Since last writing on this subject I have further determined that I can scale 0.3 and 0.4 metre pixel resolution background Linz aerial images to double the scale (0.15 and 0.2 metres) and these scales work very well with the 1:4300/4325, 1:5500 and 1:8000 scale Retrolens aerial photos which are the best ones for creating yard layouts. Furthermore Qgis can handle a tile size that is four times the original (halving the original pixel size) without difficulty when render caching is disabled.
The result is that I don't need to use grid segmenting of the original layers to create the original tile size of 4800x7200 pixels, in which each of the original tiles was split into four segments with an index grid loaded into Gimp to ensure the correct naming pattern was used. This also facilitated processing the segments in a Python script to automatically generate the correct world files and copy all the sidecar files to the segment names.
I expect this grid segmenting system will only be needed with 0.75 metre or larger pixel sizes which are most practically scaled to 5 times original size resulting in a very large 5x5 grid. At the moment I can't think of one area in NZ for which there is only 0.75 metre imagery available although having said that it was only last year that 0.4 metres was available for the whole of Central Otago and when I started mapping the Cromwell Gorge only 0.75 m was available.
Whilst it is now considerably simpler for most cases to be able to eliminate grid segmenting, there is still a need for a script that can copy all the sidecar files from the original file names because in most maps there are multiple generations of historical imagery resulting in sidecar files needing to be duplicated for all these generations and the world file's pixel size needing to be scaled accordingly. I am now working on a script to perform these tasks as a straightforward step and unlike other scripts so far which are based on serverpc, this one will be hosted on mainpc as it will be working with files off the References directory in the maps share.
