Of late we have been working on some of the rail corridors around Christchurch by amalgamating Gimp projects covering single station sites into projects that cover a whole section of corridor. This includes both the MSL and the MNL. The first one for the MNL covers from Addington to Ashley, which except for the area covering Flaxton station, which appears to be a tiny flag stop between Eyreton and Southbrook, is continuously in 0.075 metre imagery from several different layer groups.
Since we have already downloaded imagery for nearly the whole MNL, we are pushing ahead to get the entire MNL corridor organised in my computer. It is important to do this because we have so many downloaded aerial layers that are consuming massive amounts of disk space on the computers and need this space to store the mosaics that are going to be created for the project. As an example we have currently just over 200 GB of downloads for the MNL and could expect that further mosaics for this corridor might use 100 GB, giving a net gain of perhaps 100 GB.
We need to do this work for every of the 12 volumes of the maps project but few of them have as much aerial imagery downloaded as for the MNL which has probably got a lot of duplications, but also it is a long corridor and therefore will have a lot of basemaps.
After Ashley, the little station on the north bank of the Ashley River 33 km north of Christchurch, the next section of mosaics would be at 0.3 metres and take us from Sefton through to Balcairn. We skip Amberley and Waipara, which are in 0.125 metre coverage from Hurunui District Council and will be in their own project, and carry on with stations north of Waipara and I haven’t really looked at what there is good coverage of. We are not actually drawing maps but we are laying groundwork to be able to draw the base maps at a future time and the mosaics after that. Since there weren’t that many major stations on the MNL we are not yet sure what aerial coverage will exist. Addington-Ashley covers really the old suburban passenger network north of Christchurch. Amberley and Waipara have been worked on recently as a separate effort, and north of there we started looking at a few places.
Some may recall there was an earlier aerial mapping effort that produced images of Picton, amongst other areas. That is indeed the case. It was carried out using similar principles but the technique was to overlay the historical aerial images into Google Earth and then trace over them, and then import the traces into Qgis overlaying them on current Linz basemaps for the produced output. This effort has been overturned because of licensing concerns with Google’s maps system which we definitely want to avoid. We published a series of articles on Picton in October 2017. We did our first map project in Gimp, which was for Alexandra on the Otago Central Line, in March 2018. Since then we have fully embraced this combination of Linz basemaps with Retrolens overlays produced in Gimp and imported directly to Qgis for the tracing and it can produce historical map outputs as well. This means Picton will be redone in Gimp for a new set of maps sometime.
This need of completing the base maps of a corridor being urgent has caused work to be suspended on the MSL maps for the time being but we expect to be drawing some of them again this weekend even as we continue to finish off the MNL corridor, the only reason we haven’t been doing any this week is because of other work priorities.