When using a mirror cube to aid in a bundle adjustment one must know the face definitions.
Mirror face 1. Is the top of the cube, opposite the bonded surface (Face 6) and defines the positive X direction of the mirror cube frame.
Mirror face 2. Is 90° to face 1, and defines the positive Y direction.
Once face 2 is defined the numbering of the rest of the faces moves counterclockwise around the cube, when looking at face 1.
Mirror face 3. Defines the positive Z direction.
Mirror face 4. Is opposite face 2 and defines (-Y).
Mirror face 5. is opposite face 3 and defines (-Z).
Mirror face 6. Typically glued to a surface, is opposite face 1 and defines (-X).
Though it can be a bit tricky aligning the theodolites to the faces of the mirror cube, the idea is to align a theodolite normal to one face of the mirror cube. Then set up another theodolite normal to a face that is 90° from the face the previous instrument aligned to. In addition to each theodolite measuring the mirror cube and naming those shots the same in the same point group, you will also need to change the point properties from normal shots to mirror cube shots.
To change the point properties, double-click the point in question in the tree. Then click on “Measurement Details”. There should be two measurement details, one from each instrument that measured the mirror cube.
Then single-click the first point in the list and click properties. The window below will then be displayed. In this window you want to click “Advanced Options”, where you will be able to select mirror cube shot and what face of the mirror cube was measured using the aforementioned face definitions. Repeat these steps for the other mirror cube shot.
Often, users wish to construct a coordinate frame based on the faces of a mirror cube. The Mirror Cube frame assignment method provides a convenient method for repeatedly and accurately locating a coordinate frame with two instruments (typically theodolites). SpatialAnalyzer contains a powerful mirror cube frame assignment tool.
At this point, you can now begin to create the mirror cube frame. Go to Theodoilte > Mirror Cube. You will be prompted to select the mirror cube point. After making that selection, the window below will be displayed:
Choosing “Yes” will prompt SA to create a frame based on the current alignment of the two instruments to the mirror cube. Choosing “NO” will allow you to change the properties of the frame which can be done in the window below:
this dialog the user is presented with some possible options.
You may select which instrument represents the X-axis of the coordinate frame.
You may select or deselect specific shots from each instrument. When there are multiple shots, their vector directions are averaged to calculate the vector displayed below the measurement box.
The check box for coordinate axes determines whether the axes of the frame are outward along the theodolite vectors or inward toward the theodolites.
The axis weighting box allows you to control the distribution of the errors between the two axes.
Set the cube face angle (normally 90 degrees). Press “Compute Mirror Cube Frame” after doing this update.
The difference between the cube “ideal” angle and the theodolite vector angle will be displayed.
Use the slider bar to control how far the frame axes are away from the theodolite axes. Sliding the bar to the left places more of the error on the “right-side” instrument.
Choose carefully when distributing the error. In some cases, it may be better to have no error on one axis since it is difficult to tell if the “error” is coming from the theodolite measurements or the mirror cube itself.
Once you are satisfied with the frame information, you can select “Create Frame!”. This will construct a frame on the mirror cube measured point utilizing the parameters that were set.