How to avoid holes in polygons from lines?

Question

I am trying to convert a dataset of lines that contains the coast of New Zealand into a big polygon.

However using the vector -> geometry tools -> lines to polygon command in Quantum GIS results in a mess with lots of holes and lines outside of the coast.

How can I do the conversion successfully?

It worked on the 1:500k data, the 1:250k data, but not the 1:50k data.

I have a screenshot of the problem here - before and after:

The dataset is from here.

Answer 1

I eventually fixed it by using PostGIS (since this was the eventual target of my fixed polygon layer anyway).

Here is the SQL I used:

ST_MakePolygon(ST_LineMerge(ST_Collect(ST_SnapToGrid(geom, 0.001))))

It basically rounds the vertex coordinates to remove any gaps in the coastline, then zips all the lines into a single line which is used as the boundary for the polygon.

Answer 2

A couple of points. Firstly, I think that there are problems with the original line shapefile. These can be put right using GRASS v.clean using the "snap" option. Try with a threshold of 5 to start with, but just 1 might do. GRASS v.clean can be run from the Sextante toolbox (it's very easy to use).

Secondly, for polygonising I should try the excellent "Polygonizer" plug-in. This needs the Shapely (python-shapely) package to be installed, which can be done using the OSGeo4W Installer or in Ubuntu using the package manager.

N.

Answer 3

Have you seen the Topo50 LDS dataset which is already in polygon form?

We can build 250 and 500 versions if you need those as well.

Answer 4

LINZ have now added the topo250 and topo500 polygon layers.