Friday, 9 March 2018

Main North Line [10]: Aerial Photography

Most of the mapwork over the past few days has been with MNL aerial photography which has to be downloaded and then sorted through to pick up just the photos I want. This is a slow process because of the download limit of 3.5 GB per download from the Linz website and the number of tiles in each download being much greater than what is needed for the rail corridor. So there has been a lot of work to get full aerial coverage for this line.

The other thing that has to be done at the moment is to amalgamate aerial photos and remove the ones that are not actually used because it takes up a lot of disk space and I have had to reorganise the storage between between different computers. I am looking at writing a script to process the Qgis layer definition files that I am putting aerial photos into. I should learn how to script on Linux with Python but to speed things up, in this instance I will use Windows Powershell to process the QLR files and copy the photos listed in them to a specific folder path that the script will create. What happens after this is to use a Qgis library function to process the aerial images into something called a VRT (Virtual Raster Transform) which is supposed to make it easier to handle all the many aerial photos I am working with.

The aerial photo downloads are now more or less finished and just need a bit of extra little bits added on here and there to make them complete. I need to get on also and finish the Otago Central maps because I haven't done any work on them for the last week or two. I think I just did some mosaics of Auripo and haven't actually drawn any of the maps; nor have I worked on any of the other maps. However reorganising the resources on the computers has taken a bit of my time up lately.

I have been getting Retrolens coverage for the major deviations on the MNL as well as the current stuff and have got enough to map all of these accurately. Apart from Nonoti mentioned last time there were major ones done at Stewarts Gully, Ashley, Balcairn and Phoebe, and all of these will be addressed over further posts in the MNL series in coming weeks.