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Wednesday, 26 September 2012

Mapping New Zealand: Editing and merging shapefiles with QGIS

The data you want to map is not always in the format you need. I was looking for a dataset containing polygons for all land areas (islands) in New Zealand (1:500,000 scale), but I couldn’t find it on the LINZ Data Portal. So I tried to create it myself from the data I found:

Update Oct. 8th 2012: LINZ has added new polygon datasets for all land areas in New Zealand (see comments below), so you don't have to do the job described in this blog post. The techniques for editing and merging shapefiles are still valid.

NZ coastlines (Topo, 1:500k)
This dataset (nz-coastlines-topo-1500k.shp) contains coastlines for North Island, South Island and Stewart Island, - but not for offshore islands (most notably the Chatham Islands) and the smaller islands around the coast. The coastlines are represented as lines, while I need the data as island polygons. This is how I converted the lines into polygons with QGIS:

Vector -> Geometry Tools -> Lines to polygons


I named the new dataset "nz-main-island-polygons.shp".

(I wasn't able to do the same with the 1:250,000 version of the coastlines dataset, as the coastlines are divided into multiple line segments that didn’t merge in the correct order. Please let me know if anyone are able to convert this dataset into polygons! At 1:50,000 scale, LINZ is providing a dataset containing polygons for all islands.)

I’ve not found data for the offshore islands at 1:500,000 scale, but the smaller islands around the coast are available:

NZ island polygons (Topo, 1:500k)
This dataset (nz-island-polygons-topo-1.shp) also contains inland islands (islands in lakes), which I don’t want. I made a copy of this dataset named "nz-coast-island-polygons.shp" and removed the inland islands manually with "Select features by freehand" in QGIS.


I now want to merge the two shapefiles into one. This can easily be achieved in QGIS:

Vector -> Data Management Tools -> Merge shapefiles to one


We now got a polygon shapefile (nz-land-polygons.shp) containing all land areas of New Zealand, except the missing offshore islands.


Lastly, I also need this dataset in the Web Mercator projection (for my next blog post). I’m using ogr2ogr:

ogr2ogr -s_srs EPSG:2193 -t_srs EPSG:3785 nz-land-polygons-webmerc.shp nz-land-polygons.shp

(Written on a rainy day at the Hopewell lodge in Marlborough Sounds. A higly recommended place to releax a few days!)



4 comments:

Jeremy Palmer said...

Hi Bjørn,

Thanks for your great blogging. LINZ is currently working on creating topo250 and topo500 polygon layers as I speak. I know it's a little too late for you, but it should mean other users won't have to go through the polygonise process

I’ll post back here once we release the datasets on http://data.linz.govt.nz

Cheers,
Jeremy

Bjørn Sandvik said...

Thanks Jeremy!

Nice if you post back here once you release the datasets. I'm in New Zealand for another month or two, so I might even use it for my experiments.

It's great that you provide all these data for free!

Cheers,
Bjørn

Jeremy Palmer said...

Hi Bjørn,

We've updated the 50k combined coastline layer with the new data. It's on LDS here:
http://data.linz.govt.nz/layer/1153-nz-coastlines-and-islands-polygons-topo-150k

LINZ has also put up 250k and 500k versions:
http://data.linz.govt.nz/layer/1559-nz-coastlines-and-islands-polygons-topo-1250k/
http://data.linz.govt.nz/layer/1560-nz-coastlines-and-islands-polygons-topo-1500k/

Enjoy!

Cheers,
Jeremy

Bjørn Sandvik said...

Thanks Jeremy,

I've alredy tested the 250k version, at it works perfectly!