Well it is actually possible to add a scale bar to ArcScene as well as applying it to elevation and so on with a little workaround. Just export any georeferenced feature from Arcmap as a High-resolution Picture with a huge scale bar wherever u want it. (you could also add multiple ones for X Y directions) This can be imported in ArcScene. As it is georeferenced, it will work. Well this is far away from being scientifically accurate, however we are talking about visual enhancements... This picture is just for showing the result. If you wish, you could add a much wider scale bar to even have the right measurements in the very far. The excact same procedure works very well with legends title or what ever. Reference it, import it.
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