Otago Central Railway [61BB]: Alexandra-Cromwell 25: Cromwell Gorge Survey

Last time we wrote about the Cromwell Gorge section that we have been remapping. We chose to redo the aerial mosaics down this section from scratch for various reasons, but a part of that has been the discovery that the original placement of the historical photos in some places was most probably incorrect in that the alignment for the old railway was too far into the present hydro lake.

For various reasons it is extremely difficult to overlay historical aerial photos in steeply graded terrain like this gorge which has steep sides and that is primarily because of perspective distortion with terrain which is not directly below the camera lens and that is a key reason why the process called orthorectification is used by Linz and others to get aerial photos that are geometrically corrected for using in map work. We don't have that option with the old maps, so the placement is a lot of guesswork. But we have found some historical aerial photos taken between 1980 and 1992 through the Gorge and they are confirming some errors in some of the original aerial layers that were produced. A part of the issue would be with the lower resolution base layers that were used at the beginning.

Right now we are just checking with aerial photos from the late 1980s through the area to confirm the alignment and make a few readjustments where necessary to ensure the result is as good as it possibly can be. This will mean a couple more days work before having something to redo the mapping with. We did get a 1991 survey scanned for evaluation back in 2017 but didn't draw too many conclusions from it. But having reasonably good coverage of a few areas helps a lot.

Ultimately the only real way would be to have access to original survey plans or drawings that plotted the new highway route in the Gorge compared with the existing route. Maybe these exist in Archives somewhere. The issue for our maps isn't that a particular aerial photo is uncertain as to its longitudinal placement along a particular part of the Cromwell Gorge; it is mainly the question of how to align things across the width of the Gorge with limited referential viewpoints and various distortions.

We had hoped to progress this work further this week but various time commitments have prevented that. We have also been working on maps of Christchurch City this week.

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.

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.

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.

MSL Timaru-Glenavy[1]: Normanby-Pareora Realignment

Between the locations of 179.5 km and 181.5 km on the Main South Line between the stations of Normanby and Pareora there has been a realignment of the rail corridor. The corridor used to be about 50 metres closer to the beach than it is now. We are indebted to the late Colin Kemp, a train driver for Kiwirail, for bringing this to our attention.

From aerial photo data we estimate the realignment was carried out around the year 1980. No records with titles specifically referencing it have been found in Archives New Zealand's Archway portal to date.

 Overview of the area with the historical (1942) aerial imagery background.

 The same view with modern day aerial imagery.

Closeup at bridge 97.

Historical aerial imagery at Bridge 97. This appears to show a location of a previous bridge at the site of the current bridge, that was not actually in use in 1942.

A big question to be asked is why it was necessary along this mile-long section to go so close to the seashore in the first place. As far as we can discover, everywhere else along that particular coastline, the railway is at least 50 metres inland and we have not seen anything to suggest other sections of the railway in this area were historically located as close to the seashore as this part of the MSL was.

MSL Timaru-Glenavy [0A]: Intro 1

For our series of articles on the Main South Line we will break the corridor up into sections and these sections are chosen mostly on prominent stations but also in this case a geographic boundary. Glenavy is on the north bank of the Waitaki river which is the historical boundary between the provinces of Otago and Canterbury. The actual boundary nowadays for the Canterbury region is just south of the Waitaki and takes in all of the Upper Waitaki lakes as it is based on river catchment and the Waitaki is considered a Canterbury river.

From Timaru to Glenavy, 60 km, the only settlement of any substantial size is Timaru itself. There is one branch junction at Studholme for the line to Waimate that closed in 1966.  This had an extension into the Waimate Gorge that closed in 1953.

For all of the maps of the Main South Line there are two types of mosaic being produced:

• Where 0.2 metre  - 0.075 metre pixel size base (Linz) imagery is available then producing tiles without any resizing of the base layer.
• Where 0.3 metre or 0.4 metre pixel size base imagery is available then resizing this to double the original resolution (i.e. 0.15 - 0.2 metre). To avoid the need for segmented grids splitting each resized base layer into four tiles at the original physical size I am hoping turning off render caching in Qgis will prevent it from running out of resources so quickly as has been a problem in the past with larger size tiles.

In all cases we have for the historical imagery a choice of 1:4300 or 1:4325 NZR station surveys, 1:5500 NZR corridor surveys and 1:8000 highway surveys that are worth doing mosaics from as there is very good visibility of detail on the ground in most cases. We can use some older surveys at 1:10000 or smaller scales up to 1:25000 in a few cases but they are less desirable.

We are currently getting all of the background imagery we need of the MSL heading southwards and will get to Dunedin in a few days and then carry on towards Bluff gathering all the Retrolens coverage along the way and making mosaics and tiles all along the route. We expect to resume drawing the Dunedin maps when we get everything finished as it is a high priority to recover disk space from deleting all of the downloaded stuff that isn't needed.

Otago Central Railway [47J]: Current delay in OCR maps

The current effort on NZ Rail Maps to map the Otago Central Line started in November of 2017 - about 18 months ago. It was at that time with the first of the posts in series 47 that we started the widespread use of the LINZ background imagery and bringing in historical imagery with the mosaic projects. We had at that time hoped to complete the set of maps for the entire line by a series of deadlines which eventually stretched out to the end of 2018.

Since the beginning of this year we decided to temporarily set the Otago Central line aside and work on other projects. One of the reasons for this is that LGGA are still scanning the aerial images for that area and adding new stuff all the time. Rather than doing an area then finding there is new stuff and going back to add it is a good enough reason for deferring the completion of work on maps for railways in Otago.

We just recently discovered LGGA have found survey 1452 which is State Highway 8 that we have used for the Cromwell Gorge has been added so we are currently poring over that to see if their scans are better quality than what we already have. Another example is patiently waiting until finally the one and only good quality scan of Ida Valley became available which was late last year.

We don't quite know when Otago Central will be completed but it is still very much under consideration and being worked on behind the scenes from time to time.