Problem:

I am attempting to edit a terrain model that was generated by points I brought in using the SS3 Survey tools.  However, when I try to edit the terrain model, I get the error message "Not an editable terrain". 

Solution:

Before you can edit this type of terrain model, you will need to deactivate the survey processing rules.  To do this, go to the Project Explorer and click on the Survey tab.  Right-click on the survey project and select the command Deactivate Survey Processing Rules.