NZ Rail Maps Technical Review 2020-04-18

So we are now at Qgis version 3.10.4 for the NZ Rail Maps map production task. Back a way we had a lot of trouble with Qgis with WMTS layers which were unusable at that time. Since then however we haven’t been able to replicate the problem but we suspect that their code has bugs in WMTS handling and possibly wasn’t dealing with a temporary issue with the WMTS feed. We had to go back to 3.4.4 running on a Stretch VM to be able to continue editing our projects until we decided to have another go with the latest LTR and it seems to be OK for now.
Also looking back a few posts we see a reference to saving mosaic tiles as a group to reduce the overall number of them. This is one of several possible solutions to a number of situations. One of these situations is when we have to upscale Linz imagery that is at 0.3 m or 0.4 m resolution in order to match the kind of scale that we get in images we download from Retrolens. From experience, a pixel height/width no greater than 0.15 m is needed to be able to match 1:4300 or 1:5500 scale and get the full resolution of the Retrolens images when we export out the mosaic tiles to be used in Qgis.
The question then to be solved was how to change that into 4800×7200 tiles because each of the original base images is now several times larger in each dimension. For example these days we will commonly scale a 0.3 m resolution tile by 200% in each direction which makes the new pixel size 0.1 m. We did scale 0.4 m tiles to 0.1 m at one time but for the rare times we use them will try to get away with 0.2 m by again scaling the base 200% both ways. Obviously the original 4800×7200 tile now becomes four if we have doubled in both directions.
One way we attempted to resolve this was to create a special grid of 4800×7200 tiles by adding a part suffix to the original filename so that XXXX1-YYYY2 would become XXXX1.2-YYYY1.3 where the .1 or .3 suffix indicated a position in a particular grid. Special grid templates would be used in Gimp to help split the image into the correct grid segments. This was tried with Northland aerial mosaics in particular but has now been ditched in favour of simply creating a larger mosaic tile made up of multiple 4800×7200 tiles. This is much easier than running the special script to create all the special world files for each sub segment that would need to calculate the top left coordinates to tell Qgis where to draw the tile.
The great advantage of creating one big mosaic tile is that only one world file is needed and it only needs to specify the top left coordinates of the big tile. So you can make that tile cover as big an area as you need as long as you can replicate the top left coordinates and the pixel size in the world file. Typically we are just taking the world file for the top left tile which is based on the original top left tile even though it has been scaled up to a new size, and just changing the pixel resolution parameters in there to reflect the new resolution.
We have been looking at the Northland mosaics again this week and have just been removing the grids and exporting the mosaics again as one big mosaic tile for each station because we never got around to creating the grid segment world files which was very complex and hard to do so this is getting these mosaics back into our Volume 1 Qgis project so they can be put to work.