Saturday, 11 November 2017

Otago Central Railway [47B]: Cromwell Gorge revisited

Well the exercise in splicing the old aerials into the current aerial photography has been interesting so far because it has come up with different results to doing it in GE. And that is quite understandable all things considered. The aerial photography from Linz is orthorectified, which means it has been run through some complex computer software to adjust for optical distortions. This is a necessary process to produce imagery that is suitable for making accurate maps and measurements. What I started with was laying the imagery over the Google Earth satellite imagery, which is not orthorectified, so different outcomes are going to be seen. But the old 1962 aerial images are not orthorectified either so there are always going to be compromises. 

But I think that process so far looks to be good but I will have to see what happens with the next image as it may well prove to be the case that they just don't line up that well on the edges, in which case it is going to be really tricky to do them this way. So the way to make things happen is to create a Gimp xcf file for the first one, and then for the second one, bring the base image in as a new layer, and then manage the overlap, remove the base layer from the previous one, and crop and so on so then you can ensure the overlap happens properly.

So at the moment I am just making new aerial photo images, by copying the Linz ones and overlaying the 1962 aerials on them, which gives me georeferenced modified Linz images, as a new set of base imagery for drawing the maps over. But the plan is to work out how to make the lake transparent so that the old images will show through it, and that is likely something that will prove possible when I learn more about Gimp. For now the priority is just to redo the maps as they are a priority for the first article.

Meanwhile at the same time I am pressing on with a few more yard diagrams, so Lauder will be next. And as it happens using Gimp to overlay the old and new aerial photography is the easiest way, then it loads straight into Qgis. So that is what will be happening there as well.

If you haven't picked it already, Gimp is an extremely good software package, both as a graphics editor and as an open source package. It's right up there with Qgis in my book, and trumps Pinta any day. Whilst I like Pinta as a lightweight alternative to Gimp in the vein of, which is a good free open source Windows graphics editor, it falls down on stability. Unfortunately some open source software tends to have those kind of problems, and it seems to be more prevalent on the Linux platform than on Windows. It hasn't taken all that long to gear up to using Gimp so it is worth the extra effort to get used what can be a somewhat daunting program because of all the whizz-bang things it can do. Inkscape is another brilliant package, which is what I would end up using if a customised version of the maps with specifically placed captions was ever produced, since all you would have to do is open each map (as a PDF) in Inkscape and then move things around to suit. When we save a map from Qgis composer as a PDF (much better result than SVG because of severe limitations in the Qt SVG library) we get a vector graphics output not a raster, which can have things done to it without affecting the quality in the way that would be a problem if it were a rasterised output.