[mapserver-users] Impact of InValid Geometry on PostGIS queries [SEC=UNCLASSIFIED]

[Roppola, Antti - BRS]

Hi Ben,

Option "d" does exist. Proper GIS do not tolerate invalid geometries.

You could import it into Arc/INFO and use "clean" and "build" for some
heavy handed (but automatic) resolution of issues.

You may come a cropper if you (or someone) makes the right (wrong) kind
of
query against dirty geometries. Say there's a self intersection in an
arc that describes the boundary between "Black" and "White", the loop is
there being defined as Black *and* White. Not particularly good if your
code is expecting an un-ambiguous response.

(Arc/INFO would split the arcs at the loop intersect and create a new,
unlabeled
 polygon that says "Don't Know")

Unfortunately desktop GIS has also promoted the generation of dirty
data.

Antti 

-----Original Message-----
From: mapserver-users-bounces at lists.osgeo.org
[mailto:mapserver-users-bounces at lists.osgeo.org] On Behalf Of Ben Madin
Sent: Thursday, 18 December 2008 6:08 PM
To: mapserver-users at lists.osgeo.org
Subject: [mapserver-users] Impact of InValid Geometry on PostGIS queries

G'day all,

I have a roads shapefile which I imported in PostGIS, planning to merge
several different countries data eventually. It is producing a layer on
the map, and labels etc.

However, the data isn't all good - in fact, about 10% is bad :

es=# select IsValid(the_geom), count(gid) from gis_roads group by
IsValid(the_geom);

  isvalid | count
----------+-------
               |  4128
       t       | 39670
(2 rows)

My question then is :

should I ?

a) leave it - it won't do any harm, but probably won't be rendered,
b) remove it - it may be incorrectly rendered, or waste processor time
on data that is unuseable,
c) fix it by hand (lazy git!)
d) fix it automatically, using super program I wasn't aware of.  
(called?)

actually, I don't have the time or budget for option c, and I suspect
option d does not exist!

cheers

Ben


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