NZ Rail Maps: Optimising Gimp and using 4x4x4 grid for mosaics [2]

In the previous post in this section I wrote at some length about the use of grid segmentation of Linz base layers that were at resolutions of 0.3 to 0.4 metres pixel size. The grid segmentation being used to rescale these layers to 0.1 metre pixel size and then the original grid references were assigned additional letters and numbers to reflect that each original tile could be in 9 or 16 segments depending on the rescale factor.
This is felt to be necessary when using NZR station surveys that have a scale of 1:4300 to prevent losing too much detail on the ground when they have to be downsampled. In practice at 0.1 metre or smaller pixel size (0.075 is generally the smallest pixel size available from Linz) I am actually upsampling the historical imagery and it may be more evenly matched to a base layer of 0.15 metre pixel resolution. However I chose to continue with 0.1 metre as it is an even multiple of all the common sizes – 0.1, 0.2, 0.3 and 0.4 metres that Linz carries.
Lately working with other Retrolens scales, specifically the SH1 and NZR corridor surveys at a somewhat smaller scale, I have taken a look at how much detail loss occurs when downsampling these scales to match 0.3 metre base imagery and after doing a trial I have found there is not that much loss of detail and the result is still generally acceptable, especially in comparison to the 0.3 metre base images.
So I will only use segmentation in future for 1:4300 station surveys, and for the larger scale stuff that is still under 1:8000, which covers the NZR corridor surveys and state highway corridors, I expect to use the 0.3 metre or in some cases 0.2 metre base imagery without rescaling and segmenting. There are some areas where the base imagery is 0.4 metres and I expect to lose too much detail downsampling and will look at a compromise of rescaling this to 0.2 metres and a 4x tile area to avoid the need for segmentation in these cases. Since I discovered how to turn off render caching in Qgis and as there are likely to be a small number of areas using 0.4 metres and they will be small areas of a station, I think Qgis will be able to handle the larger tile sizes. Anyway we will see how that goes. Segmentation is a good system but it does take a lot more work to implement so it will just be used to get the best out of NZR station surveys which are in a small number of areas in any case, that aren’t in urban areas with coverage of 0.075 metre to 0.15 metre resolution.

Eliminating the need for segmentation will speed up and reduce resource usage for mosaics for the MNL and MSL lines where I am currently working on including as many small stations as possible in the maps of these corridors. The development of more detailed maps for these corridors is now a priority in terms of volume production for these routes and will make extensive use of the SH1 maps as well as the NZR corridor surveys so as to get good quality maps completed as quickly as I can for these two lines in the South Island.