Square polygons appear as triangles

[Stephen Woodbridge]

OK, I have a question. Does the ESRI spec require polygons to be closed
or is this just mapserver that requires it? It seems to me that if the
user says it is a polygon layer, then we should be smart enough to check
if it is closed and close it if it is not. This would make much more
sense and make mapserver much more user friendly.

-Steve

Frank Warmerdam wrote:
> On Fri, 25 Feb 2005 21:54:59 +0100, Roberto Bianconi
> <roberto.bianconi at gmail.com> wrote:
>
>>Hi,
>>I have a shp file that describes a grid plot. It is made of adjacent
>>square polygons and when viewed out of Mapserver it is displayed
>>correctly.
>>If I try to plot it with Mapserver, with something like:
>>
>>    LAYER
>>       NAME grd
>>       STATUS DEFAULT
>>       TYPE POLYGON
>>       DATA /path_to_data/grd"
>>       CLASS
>>          EXPRESSION ([VALUE] > 1.e-5 and [VALUE] <= 1.e-4)
>>          COLOR   0 151 255
>>          OUTLINECOLOR 0 0 0
>>       END
>>       CLASS
>>          EXPRESSION ([VALUE] > 1.e-4 and [VALUE] <= 1.e-3)
>>          COLOR   0 223 255
>>          OUTLINECOLOR 0 0 0
>>       END
>>  END
>>
>>I get the square polygons represented as triangles, with a line
>>connecting the lower right corner to the upper left corner (i.e. the
>>lower left vertex is not connected). If I plot this layer - as a test
>>-  as TYPE POINT and add a CLASS vector symbol and size, I can see the
>>4 corners of each polygon grid, each with the selected symbol.
>>
>>Where am I wrong ? Any hint on how to get the correct behaviour?
>>Thank you.
>
>
> Roberto,
>
> My first suspicion is that the polygons in the shapefile are not
> properly closed.  There should be five vertices for each rectangle
> as a close rectangle must have matching first and last vertices.
>
> However, your description of how it is drawn doesn't quite
> sound like I would expect.  You could inspect the file with
> shpdump or ogrinfo to see how many vertices the squares
> really have.  Lots of sloppy shapefile generating apps fail to
> close polygons.  Many apps will do it implicitly, but MapServer
> does not.
>
> Running the file through ogr2ogr may fix it up.
>
> Best regards,
> --
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent
>