Saturday 8 December 2018

Otago Central Railway [61Z]: Alexandra-Cromwell 23 - 2nd Clyde Station 2

Last time I posted the 1972/77 aerial photos of Clyde station had some problems that needed to be fixed. It looks like the aerial images of a 1977 survey were significantly distorted, perhaps the plane that took the photos was not level. Anyone who has used a data projector knows about keystone adjustments you have to make because the lens is not exactly square on to the screen and the shape of the picture is distorted. The same thing will happen when an aerial survey is made and the plane is not level either laterally or longitudinally or maybe the camera is not mounted properly in the plane. Aerial photography generally uses a special plane with a camera mounted in the bottom of it looking straight down. There are other distortions that are caused by varying terrain height and on the modern aerial photography we get from Linz they run it through a computer program called orthorectification to mathematically correct these issues caused by different ground level heights, however the Retrolens photos are just straight scans of the original negative and not orthorectified in any way so especially on hilly terrain you will see such variations. But on what is a fairly flat part of Clyde over 1 mile there wouldn't be expected to be the amount of variation seen and which wasn't seen on the other generations of aerial photos. The result was something like 30 metres displacement in one particular area which is a lot. It took all day yesterday to find a workable solution with a different image of most of the area to the one that was first used.
Here are the two images for 1972/77. The first one is the original, and the second one is the corrected one.

Now that the 1972 series has been fixed, I found that the rail corridor on the 1962 series does not line up with 1972 series, so that also has to be adjusted.