This month the blog is starting a series of posts on the history of Christchurch as seen in aerial photos. The Department of Lands and Survey, commonly known as L&S, which was the old government department responsible for maps, and which is now called Land Information New Zealand (Linz), and its predecessors began using orthogonal aerial photos in NZ in the mid 1930s commissioning a company called New Zealand Aerial Mapping (NZAM) based in Hastings. The company also maintained the Crown archive of these aerial images in climate controlled conditions and made them available for sale to interested parties on a commercial basis. NZAM went out of business in 2014 and since then the Crown aerial archive (Crown Aerial Film Archive) has been managed by Opus. It remains very expensive to access individual images from the archive on a commercial basis.
Due to a mapping hobby project I am involved in, I discovered contact prints of a lot of aerial photos from the archive are held in Archives New Zealand’s collections in Wellington and began to get scans of them for various areas of New Zealand that I was mapping the history of. At the time I began doing this in 2015, I was mapping parts of Central Otago, particularly in the Cromwell Gorge, which is a highly modified landscape these days due to the hydro development of the 1970s/1980s. This was much cheaper than accessing originals from Opus although quality varied somewhat. In 2017, I was notified by Linz of the new Retrolens site which came online that year. Linz began scanning all the images in the CAFA and supplied them to the Retrolens site for publication there, from which individual images can be freely downloaded. In the last five years I estimate I have downloaded thousands of images from the site. Although they are saved as JPEG, which means lossy compression, the quality is still extremely good and highly suited to my needs. Over time, the vast majority of CAFA images have now been added to Retrolens.
For the purposes of this article series I will be using a custom made set of georeferenced aerial tiles produced by mosaicing the historical images of each generation (e.g. 1940s, 1950s, etc) over a current set of tiles of Christchurch at 0.3 metre resolution taken in 2015. It takes a lot of work to make these mosaics so that is one of the reasons that articles will be produced about once a month. Whilst many people will be familiar with the Canterbury Maps website and consider it is basically doing the same thing as what I am doing, the CM website does not allow its historical layers to be downloaded for use in a GIS and therefore their usefulness is limited for drawing actual maps, because the ones you see in posts in this blog will be actual maps drawn in a GIS with the historical aerial mosaics as the background.
There are other aerial archives available out there but the majority of them are obliques (taken on an angle, mainly out of the window of a plane) and whilst they do provide useful detail that can be filled in on real maps, they are of no actual use for tracing from as they can’t be accurately georeferenced. Examples are the V C Browne collection and the Whites Aviation collection; the latter is held by National Library’s Alexander Turnbull collections, and the former remains in private ownership. Publicly available orthogonal aerial photography is mostly sourced from Linz. This is generally of high quality and accuracy as it is run through computer software to achieve a process called orthorectification, which corrects for various forms of distortion that are commonly found in aerial photography (or in fact any type of photography). When downloaded from the Linz website, archives of aerial photo layers are generally divided into tiles which are georeferenced according to a CRS (coordinate reference system). CRSs are created for all sorts of different purposes, often by a national agency in a particular country; for example there are a few different ones produced specifically for NZ, by Linz. I normally use the EPSG:3857 CRS, which is an international one required for web map tiling, which I also use in my map hobby projects. The tiles downloaded in this CRS come in multiples of 4800×7200 pixels, usually in full colour, whereas most of the historical ones from Retrolens, which are not georeferenced, are in black and white and are various sizes. Linz aerials also vary in resolution; the highest ones are now 0.05 metres pixel size (one pixel covering 5 cm on the ground) and they go all the way up, the smallest resolution ones I use regularly are 0.75 metres (i.e. a pixel width of 75 cm) but there are some which are smaller than this available.
The maps I am producing for these articles are generally being georeferenced using a grid of about 400 tiles at 0.3m resolution that covers what would more or less be recognised as Christchurch in 1990 (after the local government amalgamations of 1989). The actual boundaries of the city today are further south due to the amalgamation of Christchurch City Council and Banks Peninsula District Council in 2006, and although my map includes Lyttelton, this was in fact part of BPDC in 1990, so the boundaries are somewhat approximate. It is more or less correct in the northern, eastern and western boundaries however, subject to the edges of tiles following the tile’s normal physical boundaries rather than the actual geographical boundary. The choice of 0.3m resolution imagery is a practical compromise as the canvas to display the tiles occupies 25,000,000,000 pixels (i.e. 25 gigapixels). Since each pixel can be any of 24 million colours and need therefore at least 3 bytes for storage, it follows that the computer needs at least 75 GB of storage to load this canvas into memory and perhaps the same amount to store the canvas onto disk (if uncompressed). So a lot of computer storage resources are needed just at 0.3 metre resolution.
If I had used 0.075 metre imagery I would have needed 16x the memory and disk storage to handle this canvas, and that is only with the background, let alone any additional layers that could be added to the map. It so happens that for my map hobby project I have used 0.075 metre resolution aerial imagery of just some parts of Christchurch, which has been an intensive effort involving a lot of computer resources. That can only work because very small areas have been mapped (all the railway stations and yards across the city) and they are still using up a lot of storage. Around two years ago I first took a look at historical aerial photos on this blog with the first post of a series on the northwest of Christchurch where I grew up. This series has not progressed, but I now intend to incorporate its proposed content into this series of articles. The first post in HCIAP series will appear next week, as it will cover some of the railway detail that I have already mapped historically, without having to wait for the maps of the rest of the city to be ready.