Similar to other MX processes, there is more than one way to generate this string.  The following technique utilizes the Profile Viewer.  

From the Profile Viewer, select the Select Reference String button to obtain the Select Reference String panel.  From the Details tab, supply the reference model and reference string of the proposed design.  The Start and End tab allows one to limit the extents of the reference string,  leave these fields blank to address the entire string.

Select OK to return to the profile viewer.  The view port should show the profile of the reference string, a null reference string should display as a horizontal line.

From the Profile Viewer, select the Create Section String button to obtain the Define Section String panel.

From the Details tab, specify the project's ground model (preferably the triangles model).  Supply the corresponding existing ground triangulation string.  At the bottom of the Details tab, verify that Apply Elevations to Reference String is not checked.  Specify the Section String Name "OCxx" where the last two characters should match the last two characters of the reference string.  For example,  if the reference string is MCAM, then specify OCAM.

To limit the extents with respect to the reference string,  go to the Start and End tab.  To activate Secondary Interpolation (not available when using triangulation) go to the Options tab.  Select OK to generate the new string, this will also return you to the profile viewer.

The view port should show the profile of the new OCxx string.

Shut down the Profile Viewer to finish.  However, before returning to MX, the Rename Temporary Model panel appears.  The old ground string "OCxx" currently resides in this temporary model.  Select the Copy Strings to Existing Model option and specify the reference model.  Click Finish to fully return to MX.

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