For our series of articles on the Main South Line we will break the corridor up into sections and these sections are chosen mostly on prominent stations but also in this case a geographic boundary. Glenavy is on the north bank of the Waitaki river which is the historical boundary between the provinces of Otago and Canterbury. The actual boundary nowadays for the Canterbury region is just south of the Waitaki and takes in all of the Upper Waitaki lakes as it is based on river catchment and the Waitaki is considered a Canterbury river.
From Timaru to Glenavy, 60 km, the only settlement of any substantial size is Timaru itself. There is one branch junction at Studholme for the line to Waimate that closed in 1966. This had an extension into the Waimate Gorge that closed in 1953.
For all of the maps of the Main South Line there are two types of mosaic being produced:
- Where 0.2 metre - 0.075 metre pixel size base (Linz) imagery is available then producing tiles without any resizing of the base layer.
- Where 0.3 metre or 0.4 metre pixel size base imagery is available then resizing this to double the original resolution (i.e. 0.15 - 0.2 metre). To avoid the need for segmented grids splitting each resized base layer into four tiles at the original physical size I am hoping turning off render caching in Qgis will prevent it from running out of resources so quickly as has been a problem in the past with larger size tiles.
In all cases we have for the historical imagery a choice of 1:4300 or 1:4325 NZR station surveys, 1:5500 NZR corridor surveys and 1:8000 highway surveys that are worth doing mosaics from as there is very good visibility of detail on the ground in most cases. We can use some older surveys at 1:10000 or smaller scales up to 1:25000 in a few cases but they are less desirable.
We are currently getting all of the background imagery we need of the MSL heading southwards and will get to Dunedin in a few days and then carry on towards Bluff gathering all the Retrolens coverage along the way and making mosaics and tiles all along the route. We expect to resume drawing the Dunedin maps when we get everything finished as it is a high priority to recover disk space from deleting all of the downloaded stuff that isn't needed.