Friday, 24 May 2019

Otago Central Railway [61AA]: Alexandra-Cromwell 24: Cromwell Gorge Survey

So here we have the first post on the Otago Central Railway for 2019. Last week I started redoing the Cromwell Gorge maps using the same 1962 historical survey images as previously, except this time sourced from Retrolens. They are a slightly higher quality, but the main change is harmonising everything with the other surveys by using the 0.4 metre tiles instead of the old 0.75 metres and also using the 1972 images of both Clyde and Cromwell stations which helped to fix a few issues. I have now finished off the survey and am about to produce the map tiles for use in Qgis. As a result some of the already concluded and drawn maps will need to be altered slightly as I have reached some slightly different conclusions about the position of the railway in relation to the hydro lake.

These images of Cromwell were produced last week and shared on Facebook.


Apart from the new mosaics of the section through the Gorge I am also putting all the station layouts for Omakau-Cromwell together into another Gimp project and the Cromwell and Clyde station maps will be tied into the CG maps so that everything joins together properly.

Friday, 17 May 2019

Project Diary 2019-05-17

We try to post at least once a week depending on what we are doing in this project. Most of the posts are about specific volumes or regions we are working on. Other times there will be a progress report for the whole project like there was last week. This week it will just be a diary entry because whilst we have been doing a lot of map work this week, not a lot of it has related specifically to NZRM.

This week the majority of our time has been taken up with getting a complete set of maps for Christchurch City. This meant downloading a lot of stuff from Land Information New Zealand, because they have a 3.5 GB limit for the estimated size of each download. As it happens, it is more like 5 GB actual download in a lot of cases. But it meant that at a pixel spacing of 0.075 metres, there are a LOT of layers for the whole city. It works out to something like 60 GB. If you have a few dollars you can get them to courier the data layers to you. With the fact we don't have any spare cash to spend, we just settled for splitting up the city layers into a series of downloads to get all of it. There are some overlaps which means we don't quite have the expected efficiency and because we inadvertently got some gaps between downloads we had to make some extra downloads to be able to fill in those gaps.

So we now have Christchurch City covered for 2018 (central only), 2015 (whole city), 2011 just after the quake (central only) and c.2010 (actually between 2004-2010 and for some reason the dates aren't more precise) which again is the whole city. We also have some historical retrolens layers to overlay at some point. We did consider downloading all the 2011 imagery and in fact did set up a series of downloads in Linz but haven't actually executed those downloads because that imagery is at 0.1 metre pixel spacing and while it is very good it is also just after the quakes so it does have limits. But it is a shame that there isn't high resolution imagery before then that can be downloaded. Even 0.3 metres would be better than the 0.75 metres 2004-2010 stuff.

As far as the NZRM project actually goes the work this week has been on the Otago Central line. First up is to redo the Cromwell Gorge section with the Retrolens scans of survey 1452 which are distinctly sharper than the original ones we paid for a few years ago, and then particular stations. At the moment Omakau-Cromwell is being considered as one Gimp project covering a number of stations. It is separate from the Cromwell Gorge mosaics because of differing resolutions. The Cromwell Gorge part was originally done at 0.75 metre base imagery because that was all that was available for the whole section, but in 2017 the full area became available at 0.4 metres. But initially all that we did was scale the 0.4 metres to 0.75 metres and overlay it with the 0.75 metre stuff. Now we have scaled the 0.75 metre base to 0.4 metres so as to be able to use this scale base imagery for the Gorge section.

Monday, 13 May 2019

Project Development Report [2019I]

This week we have continued working on downloading Linz base imagery and Retrolens historical imagery for the entire MSL corridor. We currently have the base imagery down to about Tumai (just south of Moeraki) and Retrolens down to Pareora, and progress has been a little slow this week, because we took some time on the following subprojects:
  • testing the replacement for grid segmenting for most parts of the maps project, which is to scale 0.3 or 0.4 metre imagery to 0.15/0.2 metre size and use doubled dimension tiles in Qgis.
  • writing a script to create and copy the sidecar files for the doubled dimension jpeg tiles automatically to save on a lot of manual steps especially with multiple generations of historical images that all need these files duplicated and having to change the pixel size in the world files to match the larger mosaic layer sizes.
  • converting the Dunedin-Burnside part of the Dunedin-Mosgiel mosaic tiles Gimp project from a sectioned canvas format to a linear canvas format. 
The Dunedin-Burnside Gimp project is something we have had to work on this week because of wishing to extract the layers for Hillside Workshops, an earlier version of which are displayed at the bottom of this post. Being able to convert to a linear canvas means being able to remove a lot of overlapping layers between the canvas sections. As all of the base layers are always displayed, it speeds up selecting layers for display as well. Since extracting these views of Hillside, we have fixed a few issues with the overlapping of different layers so that one place in particular where quite a rough cut is visible between two layer going across the middle of the workshops site, this has now been made much smoother.

As well as the southern part of Dunedin going out as far as we can with the 0.125 metre resolution urban imagery until we have to switch to the 0.3 metre or 0.4 metre stuff that covers probably the rest of the way to Invercargill except for probably a small number of townships that will have something of higher resolution, we also have to go north to the limits of the Dunedin coverage in that direction so we are working to get that sorted today.

Last week we also downloaded 0.075 metre coverage of Timaru City and will have a look to see what is available going further south. Retrolens coverage downloaded is currently down to Pareora, and rural base imagery down to Tumai. Finishing all the downloads for the MSL corridor and retrieving the 100 GB or so of disk space used remains a high priority so that is one reason why the work seems slow at the moment.

 Hillside Workshops as seen in 1972. As mentioned above a rough overlap can be seen between two layers vertically in the left hand third of the image, this has been correct in current aerial mosaics.
The same Hillside site seen in 2013, which is the latest aerial imagery of Dunedin at high resolution. Now a bit out of date with further changes at Hillside Shops since it closed a few years ago.