This report as always with these reports is a general overview of the project as a whole across all volumes. Our earlier report 2020A was written on the first day of the year and outlined our expected programme for map development across the whole project this year.
At present with the development of the maps across all volumes, the following sequence is proposed at this time, but could change later:
- Volume 5 (PNGL) – on track to be completed by the end of January
- Volume 6 (WL) – February
- Volume 4 (MNPL) – March
- Volume 3 (ECMT) – Apri
- Volume 1 (NAL) – May
- Volume 2 (NIMT) – June
This means we have priorisitised the 6 volumes of the North Island for the first half of the year, leaving the 6 volumes of the South Island for the second half of the year. Most of these volumes are already completed to some extent from previous work. The largest amount of work is likely to be needed in the two longest corridors apart from PNGL – these are ECMT and NIMT of course. NIMT is one of the more challenging ones because of the length of the corridor and the fact it runs between two of our largest cities.
We now turn to a look at some evolving changes in the way certain technical aspects of the maps themselves have been developed. What is becoming more commonplace in the maps in general in the last year or two is widespread use of aerial photography to provide references for historical and contemporary content. How that aerial photography is used to provide finished maps has varied to date. Now that we are moving towards completed volumes for all areas, this is very relevant.
Map data in a GIS is handled in layers. Each layer manages a particular type of object and generally for convenience we can further divide the types of data into our own personal classifications. Here are some examples:
- Locations (stations, distance pegs etc)
- Bridges and Tunnels
- Features (such as premises that contain sidings)
- Main Line Corridor(s)
- Branch Line Corridor(s)
- Yards and Sidings
- Aerial Photos
All of the map volume GIS projects follow that general style.
The maps are presented in two overall types. Diagram maps are drawn against a terrain background. The terrain is rendered greyscale relief imagery that shows the type of terrain for the area (i.e. flat or hills). It does not have a significant visual impact upon the finished maps as most of the grey shades are less than 50% density.
Aerial maps are drawn against an aerial photography background. With this type of background there are many more challenges mostly in making data layers stand out against a background that is much more dense and often quite dark. There is also the issue of the data layers obscuring visible content in the aerial photography itself. Another issue is the alignment of some of the general (non rail) data layers such as roads.
Because of these issues with aerial photography in particular, the work on Volume 5 has begun to classify layers as being diagram specific. This means when we produce a map of one particular area, such as one station, a diagram map will show much more data than an aerial map. This means the aerial map will be visually a lot less cluttered and more of the aerial background will be visible.
Another important aspect specifically relating to diagrams has been the production of diagrams that cover more than one historical era. We developed a system a year or two back using date range fields to enable us to determine where possible when a particular object became visible and when it became invisible, driven by aerial photography reference. By applying filter formulas, we could cause these objects to appear and disappear on the resulting maps. The main drawback however is the amount of work needed to create these different historical eras. In major yards a lot of changes can take place over a long period.
It has been decided that most diagrams going forward will be composites that show multiple eras in one diagram, with labelling to clarify certain aspects. Eras will be solely represented by producing an aerial map for each era, but as the map information on an aerial map is not the full range of data visible on a diagram map, much of the information that is era specific will not be marked on the aerial map itself. Instead, a reader will use the diagram map to cross reference specific features which are visible on an aerial map.
A small change has also occurred where aerial photo tiles are partly filled by empty canvas in the graphics editing software that produces (mainly) historical tiles, but also is used to remove some black edges that we get in the tiles we download from Linz. For some time we have filled these gaps with current aerial photos. However, to avoid confusing different eras of aerial photography, starting with Volume 5 we will fill these gaps with a white background.
We are also moving towards producing combined map sets that incorporate all aerial and diagram maps together in sequence. This is in addition to producing separate aerial and diagram sets, which has been the norm up until now.