Sunday, 22 April 2018

Project Development: Aerial Photography Selection

Progress on the OCR maps has been slow this week because of other work. As far as the maps go, work on the Main South Line maps around Dunedin has been a greater priority. I had an enquiry for the maps around Dunedin Locomotive Depot and it was decided at the same time to download the full section of the 0.125 metre LDS coverage for the suburban area of Dunedin. This covers the MSL from about Owhiro at the south end, to as far north as Warrington at the north end, but inexplicably omits around 4 km in the Mihiwaka area. 

As usual after assembling together the areas covered by several different downloads totalling about 12 GB, selecting the images actually needed using Qgis has been the time consuming aspect. This time around instead of manually copying the files, which has always been a problem because some are always missed, I spent a bit of time extracting the filenames and running them through a simple PowerShell script to copy them to a new folder. Although there are several manual steps needed to get the list of filenames, with more work I hope to get to a stage of automating more of the process starting from saving the list of files as a layer definition file (*.qlr) as there is a lot of aerial photography I need for various aspects of the overall maps project and the selection of just the files needed for rail corridors will save a lot of disk space on MainPC where the files are stored for use in the maps. 

The process for the files for Dunedin is a typical example as the selection and copying process reduced this from nearly 1200 individual rasters down to about 250 saving about 10 GB of disk space on this small area. Whilst most of the areas being covered will not be at such a high resolution, it is to be appreciated that there is a great deal of aerial photography now required for the entire country and this does use an appreciable amount of disk space on the computers so anything which can be done to reduce this disk usage is very valuable.

Whilst I have spent a lot of time optimising the disk usage for this project, some of the other projects also need this work and after Otago-Southland project there is also the CWNM project for the rest of the South Island that I worked on recently with the Addington coverage. There is a large volume for the Main North Line that currently totals some 5000 individual rasters occupying 53 GB of disk space. Another example is the coverage needed for the Napier-Gisborne Line, a future aerial project example I have done some preliminary work on where I have obtained Retrolens aerials for many stations and LDS coverage for Napier-Waikokopu which is around 1000 layers gross and needs to be graded down to just the corridor which will be much less hopefully. As time goes on naturally there will be further usage around the country which will keep things happening.

I have also recently had an enquiry about the coverage of parts of Southland and I think there will not be a lot of the area covered. Once the Central line maps have been completed the other section required for Volume 12 is the Kingston Branch and it is natural I will want to look at maps of this line because I would want to proceed to assemble the complete volume for the PDF. 

The Kingston Branch started originally at Invercargill and ran to Kingston over a total length of 140 km. There is good Retrolens aerial coverage of only a few stations. It is possible I will do mosaics for Invercargill, Makarewa Junction, Winton Junction and Lumsden Junction, and as this line also has a few branches then there is the possibility of adding in Mararoa and Mossburn from the Mossburn Branch, Gore Junction from the Waimea Plains Railway, Thornbury Junction  and Tuatapere from the Tuatapere Branch and Wairio from the Wairio Branch. 

Since I have been asked about Southland in general the other areas for mosaics (subject to availability) could include Bluff, McNab Junction and Edendale Junction. These fall under the MSL which is Volume 11.

After all that, I will carry on with the Otago Central in the coming week with the Gimp editing of the coverage for Alexandra to Cromwell being the high priority so that the maps for that section can be generated, and as previously mentioned I just need to add in some detail of Alexandra itself. There are not any new aerial images presently available around Alexandra. It seems the scanning of the Otago region is very slow as they have not yet added the survey that I used of the Cromwell Gorge, although other surveys are available.