We are pleased to announce a key milestone has been reached in the Greater Christchurch maps we are producing as part of the NZ Rail Maps project. We have now completed the historic aerial photo mosaic projects for all of Greater Christchurch, that is, the following four sections of railway:

The extraction step is a slow one because of the use of multiple generations of historical aerial photography in the mosaic projects – the same reason why it has taken so long to produce those mosaics in the first place. In addition, once the resulting tiles have been imported into the GIS, it is common for mistakes to be detected at this stage, requiring more work to rectify.

The historical research stage is also slow because of the number of files that need to be consulted. At this stage we have been researching up to 50 files in a typical week over the past 3 months, which in total has been in actuality 500 files to date. We estimate it will take another 6 weeks of work to complete this but there may not be more than 200 actual files left to look at.

After obtaining the various diagrams that we have copied as part of the historical research, some of these will need to be imported into map mosaics where they are the only source of information for a particular historical feature. An example is the pumping station siding at 2.81 miles on the MSL that was the subject of a recent post. Since this did not appear on an aerial photo, the copied drawing was added into the existing aerial photo mosaic and another set of tiles (only 2 in this case) dated 1924 were produced for the GIS in order to be able to draw the maps that accompanied the blog post. There is likely to be more of this needed, so the mosaics will have to be added to with the diagrams that have been copied. However, unlike the aerial photos that have been used in the projects to date, most of these diagrams only cover small areas and therefore don’t take very long to be added in. So that will not in actuality cause much additional delay, as most of the diagrams we have copied in actuality are just for labelling features that we can already see on aerial photos.

Then finally the maps get drawn and labelled. How much work is needed for each yard depends on how big the yard was. This in turn informs the complexity of the task. In the past we have gone to great lengths to document as much as possible historical changes visible on different generations of aerial photography. Our focus has changed with Retrolens because of being able to get the NZR yard and corridor surveys that are not available on our previous main source for the region, which was Canterbury Maps. This means we have changed focus into having really only one or two complete historical layers for a yard or section, which will be a yard survey if there was one, and a corridor survey if there was one that shows significant changes from the yard survey. There are a very small number of yards that have had two NZR surveys, and the corridor surveys are additional to these. In the case of a small number of yards there is no yard survey (Heathcote for example) and we are relying on other historical surveys that are available of the general area. For some stations these will be highway surveys if the yard was alongside State Highway 1 or another highway corridor.

Previously we would go to great lengths to try to document which parts of a yard in the modern era were based on historical features visible in the yard survey that might have been done 40 or 50 years ago. However as you can imagine this requires a lot of work, not the least because it is difficult to exactly align the historical aerial photos to modern ones, and because in the case of some yards (Lyttelton, Middleton and a few others that come to mind as examples) they have changed so much that there has been major track realignment. So right now the focus is on drawing every track in the historical layout as a “former” or “closed” track and then drawing the tracks again as “current” or “open” on the contemporary aerial background, and not bothering trying to work out which bits are common to both. Just that we will use the filtering capabilities built into the GIS to display the appropriate generation of track and structures etc with the aerial photo generation that is applicable.

As we have noted in previous posts the actual map drawing is the furtherest behind at the moment as we have focused most of our time in the last few months on the mosaics and research work. This has been changed this week with the completion of the mosaics and even though research is not completed, we are currently progressing as much map work as possible, with Lyttelton and Heathcote progressing well at present. There has just been a post about part of Heathcote and our next post will be about Lyttelton. However we still have to do historical research for Lyttelton so the maps posted in the next post will be necessarily limited until we can find out more historical information about the general layout of Lyttelton. On the other hand it is possible to push ahead with Heathcote to full completion because we have done most of the research on that yard although there is still a small amount, maybe one day’s worth, to be carried out.

So expect to see a lot more posts about specific yards and locations in Greater Christchurch in the coming weeks. Once that is done, then the overall project work will shift back to the rest of NZ as detailed in other posts in the last few months with the total project schedule. We can then expect to see the volumes produced for all of the other maps for the whole country.