Tuesday, 29 August 2017

Significant changes in maps unlikely as result of new aerial photography availability

In recent posts I have talked about Retrolens and their new aerial photography. One of my recent posts made the point that I don't expect the work done with the Nelson Section maps to be replicated across all the maps I am drawing.

Whilst the aerial photography has been very useful and I will be posting aerial images on the Flickr site shortly for the Otago Central Railway as well as the remaining ones I have obtained for Nelson, it is correct to say that I don't have time to get aerial photography for every single route I have drawn. Only the Linz aerial footage is being used at a base level to draw the maps, and that is because it is georeferenced, meaning it can be opened as a layer in the GIS and directly traced over. I can download the aerial images for a large section of country at once and simply open them, so it is straightforward enough to obtain and use.

Whilst I have been making significant use of aerial imagery both from Retrolens and earlier sourced scanned imagery from the Linz aerial photography contact prints collection held at Archives New Zealand, this is a time consuming and potentially costly exercise. The imagery isn't orthorectified or georeferenced, meaning Qgis can't simply load it as a layer in the way that for example Google Earth would be able to. Even if I was able to get the Qgis wizard to work in placing the images into a layer (which I haven't as yet), it would still be a time consuming process for each image in turn.

Because of this extra time and money cost to get imagery (even allowing for the fact that Retrolens content is free), it would be too slow to bring in the non-georeferenced content which requires extra steps that have to be taken for every single image. The Linz imagery has the georeferencing already done per tile (image) and loading it is all that has to be done to use it. Being orthorectified also means you don't have the time consuming process of shaping it to fit.

The Otago Central imagery I have which includes some quite sharp hi res images from the 1960s for some of the stations that I didn't have tiles of before, will be used to improve the diagrams I have already drawn for every station that was in the chainage charts. For example, the one of Omakau shows where the old turntable pit was, quite well. So it will be good for doing that. In this case it will mainly be copying rather than tracing. As I already said I won't be drawing any new track diagrams anywhere except where details can be directly traced in Qgis off the Linz imagery. That remains my present policy. Next week I'll be working on the SNL again.