As some will know, this week CHAT Club released their "MaRTI proposal", which suggests the Middleton rail yards could be turned into a housing development. A couple of posts about it have appeared on Christchurch Transport Blog, but the indepth analysis hasn't been done until the historical maps of Middleton are ready. As per usual this has taken longer than expected but right now they are about to be saved in their final form.
The Gimp project covering this part of the MSL takes in from Middleton to Sockburn in order to keep the project at a reasonable size but it still has nearly 80 layers and a file size of just over 30 GiB. As per usual we start with 1940 as the first era covered, then 1950 and 1961. These three eras in particular are pretty much what can be found to cover any part of the railway landscape in Christchurch, but they are not rail specific. They vary up to about 1:16,700 scale. What is rail specific for this particular part of the MSL rail corridor are the NZR station surveys, which are in particular for Middleton in 1967, Hornby in 1972, Middleton again in 1975 and Sockburn in 1980. The actual areas covered extend outside those named stations. Typically the station surveys have a scale of around 1:4300.
Then we have the MSL corridor survey starting at Lyttelton and heading to Ashburton in this case, and it dates from 1984. Scale for these corridor surveys is usually 1:5500. To finish off we have two more non-rail surveys which are again standardised over much of Christchurch, and they are from 1994, and the only colour one which is from 2000. These last two are at a scale of 1:50,000 and because they cover such a large area at that scale, they have to be trimmed down to covering only the base tiles that we actually use in order to avoid a lot of unnecessary pixels having to be saved in the file and blowing it up too big. To explain further, these are the only layers that cover the whole area of the project canvas in one single layer, and with a canvas that is only half filled, due to the shape of the base not being a nice and regular, we have to avoid covering areas of the canvas that aren't occupied by the base layers, because it just wastes file space unnecessarily and it can add up to a few extra GiB each time we save, which is a big deal.