http://www.ptgui.com/support.html#5_21
abs5.21. How can I retouch the nadir or zenith of a stitched panorama?
It's often difficult to retouch the nadir and zenith of a panorama, due to the stretching that occurs at the bottom and top of the image. A solution is to reproject the panorama such that the nadir or zenith is in the middle:
Start a new PTGui project
Load your stitched panorama (a 360x180 degree equirectangular tiff or jpeg file) as a source image
If the 'Camera / lens data (EXIF)' window pops up, cancel it
In the Project Assistant, Camera/lens parameters box: uncheck Automatic, set the Lens Type to Equirectangular and Horizontal field of view to 360 degrees
Switch PTGui to Advanced mode by clicking the Advanced button in the Project Assistant
Go to the Panorama Editor. Choose 'Projection - Spherical: 360 x 180 Equirectangular' from the Panorama Editor menu bar.
Open Numerical Transform, enter a Pitch value of -90 and press Apply. The panorama is now reprojected with the nadir in the middle.
Go to Create Panorama, choose Optimum Size -> Maximum Size and create the panorama
After retouching the nadir in the reprojected panorama, use the same process, but now with a pitch correction of +90 to reproject it back to the original orientation.
