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.
P.S. additional attachments in followup posts