world x/y plane.

So for Orient line to line you would pick the following points to orient your object (here I turned scaling mode to Scaling:none):



So using orient line to line, I picked the first 2 points on the object, the first one on the midpoint of the edge and the second one further along it to define the base line, then the target points on the curve, first target point on the curve to control the placement, and then the second point snapped on to the curve tangent to define rotation.

That is probably an easier tool for you to use for this case because it has less need to worry about which directions all the x,y,z axes are pointing at which you need to be aware of (and sometimes adjust to get what you want) for the full 3D axis orient tool.

- Michael