Last time I wrote in this series I had been experimenting with the linear and segmented methods of covering a large geographical area in Gimp, such as I do with map mosaics for NZ Rail Maps. In summary the linear method consists of creating a large canvas that can lay out all the tiles in a continuous sequence with the canvas area used once, while the segmented method consists of creating a small canvas that can lay out tiles for each area over the top of each other re-using the canvas area multiple times.
The previous posts commented on the segmentation method using maps for Dunedin to Mosgiel, a distance of around 17 km, and the canvas size used was 7x7, where each tile is 4800 pixels wide and 7200 pixels high. About 200 layers were used in total for a file size of around 60 GB. I am now testing the linear method for Horotiu-Hamilton-Claudelands. The canvas size used is 20x13, a total of 260 tiles, but many of them are not used because of the L shaped corridor area. There are 96 layers at present and the file size is 12 GB. The base aerial photography resolution is 0.1 metres per pixel, which means that the canvas covers an area of 9.6 x 9.36 km.
It is therefore technically possible that a long linear area could possibly be laid out (for example like the MSL corridor in Christchurch in some of the sections used) which was up to 65x4 giving a linear length of 31 km. However since there would be a much higher tile occupancy ratio, it is unlikely we could get to 31 km before running out of system resources. This is not really that big a deal since 8 km or so is plenty and long straight sections of corridor that are perfectly aligned to latitude or longitude are pretty rare anyway.
Nevertheless it does look like a linear canvas usage method is achievable as an alternative to a segmented canvas usage method and with similar resource usage considering that I did split my original Dunedin file into two and therefore each individual file around 30 GB covers about an 8 km section and has 100 layers. Also, the Hamilton file will be able to have some base layer tiles removed once I have finished putting in the mosaics as some of the tiles are superfluous. Linear has a considerable speed advantage for corridors covering a continuous section because overlaps are much easier to work with, not having to be duplicated across both ends of a segmented section.
