Help with creating scale bar with only a point cloud

[Byrje]

Hi I’m creating a model from points i get from a 3d-sonar, I can create the model without a problem but when I’m gonna create a scale bar I just got the message that I don’t have enough reference data? Is it possible to create a scale bar without photos?

I’m creating a model from a underwater structure where there is zero visibility that’s why a use the 3-d sonars instead of photos.