Otago Central Railway [61M]: Cromwell-Alexandra 10

Last time I wrote about the process of fixing up overlap issues in the Linz aerial photography background tiles, specifically with relation to the Cromwell Gorge section of the maps, which is about half of the total distance from Cromwell to Alexandra. The work on that has comprised Steps 1 to 9 out of 10.

Step 10 is the optional, and more intensive, part of the work. Because all the retro aerial layers have been scaled to fit the 0.75 metres per pixel aerial photography layers that were used at the time. I have two choices. The harder one by a great many times is to rescale the aerial layers to fit the new tiles, although the tiles have the same number of pixels, 0.4 metre pixels are quite a bit smaller, and therefore each tile in the 0.4m resolution occupies less physical space when displayed on a map. This is all set out in the world file (.jgw) which I mentioned in a previous post, because this tells the map software the size of each pixel, and therefore how much area it needs to display the whole layer.

The other, much simpler, option, is to rescale the 0.4 m tiles to the same proportions as the 0.75 m tiles. This means each of the higher resolution tiles gets rescaled to 2560×3840 compared to the original resolution of 4800×7200. Once this has been done they can all be properly lined up and joined just as the regular tiles are. Given the size of the original tiles I have opted to merge the resized layers in side by side pairs into one layer, because it simplifies things like copying the masks. Copying a mask from one tile to another is fortunately quite straightforward, and just as it has been necessary to line up the first layer on the 0.75 metre layer underneath, the mask needs to be lined up on the original as well. So doing it this way although somewhat fiddly is probably a lot easier than realigning all the aerial photo layers that don’t inherently line up, because the maps tiles do inherently line up and once I have the first ones aligned then I can just drop in the extra ones and line them up on the first ones. 

Once having got the 0.4 metre tiles all in place and the masks copied across then the next step is to export the tiles on the 0.75 m boundaries which lets us use their original world files to specify the placement and size of the layer. So the new layers will be 4800×7200 and will actually be the 0.75 m tiles with the 0.4 metre tiles overlaid on top of them, and using copies of the original masks in the new layers to ensure the retro aerial content is still visible exactly the same way.

If you look closely at the above image you can see the greenish original layer at the very top, with the brownish high resolution content most of the way down and some more of the green stuff at the bottom, that shows how the tiles are overlaid onto each other. A couple of gaps can be seen at the bottom where I have to fix some mask positioning issues but it should be OK.