The Addington mosaic has been a lot of fun. Working with so many large layers in one image is taxing on computer resources, and one relatively fast PC (its CPU is a Pentium G with 2 cores but no hyperthreading) with nothing else open and 16 GB of RAM is still quite slow working with what is now a 10 GB Gimp project and which keeps growing as the historic aerial layers have to be scaled up in size. Since I ended up with two large layers (scaled from around 9000 to around 15000 width) on the west side of Addington with a small slice in the middle where another layer has to be pasted in to fill the gap, I resorted as in the past to cropping the required piece out of the gap layer, otherwise it takes all day to get it scaled to the right size.
The idea of running Gimp side by side on two computers in order to get the smallest number of steps to maximise quality has been useful but the scanned images in this case are so sharp (the original scale is 1:3400) that it's been unnecessary to continue with this. At the same time it is hard to get much memory available on MainPC for this kind of thing because browsers and other stuff I do a lot of on it just seem to gobble up all the available memory. Because of this I put Qgis 2.18 onto mediapc as well and have been using that because the media player stuff doesn't use much RAM. This shows the great benefits of an OS like Linux which is designed to work with resource limited hardware, being resource efficient on good hardware that you might be using do do resource intensive tasks.
As it turned out my strategy outlined above did not work and I have changed to using two big layers next to each other with a small slice at one end instead of in the middle, this is because the usual skewings and distortion are better seen at one end rather than in the middle of a critical area of the railway depot. However this means a lot of yesterday's work has had to be redone. This sort of outlines why only a few areas are going to get this aerial photography coverage - it takes a huge amount of time to create these mosaics especially for big areas like a major railway yard or depot. The small stations on the Otago Central line which usually only involve one or two base images and maybe the same number of aerials are child's play by comparison. In this case having a slice on the end is much simpler because it can simply be cropped off when the mosaic georeferenced copies of the original base images are saved, so the full size image can be used instead of cropping a piece of it. We will have to see if Gimp copes with all the historic images once they are being displayed and it may well prove necessary to split the project into two or three pieces.