Unified Transform is a very useful capability in Gimp since about 2.10 that combines a number of transformation tools together, such as Scale, Rotate and Shear. I have used this tool extensively when creating aerial map mosaics for the NZ Rail Maps project.
To create an aerial map mosaic, I start with downloading the current aerial imagery layer for the part that I want to create a mosaic from, and then extract the map tiles from it (each is 4800x7200 although if at the edges of the layer they can be smaller) and lay them out in Gimp. Using a grid size of 4700x7200 and snapping to grid makes this quick and easy. If the tiles need to be scaled to a multiple (usually this is x2 in each direction) then use the Scale Layer function to achieve this with Cubic interpolation.
Then import the historical tiles, and the first thing you need to do with downloads from Retrolens is to crop out the borders. Apart from the copyright banner along one border there is also the negative mounting frames, which I presume are cardboard like slides. Sometimes I need to get so close to an edge that I'll be forced to include part of the border and then use masking to hide it. When I first started doing these mosaics, I didn't bother cropping the borders and instead I just masked them off every time.
The next step is to overlay the historical tiles on to the current background tiles. This is where the Unified Transform tool comes into play. The first thing to do is to find a common point near one of the overlay's borders. Then drag the pivot from the centre of the overlay to that point on the overlay, and then set the overlay's opacity to zero. Then drag the overlay until the pivot is in the same place on the base layer.
Then ensuring that the Constrain and From Pivot options are set properly (Scale option turned on), and then scale and rotate the overlay until the rest of it lines up with the base. I have set keyboard shortcuts to control the opacity without having to set this with the mouse (this is Tool Opacity in the keyboard shortcuts section) and also canvas rotation with more shortcuts so I can quickly turn the canvas to make the view work better for me.
The new "Readjust" feature addresses a fairly common issue I have with lining up the overlay with the base where the part I want to match up is nowhere near the edge or the scale handles. It can be somewhere near the middle, for example, and I need to be able to rotate or scale without having to zoom out to get the edge or scale handle in view and then lose sight of what I am trying to line up. Clicking the "Readjust" button creates a new transform control frame that fits in the current view in the window instead of being aligned to the edges of the image, which means you can get a set of edges and adjustment handles anywhere on the overlay. This is particularly useful with some of the quite big images I am working with for ensuring there is perfect alignment especially where two overlays overlap each other and will make a big difference in creating these mosaics. The only issue I have found with Readjust is that it moves the pivot when used, and the option for locking the pivot doesn't seem to achieve this, so I have to drag the pivot back to where it was before but it still seems to be OK. This is likely to make a huge difference in creating more accurate alignments with mosaics than I have been able to achieve in the past.
Normally for Unified Transform these days I will use Cubic interpolation for everything. I used to use LoHalo but I doubt that any extra quality it might have produced was significant so Cubic seems to have little effect on the final quality.
