•
Switch into Triangle Surface Creation mode by
pressing the Triangle Mode Tool 
or by selecting "Triangle Mode" from
the Surfaces sub-menu (ribbon toolbar’s Create Tab).
The cursor will change to the triangle surface cursor.
• Select the first object (mark point, object point, line, edge mark or edge) of the triangle from a photo, 3D view or table.
The selected object will highlight.
• Select the second object of the triangle from a photo, 3D view or table.
The second object will highlight and a path will be drawn on any open photographs with marks for all items in the triangle . Once you have selected the minimum objects required to form a triangle the surface will automatically finish.
• Continue with selecting objects to complete the triangle.