3D OpenCities Map users engaging the Create Buffer tool for a given layer displayed within the Map Manager are able to set the resulting buffer's height based upon comparison to an attached reality mesh.

During such an operation, the user may experience the warning "Buffer operation requires at least one reality mesh in model".

This may occur without apparent cause, as the expected reality mesh for comparison is plainly visible, and display accessible from the Map Manager. 

The likely cause in these cases is that the reality mesh in question is a nested attachment, residing within a reference attachment and thus not as an attachment to the active model. 

To resolve the condition, attach the reality mesh to the active model or exchange the active map (e.g. *.dgn) for the reference providing the reality mesh attachment.