1 Topic by SK53

  • SK53
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Topic: Errors in MLWM Vice County Shape Files

I've been using the MWLM versions of the Vice County shape files rather more seriously (see //www.flickr.com/photos/mausboam/sets/72157631967110193/with/8169458447/) than in the past and have discovered that several Vice-counties do not have correct geometries. These are:

  • 28 West Norfolk: the geometry crosses itself near Blakeney (TG 014 345). See attachment vc28_bad_geometry

  • 36 Herefordshire: can't diagnose problem.

  • 40 Shropshire: can't diagnose problem.

  • 47 Montgomeryshire: can't diagnose problem.

  • 105 West Ross and Cromarty: island off Loch Broom (Isle Martin) turned inside out (NH 098 992), see vc105_bad_geometry

  • 110 Outer Hebrides: geometry crosses itself on N shore of Eriskay (NF 791 121). See attachment vc110_bad_geometry

  • 111 Orkney: Island S of Sanday geometry not closed (HY 662 373.

  • 112 Zetland: complete mess with land and sea been interchanged on W coast of Yell (HP 477 050). See attachment vc112_bad_geometry.

The grid refs of geometry problems reported by PostGIS are as follows:

  • SO 37392 74844 (Mont, Salop & Heref)

  • SO 25065 99999 (Mont, Salop & Heref)

  • TG 01418 45604 (West Norfolk)

  • NF 79123 12181 (Outer Hebrides)

  • NH 09824 99276 (West Ross and Cromarty)

  • HY 66254 37333 (Orkney)

  • HP 47699 05029 (Zetland)

I haven't checked the MapInfo format files to see if they have the same problems.

A simple way to find bad geometries is to use ST_ISVALID(geometry) within PostGIS. I've tried to autocorrect the geometries with a PostGIS function by called CleanGeometry(), but without success (although I'm not sure I'm using a good version of this routine since I upgraded to PostgreSQL 9.2). See an old blog post of mine for issues concerned with bad polygons in other contexts (see //sk53-osm.blogspot.co.uk/2011/02/exploration-of-bad-polygons.html).

I'll probably edit the offending polygons for my own purposes, but it would be good if these could be fixed for one and all.

Jerry Clough

P.S. additional attachments in followup posts

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2 Reply by SK53

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Re: Errors in MLWM Vice County Shape Files

Orkney bad polygon attachment

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3 Reply by SK53

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Re: Errors in MLWM Vice County Shape Files

West Ross bad polygon

4 Reply by SK53

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Re: Errors in MLWM Vice County Shape Files

Outer Hebrides bad polygon

5 Reply by Mandy Henshall

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Re: Errors in MLWM Vice County Shape Files

Hi Jerry,

Thank you for raising these issues.  I am trying to find out how we resolve the problems and will let you know as soon as possible.

Many thanks for your patience.

Regards,
Mandy

6 Reply by SK53

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Re: Errors in MLWM Vice County Shape Files

Hi Mandy,

Thanks for the response. I've sorted out Norfolk & can attach a shape file to this thread. Simple editing of the shapes is out for the island VCs: I suspect the polygons need to be decomposed and reassembled from their segments. I'll return to the problem at some time.

AFAIK the original digitisation was done by Landmark in the early 2000s. Landmark digitised all the old 6 inch maps which appear on the oldmaps site so obviously had the important intellectual property as well as skills for the digitisation process.

Jerry

7 Reply by Mandy Henshall

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Re: Errors in MLWM Vice County Shape Files

Hi Jerry,

Graham French is going to look into making the necessary corrections.  Unfortunately due to all the work relating to the new version of the NBN Gateway, this will not be possible for a few months.  If you could attach the Norfolk shape file in the meantime, it would be really helpful.

Many thanks and we will let you know how we get on in due course.

Kind regards,
Mandy

8 Reply by SK53

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Re: Errors in MLWM Vice County Shape Files

Time to restart this thread.

I have recently created a set of Vice County boundaries for Ireland using OpenStreetMap. These are available at https://github.com/SK53/Irish-Vice-Coun … e_counties in various formats (Shapefile & Geojson) and projections (WGS84 and Irish Grid). These are still a work in progress. For a detailed account of their creation and some idea of where they might be improved see my blog post.

I exhibited this work at the BSBI Annual Exhibition meeting on Saturday (28th Nov).  Jim Mcintosh (Scottish Officer) & Tom Humphries (software/DB ) of the BSBI were keen to get these known issues fixed. The experience of working on the Irish boundaries has given me a better insight as to how this might be done. In particular I think using GitHub has several benefits:

  • Changes can be tracked

  • Changes can be checked & reviewed before release

  • Releases can be managed

  • Mulitple people can share the work

  • Anyone can raise Issues

  • Partially done useful work is always available for others to build on

The downside is that GitHub is rather mysterious .

So this is really a little bit of advance notice of intent.

Regards,

Jerry Clough
@SK53onOSM on twitter