Frank et al, I've had great success with simple polygons, and with single polygons with holes. Everything seems to follow expectations. However, the data I'm working with sometimes has multiple polygons, but sometimes doesn't. According to the OGC simple features spec 05-134, a MULTIPOLYGON statement following the same format as a simple POLYGON statement should work. So, the following 2 statements should be equivalent:
'POLYGON( ( 56 34, 62 48, 84 48, 84 42, 56 34) )' or 'MULTIPOLYGON( ( 56 34, 62 48, 84 48, 84 42, 56 34) )' In practice, using the latter format failed in mapserver - no geometries shown. By adding a 3rd set of brackets the problem is solved: 'MULTIPOLYGON( (( 56 34, 62 48, 84 48, 84 42, 56 34)) )' I am now ensuring that all text geometries are added in the database with that extra set of brackets, so it is no longer a problem for me, but it might cause others some pain - seems like a deviation from the spec. (Or I might be reading it wrong). Otherwise, publishing text-based polygon data is working out very well, so thanks again for the support. ------------------------------ Mark Adams Senior Analyst & Project Manager Cuesta Systems (DPRA Canada) 5230 South Service Road Burlington, ON L7L 5K2 Phone: 905-333-4544 x14 Fax: 905-333-0455 Email: [EMAIL PROTECTED] -----Original Message----- From: Frank Warmerdam [mailto:[EMAIL PROTECTED] On Behalf Of Frank Warmerdam Sent: February 20, 2006 11:56 AM To: Mark Adams Cc: [email protected] Subject: Re: [UMN_MAPSERVER-USERS] Solution for OGR Virtual polygon data Mark Adams wrote: > Emil, > > Thanks, you are right, that works. I'd had the same response from Frank W. > I'm not sure where the format for these strings is specified or documented - > the only information I was able to find said to do things differently. > > For the benefit of anyone else trying to use this functionality, I also > discovered that you could create a multi-polygon as follows: > > POLYGON (( 501453.34 4800585.97, 501576.14 4800590.44, 501569.85 4800522, > 501471.61 4800521.63, 501453.34 4800585.97 )) Mark, The format is OpenGIS Well Known Text geometry format from the Simple Features for SQL Specification (99-049 I think). It is the same format reported by ogrinfo, and the textual format of PostGIS or MySQL Spatial. Technically the above is not a multi-polygon. A multi-polygon (in simple features) is a collection of polygon objects. A polygon is one or more rings, with one being the outer ring and the rest being inner rings. So in the parliance I was used to in the old days, a polygon is an island potentially with lakes cut out of it. A multipolygon is a much of such islands. Multipolygons use the MULTIPOLYGON keyword. A 'polygon' object in a shapefile may be either a simple features POLYGON or MULTIPOLYGON depending on whether there are multiple outer rings. Best regards, -- ---------------------------------------+------------------------------------ -- I set the clouds in motion - turn up | Frank Warmerdam, [EMAIL PROTECTED] light and sound - activate the windows | http://pobox.com/~warmerdam and watch the world go round - Rush | Geospatial Programmer for Rent
