[postgis-users] shp2pgsql and clockwise vertices?

[Jeff Lounsbury]

This is what happens, pgsq2shp verifying clockwiseness and reverses it if it is
incorrect(correct being, ring-clockwise, hole-counter)
shp2pgsql does not reverse vertice order, it assumes it should be correct in the
database.
It is possible however that the shapelib library used in shp2pgsql to generate the
shapefile does not guarantee the clockwise/counter-clockwiseness of the rings?
see http://mapserver.gis.umn.edu/data2/wilma/mapserver-users/0111/msg00079.html
(it's a little old, but seems to say that shapelib doesn't care about
clockwiseness)

So if all you have is single polygons(rings) in clockwise order, they shouldn't be
touched on the way into the database since they are all correct. (is everything
correct in the DB?)
Then on the way out, possibily shapelib reverses some of them, and I can't really
force it not to if that is the case...
-Jeff

Hubert Fröhlich wrote:

> Hi list,
>
> I use PostGIS 0.7.5 and PostgreSQL 7.3.4.
>
> I produced a PostGIS table with MULTIPOLYGONS (150 rows) and wanted to
> export them to shape with pgsql2shp .
>
> When filling the PostGIS table I t took care about:
> - filling only with multipolygons containing 1 polygon with vertices in
> clockwise order.
>
> However, when I produced the shapes and examined the data, I realized
> that  in some (few) shapes the orientation changed to counterclockwise.
>
> Then I tested the same with a modified PostGIS table by deleting some 30
> rows. Result: again some shapes with changed orientation, but not the
> same shapes as in the first test with the 150 shapes.
>
> What is the reason for this mixing up the shape orientation?
>
> Any hints will be appreciated.
> Thanks.
>
> Hubert
>
> --
> -------------------------------------------------------------------------------
> Dr.-Ing. Hubert Fröhlich
> Bezirksfinanzdirektion München
> Alexandrastr. 3, D-80538 München, GERMANY
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> Fax  :+49 (0)89 / 2190 - 2459
> hubert.froehlich at bvv.bayern.de
>
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