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NZ Populated Places - Points

Peter Scott
  • Added 16 Jun 2011
  • 4441 Views
  • 1912 Downloads

ps-places-metadata-v1.01

SUMMARY

This dataset comprises a pair of layers, (points and polys) which attempt to better locate "populated places" in NZ. Populated places are defined here as settled areas, either urban or rural where densitys of around 20 persons per hectare exist, and something is able to be seen from the air.

RATIONALE

The only liberally licensed placename dataset is currently LINZ geographic placenames, which has the following drawbacks:
- coordinates are not place centers but left most label on 260 series map
- the attributes are outdated

METHODOLOGY

This dataset necessarily involves cleaving the linz placenames set into two, those places that are poplulated, and those unpopulated. Work was carried out in four steps. First placenames were shortlisted according to the following criterion:
- all places that rated at least POPL in the linz geographic places layer, ie POPL, METR or TOWN or USAT were adopted.
- Then many additional points were added from a statnz meshblock density analysis.
- Finally remaining points were added from a check against linz residential polys, and zenbu poi clusters.

Spelling is broadly as per linz placenames, but there are differences for no particular reason. Instances of LINZ all upper case have been converted to sentance case. Some places not presently in the linz dataset are included in this set, usually new places, or those otherwise unnamed. They appear with no linz id, and are not authoritative, in some cases just wild guesses.

Density was derived from the 06 meshblock boundarys (level 2, geometry fixed), multipart conversion, merging in 06 usually resident MB population then using the formula pop/area*10000. An initial urban/rural threshold level of 0.6 persons per hectare was used.

Step two was to trace the approx extent of each populated place. The main purpose of this step was to determine the relative area of each place, and to create an intersection with meshblocks for population. Step 3 involved determining the political center of each place, broadly defined as the commercial center.

Tracing was carried out at 1:9000 for small places, and 1:18000 for large places using either bing or google satellite views. No attempt was made to relate to actual town 'boundarys'. For example large parks or raceways on the urban fringe were not generally included. Outlying industrial areas were included somewhat erratically depending on their connection to urban areas.

Step 3 involved determining the centers of each place. Points were overlaid over the following layers by way of a base reference:

a. original linz placenames
b. OSM nz-locations points layer
c. zenbu pois, latest set as of 5/4/11
d. zenbu AllSuburbsRegions dataset (a heavily hand modified) LINZ BDE extract derived dataset courtesy Zenbu.
e. LINZ road-centerlines, sealed and highway
f. LINZ residential areas,
g. LINZ building-locations and building footprints
h. Olivier and Co nz-urban-north and south

Therefore in practice, sources c and e, form the effective basis of the point coordinates in this dataset. Be aware that e, f and g are referenced to the LINZ topo data, while c and d are likely referenced to whatever roading dataset google possesses. As such minor discrepencys may occur when moving from one to the other.

Regardless of the above, this place centers dataset was created using the following criteria, in order of priority:

  • attempts to represent the present (2011) subjective 'center' of each place as defined by its commercial/retail center ie. mainstreets where they exist, any kind of central retail cluster, even a single shop in very small places.
  • the coordinate is almost always at the junction of two or more roads.
  • most of the time the coordinate is at or near the centroid of the poi cluster
  • failing any significant retail presence, the coordinate tends to be placed near the main road junction to the community.
  • when the above criteria fail to yield a definitive answer, the final criteria involves the centroids of:
    . the urban polygons
    . the clusters of building footprints/locations.

To be clear the coordinates are manually produced by eye without any kind of computation. As such the points are placed approximately perhaps plus or minus 10m, but given that the roads layers are not that flash, no attempt was made to actually snap the coordinates to the road junctions themselves.

The final step involved merging in population from SNZ meshblocks (merge+sum by location) of popl polys). Be aware that due to the inconsistent way that meshblocks are defined this will result in inaccurate populations, particular small places will collect population from their surrounding area. In any case the population will generally always overestimate by including meshblocks that just nicked the place poly. Also there are a couple of dozen cases of overlapping meshblocks between two place polys and these will double count. Which i have so far made no attempt to fix.

Merged in also tla and regions from SNZ shapes, a few of the original linz atrributes, and lastly grading the size of urban areas according to SNZ 'urban areas" criteria. Ie: class codes:

  1. Not used.
  2. main urban area 30K+
  3. secondary urban area 10k-30K
  4. minor urban area 1k-10k
  5. rural center 300-1K
  6. village -300

Note that while this terminology is shared with SNZ the actual places differ owing to different decisions being made about where one area ends an another starts, and what constiutes a suburb or satellite. I expect some discussion around this issue. For example i have included tinwald and washdyke as part of ashburton and timaru, but not richmond or waikawa as part of nelson and picton. Im open to discussion on these.

No attempt has or will likely ever be made to locate the entire LOC and SBRB data subsets. We will just have to wait for NZFS to release what is thought to be an authoritative set.

PROJECTION

Shapefiles are all nztm. Orig data from SNZ and LINZ was all sourced in nztm, via koordinates, or SNZ. Satellite tracings were in spherical mercator/wgs84 and converted to nztm by Qgis. Zenbu POIS were also similarly converted.

ATTRIBUTES

Shapefile: Points
id : integer unique to dataset
name : name of popl place, string
class : urban area size as above. integer
tcode : SNZ tla code, integer
rcode : SNZ region code, 1-16, integer
area : area of poly place features, integer in square meters.
pop : 2006 usually resident popluation, being the sum of meshblocks that intersect the place poly features. Integer
lid : linz geog places id
desc_code : linz geog places place type code

Shapefile: Polygons
gid : integer unique to dataset, shared by points and polys
name : name of popl place, string, where spelling conflicts occur points wins
area : place poly area, m2 Integer

LICENSE

Clarification about the minorly derived nature of LINZ and google data needs to be sought. But pending these copyright complications, the actual points data is essentially an original work, released as public domain. I retain no copyright, nor any responsibility for data accuracy, either as is, or regardless of any changes that are subsequently made to it.

Peter Scott 16/6/2011

v1.01 minor spelling and grammar edits 17/6/11

Information

CategoryTopographic
Tagsgeographic placenames
RegionsNew Zealand

Technical Details

Layer ID 3657
Data type Vector point
Feature count 929
Attributes gid, name, lid, desc_code, pop, rcode, tcode, region, tla, class
Services Vector Query API

History

Added16 Jun 2011
Revisions 1 - Browse all revisions
Current revision Imported on June 16, 2011 from Shapefile in NZGD2000 / New Zealand Transverse Mercator 2000.
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