Improved NZ Road Centrelines (August 2011)

Extensively updated, improved and value-added road center lines originally based on LINZ data. NZ Open GPS Maps nzopengps.org/ is an open source project so contributions and feedback are welcome at gwprojects.org/forum.

The maps are constantly being updated. If you want the very latest data you can follow the instructions on this page to make your own shape files: gwprojects.org/forum/viewtopic.php?f=3&t=348

A lot of the additional attributes to do with routing come through from the original data source in Polish Format (.mp). GPSMapEdit is the definitive program used to edit this format: www.geopainting.com/en/

Speed limit

• 0 = 5km/h
  
• 1 = 20km/h
  
• 2 = 40km/h
  
• 3 = 60km/h
  
• 4 = 80km/h
  
• 5 = 100km/h
  
• 6 = 110km/h
  
• 7 = no limit
  

Route Class

• 0 = Residential
  
• 1 = Collector
  
• 2 = Arterial
  
• 3 = Principal HW
  
• 4 = Major HW
  

The speed limit attribute does not refer to legal speed limits. It may be interpreted as an attempt at capturing the ‘actual’ speed a car would travel on a given road, but actually relates to the routing systems used by in-car GPS units so that trip-routing is optimised. For example, a road with speed bumps may be classified as having a speed attribute of 1 (20km/h) so that it is distinct from adjacent roads of category 2 or 3 (40km/h and 60km/h, respectively), even if these are not the legal speed limits for these sections of the road network.

Note that for the majority of residential streets in major cities, roads are classified as 2 (40km/h). If an accurate representation of speed is your intention (rather than solving computationally-simplified routing problems), interpreting 2 as being 50km/h can give a more realistic picture of the road network. An important exception to this is Wellington/Hutt Valley, where most roads are given the attribute 3 (60km/hr) where they are mostly 50km/h legally.

There are some legacy issues with the speed attribute, with many roads being populated with attributes en masse some time ago, and have not been updated since. There is also the possibility of inconsistencies in the assignment of speed limit attributes between regions for the same type of roads.

The roadclass attribute is similar to the speed limit attribute, but is intended for limited-performance GPS units that would not be able to determine the most appropriate route if considering the more realistic speed attributes. It might solve a routing problem with a marginally shorter route (avoiding, say, a highway) as the nominal speed would be the same, although intersections and other characteristics of the route would actually make it less favourable. A GPS unit using roadclass to solve a routing problem will attempt to find a ‘backbone’ of high-order routeclass, and only use lower-priority roads to get to and from this backbone. In a GIS, routeclass may be appropriate to use in order to solve a large number of network problems more quickly than using the speed attribute. Given that this attribute has fewer categories than the speed limit attribute, it also makes for excellent visualisation of the road network that could be used in map outputs even if the underlying attributes are neglected in favour of speed.