Monday 17 December 2018

Palmerston North Gisborne Line [0C]: PNGL maps update project [3]

The most complex task of assembling a collection of aerial photography to support a NZ Rail Maps project is almost complete. This one is extremely complex because it has been necessary to access multiple generations of base aerial photography for the same location, simply because after six years of closure much of the current aerials make it difficult to pick out much detail on the ground in places. 

As well, there is higher resolution coverage of Napier, Gisborne and a few smaller locations along the way. Having discovered the 0.15 metre coverage of Gisborne in particular means I have to redo the mosaics for Gisborne City with the larger scale stuff, but it will be worth it to be able to pick out and display more detail on the ground, because these aerials also date from when the line was open.

What makes getting the Linz base aerial photography together tedious is their limit of 3.5 GB for a free download. The workaround is to pay hundreds of dollars to have the stuff shipped out on a pen drive. So of course I have to split each source layer into multiple downloads of around 3.5 GB (sometimes they are actually larger, as much as 1 GB greater) and then reassemble them. And because I can only draw a rectangle on the screen to select the area I want, I then have to go through the downloaded tiles and select only the ones I need to cover the railway corridor. This is done in part using a script that reads a layer list and then copies just the files needed.

So all those tasks added together makes a lot of work to get what we need together, for just 200 km of track length. So far I have only done the full base aerial imagery for a small number of volumes. The aerial photos and mosaics do take up a lot of disk space on my computer and both are due for a cleanout to free up some disk space.