Saturday, 18 November 2017

Otago Central Railway [47I]: CG maps fix

Well today I had hoped to get the revised maps of Cromwell Gorge out. Unfortunately a mistake was found with the positioning of one of the aerial photos, which you can see reflected in the wrong position of the riverbed (light blue) as seen in the lower left of the image below.

Fixing this has been a non trivial task involving repositioning and realigning three or four different aerial photos (not just the one that covers the railway yard), because as soon as you move one, you have to move the others to keep them aligning at the edges, and then you run into problems ensuring they all keep aligning at the edges. If you have ever seen in Google Earth where features are misaligned at the edges of different image runs, you can understand what the issue is. This is why orthorectified aerial imagery like the stuff Linz supplies for free download is so valuable, because the orthorectification overcomes those alignment problems by correcting the distortions in the aerial photos that cause the alignment issues in the first place. But working from historical photos that haven't been corrected, I just have to make them fit somehow. Fortunately I have had enough of them scanned that there are sufficient overlaps in many cases that a bit of stretching and distorting of an image or a part of it can be done without it being too obvious.

Because of the lost time fixing the aerial imagery, and then the time I will need to realign the railway yard map and the riverbed and the Cromwell township street map, I won't have the CG maps ready before tomorrow. 

This aerial image fixes the positioning errors, but I still have to redraw everything I traced off it to finish the maps. Hence the riverbed and the railway line / yard etc are all in the wrong place.

I have pretty much decided that the maps will be drawn as two sets, as now: the map tiles, which will look somewhat like the image shown at the top, and the overlay tiles, which will look more or less like the image shown above.