Monday, 22 June 2015

Aerial Photography of the Cromwell Gorge

In previous posts, here and here, I detailed my investigations into aerial photography of the Cromwell Gorge and the costs expected. Having found that the Crown Aerial Film Negative Collection would be very expensive to access, the option I am now working with is Archives New Zealand's collection of contact prints that were made from these negatives for Land & Survey's use. Obviously there has been a lot of doubt that these would be of sufficient quality to give a good scan. The negatives for the particular survey I chose were at 1:8000 scale, so 1 mm on the contact print (18x18 cm approx size) would equate to 8 metres on the ground.

Having received my scans today I am very pleased to report that even the low resolution scans are of sufficient quality to pick out the railway line quite clearly on the ground as well as various features. It is not up to the standard of some of the sharpest coverage that Canterbury Maps has of Christchurch, but it does provide enough to be able to pick out the individual tracks in a yard setting.

From here the big difficulty is to overlay the images accurately in GE given that the gorge has changed so much since the advent of the Clyde Dam. It is not only the raising of the two rivers to form Lake Dunstan, but also the extensive works done along the banks in stabilisation work that has changed the appearance of the landscape quite significantly. Getting the overlays in place correctly is essential in order to be able to trace out the various features and place them accurately onto a map.

I am very busy until Saturday so the weekend is probably the time when I will be exploring the whole business of the Cromwell Gorge and the stations along it in more detail. Because the aerials have given such a satisfactory result and have been extremely good value for money so far, I expect that in coming years I will be making extensive use of this Archive New Zealand aerial collection to aid the production of maps. The first priority is the Otago Central line and I expect to obtain aerial images from around the same era of every yard on the line in order to be able to accurately diagram them.